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  Plug-and-Play-HOWTO
  David S. Lawyer
   <mailto:dave@lafn.org>
  v1.15, August 2007

  Explains in detail low-level resources such as addresses, interrupts,
  etc.  Covers both the PCI bus, which is inherently Plug and Play (PnP)
  and PnP on the old ISA bus.  If PnP did it's job right, you wouldn't
  need this howto.  But in case it doesn't, or if you have old hardware
  that doesn't use PnP for all the cards, then this HOWTO should help.
  It doesn't cover what's called "Universal Plug and Play" (UPnP).
  ______________________________________________________________________

  Table of Contents



  1. Introduction
     1.1 1. Copyright, Trademarks, Disclaimer, & Credits
        1.1.1 Copyright
        1.1.2 Disclaimer
        1.1.3 Trademarks.
        1.1.4 Credits
     1.2 Future Plans; You Can Help
     1.3 New Versions of this HOWTO
     1.4 New in Recent Versions
     1.5 General Introduction.  Do you need this HOWTO?

  2. What PnP Should Do: Allocate "Bus-Resources"
     2.1 What is Plug-and-Play (PnP)?
     2.2 Hardware Devices and Communication with them
     2.3 Addresses
     2.4 I/O Addresses (principles relevant to other resources too)
     2.5 Memory Ranges
     2.6 IRQs --Overview
     2.7 DMA (Direct Memory Access) or Bus Mastering
     2.8 DMA Channels (not for PCI bus)
     2.9 "Resources" for both Device and Driver
     2.10 Resources are Limited
        2.10.1 Ideal Computers
        2.10.2 Real Computers
     2.11 Second Introduction to PnP
     2.12 How Pnp Works (simplified)
     2.13 Starting Up the PC
     2.14 Buses
     2.15 How Linux Does PnP
     2.16 Problems with Linux PnP

  3. Setting up a PnP BIOS
     3.1 Do you have a PnP operating system?
        3.1.1 Linux prior to the 2.4 kernel
        3.1.2 Windows 2000 and XP
        3.1.3 MS Windows 95, 98 (and Me ?)
     3.2 Assigning Resources by the BIOS
     3.3 Reset the configuration?

  4. How to Deal with PnP Cards
     4.1 Introduction to Dealing with PnP Devices
     4.2 Device Driver Configures, Reserving Resources
     4.3 /sys User Interface Configures
     4.4 BIOS Configures
        4.4.1 Intro to Using the BIOS to Configure PnP
        4.4.2 The BIOS's ESCD Database
        4.4.3 Using Windows to set the ESCD
        4.4.4 Adding a New Device (under Linux or Windows)
     4.5 ISA cards only: Disable PnP ?
     4.6 ISA Bus: Isapnp (part of isapnptools)
     4.7 PCI Utilities
     4.8 Windows Configures
     4.9 PnP Software/Documents

  5. Tell the Driver the Configuration ??
     5.1 Introduction
     5.2 Serial Port Driver Example

  6. How Do I Find Devices and How Are They Configured?
     6.1 Finding and How-Configured Are Related
     6.2 Devices May Have Two "Configurations"
     6.3 Finding Hardware
     6.4 Boot-time Messages
     6.5 The /proc Tree
     6.6 The /sys Tree
     6.7 PCI Bus Inspection
     6.8 ISA Bus Introduction
     6.9 ISA PnP cards
     6.10 LPC Bus
     6.11 X-bus
     6.12 Non-PnP Cards
     6.13 Non-PnP Cards with jumpers
     6.14 Neither PnP nor jumpers
     6.15 Tools for Detecting and/or Configuring all Hardware
     6.16 Tools for Detecting and Configuring One Type of Hardware
     6.17 Use MS Windows

  7. PCI Interrupts
     7.1 Introduction
     7.2 History: From ISA to PCI Interrupts
     7.3 Advanced Programmable Interrupt Controller (APIC)
     7.4 Message Signalled Interrupts (MSI)
     7.5 Sharing PCI Interrupts
     7.6 Looking at Routing Tables
     7.7 For More Information
     7.8 PCI Interrupt Linking

  8. PnP for External and Plug-in Devices
     8.1 USB Bus
     8.2 Hot Plug
     8.3 Hot Swap
     8.4 PnP Finds Devices Plugged Into Serial Ports

  9. Error Messages
     9.1 Unexpected Interrupt
     9.2 Plug and Play Configuration Error (Dell BIOS)
     9.3 isapnp: Write Data Register 0xa79 already used (from logs)
     9.4 Can't allocate region (PCI)

  10. Interrupt Sharing and Interrupt Conflicts
     10.1 Introduction
     10.2 Real Interrupt Conflict
     10.3 No Interrupt Available

  11. Appendix
     11.1 Universal Plug and Play (UPnP)
     11.2 Address Details
        11.2.1 Address ranges
        11.2.2 Address space
        11.2.3 PCI Configuration Address Space
        11.2.4 Range Check (ISA Testing for IO Address Conflicts)
        11.2.5 Communicating Directly via Memory
     11.3 ISA Bus Configuration Addresses (Read-Port etc.)
     11.4 Interrupts --Details
        11.4.1 Serialized Interrupts
        11.4.2 DMA
        11.4.3 Soft interrupts
        11.4.4 Hardware interrupts
     11.5 How the Device Driver Catches its Interrupt
     11.6 ISA Isolation
     11.7 Bus Mastering and DMA resources
     11.8 Historical and Obsolete
        11.8.1 OSS-Lite Sound Driver
        11.8.2 ALSA (Advanced Linux Sound Architecture) as of 2000
        11.8.3 MS Windows Notes


  ______________________________________________________________________



  1.  Introduction

  1.1.  1. Copyright, Trademarks, Disclaimer, & Credits

  1.1.1.  Copyright

  Copyright (c) 1998-2007 by David S. Lawyer  <mailto:dave@lafn.org>

  Please freely copy and distribute (sell or give away) this document in
  any format.  Send any corrections and comments to the document
  maintainer.  You may create a derivative work and distribute it
  provided that you:


  1. If it's not a translation: Email a copy of your derivative work (in
     a format LDP accepts) to the author(s) and maintainer (could be the
     same person).  If you don't get a response then email the LDP
     (Linux Documentation Project): submit@en.tldp.org.

  2. License the derivative work in the spirit of this license or use
     GPL.  Include a copyright notice and at least a pointer to the
     license used.

  3. Give due credit to previous authors and major contributors.

  If you're considering making a derived work other than a translation,
  it's requested that you discuss your plans with the current
  maintainer.


  1.1.2.  Disclaimer

  While I haven't intentionally tried to mislead you, there are likely a
  number of errors in this document.  Please let me know about them.
  Since this is free documentation, it should be obvious that I cannot
  be held legally responsible for any errors.


  1.1.3.  Trademarks.

  Any brand names (starts with a capital letter such as MS Windows)
  should be assumed to be a trademark).  Such trademarks belong to their
  respective owners.



  1.1.4.  Credits


  �  March 2000: Daniel Scott proofread this and found many typos, etc.

  �  June 2000: Pete Barrett gave a workaround to prevent Windows from
     zeroing PCI IRQs.

  �  August 2004: Ross Boylan found typos, etc. and pointed out lack of
     clarity in telling the BIOS if it's a PnP OS


  1.2.  Future Plans; You Can Help

  Please let me know of any errors in facts, opinions, logic, spelling,
  grammar, clarity, links, etc.  But first, if the date is over a
  several months old, check to see that you have the latest version.
  Please send me any info that you think belongs in this document.


  I haven't studied the code used by various Linux drivers and the
  kernel to implement Plug-and-Play.  But I have sampled a little of it
  (especially some of the comments).  Thus this HOWTO is still
  incomplete.  It needs to explain more about "hot swapping", "hot-plug"
  and about the new PnP software for kernel 2.6.  The history of Linux
  PnP is not well covered.  Also, it doesn't cover firewire.  It likely
  has some inaccuracies (let me know where I'm wrong).  In this HOWTO
  I've sometimes used ?? to indicate that I don't really know the
  answer.


  1.3.  New Versions of this HOWTO

  New versions of the Plug-and-Play-HOWTO should appear every year or so
  and will be available to browse and/or download at LDP mirror sites.
  For a list of mirror sites see:  <http://tldp.org/mirrors.html>.
  Various formats are available.  If you only want to quickly check the
  date of the latest version look at:  <http://tldp.org/HOWTO/Plug-and-
  Play-HOWTO.html>.  The version you are now reading is: v1.15, August
  2007 .


  1.4.  New in Recent Versions

  For a full revision history going back to the first version see the
  source file (in linuxdoc format) at
  <http://cvsview.tldp.org/index.cgi/LDP/howto/linuxdoc/Plug-and-Play-
  HOWTO.sgml>


  �  v1.15 Aug. 2007 Revised interrupt sections.  Removed 2 redundant
     and confusing paragraphs containing a mystery function "h()"

  �  v1.14 Feb. 2006: Revised "How Linux Does PnP";  LPC was intended to
     be config. by the BIOS.  Balancing IRQs.  Linux can find drivers
     for detected devices.

  �  v1.13 July 2005: IRQ conflicts. Better clarity in resource
     descriptions.  /proc/bus.  PCI configuration space accessed via IO
     address space.  More hardware detection tools.  "Can't allocate
     region" error message.

  �  v1.12 March 2005: /dev/eth0 doesn't exist anymore. Info in /sys and
     /proc changed for kernel 2.6.  PCI Config. address space is
     "geographic".  scanpci may find a device that lspci can't.  Kernel
     may assign addresses at boot-time.


  1.5.  General Introduction.  Do you need this HOWTO?

  Plug-and-play (PnP) is a system which automatically detects devices
  such as disks, sound cards, ethernet cards, modems, etc.  It finds all
  devices on the PCI bus and all devices that support PnP on the old ISA
  bus.  Before PnP, many devices were automatically searched for by non-
  PnP methods, but were sometimes not found.  PnP provides a way to find
  all devices that support PnP.  It also does some low-level configuring
  of them.  Non-PnP devices (or PnP devices which have not been
  correctly PnP-configured), can often be detected by non-PnP methods.
  The PCI bus is inherently PnP while the old ISA bus originally wasn't
  PnP but had PnP support added to it later.  So sometimes PnP is used
  to only mean PnP for the old ISA bus.  For example, when you see a
  boot-time message from "isapnp" and it reads: "Plug & Play device" it
  only means an ISA Plug & Play device.  In this HOWTO, PnP means PnP
  for both the ISA and the PCI bus.


  As time goes by the Linux kernel is became better at supporting PnP.
  In the late 20th century, one could say that Linux was not really a
  PnP OS.  But the claim is made that with version 2.6 of the kernel,
  Linux is now fully PnP (provided the kernel is built with appropriate
  PnP support).  While the PnP system is not centralized like it is in
  MS Windows (with its registry) the decentralized Linux PnP seems to
  work OK.

  Linux does keep track of resource assignments requested by device
  drivers and refuses any request if it thinks it would cause a
  conflict.  The kernel also provides programs that device drivers can
  call on to do their own plug-and-play.  The kernel also reads all
  configuration registers of all PnP devices and maintains tables of
  them that device drivers can consult.  This table helps drivers find
  their hardware.  Kernel 2.6 provides better support for "hot plug".

  The BIOS hardware of your PC likely does some plug-and-play work too.
  Thus if everything works OK PnP-wise, you can use your computer
  without needing to know anything about plug-and-play.  But if some
  devices which are supported by Linux don't work (because they're not
  discovered or configured correctly by PnP) then you may need to read
  some of this HOWTO.  You'll learn not only about PnP but also learn
  something about how communication takes place inside the computer.  If
  you have a modern computer with a PCI bus but no ISA bus, you may skip
  over or skim the parts about the ISA bus.

  If you're having problems with a device, watch the messages displayed
  at boot-time (go back thru them using Shift-PageUp).  If this doesn't
  also display early messages from the BIOS use the "Pause" key.  See
  ``Pause''

  Check to see that you have the right driver for a device, and that the
  driver is being found and used.  If the driver is a module, type
  "lsmod" (as the root user) to see it it's loaded (in use).  If it's
  not a module then it should be built into the kernel.

  This HOWTO doesn't cover the problem of finding and installing device
  drivers.  Perhaps it should.  One problem is that a certain brand of a
  card (or other physical device) may not say what kind of chips are
  used in it.  The driver name is often the same as the chip name and
  not the brand name.  One way to start to check on a driver is to see
  if it is discussed in the kernel documentation, in another HOWTO, or
  on the Internet.  Warning: Such documentation may be out of date.

  The PCI bus computers (no ISA bus) have significantly reduced the
  number of things that can go wrong.  For the ISA bus and the lack of
  kernel support for ISA Pnp (before kernel 2.4), there was much more
  that could go wrong.  Remember that sometimes problems which seem to
  be PnP related are actually due to defective hardware or to hardware
  that doesn't fully conform to PnP specs.


  2.  What PnP Should Do: Allocate "Bus-Resources"

  2.1.  What is Plug-and-Play (PnP)?

  If you don't understand this section, read the next section ``Hardware
  Devices and Communication with  them''


  Oversimplified, Plug-and-Play tells the software (device drivers)
  where to find various pieces of hardware (devices) such as modems,
  network cards, sound cards, etc.  Plug-and-Play's task is to match up
  physical devices with the software (device drivers) that operates them
  and to establish channels of communication between each physical
  device and its driver.  In order to achieve this, PnP allocates and
  sets the following "bus-resources" in hardware: I/O addresses, memory
  regions, IRQs, DMA channels (LPC and ISA buses only).  These 4 things
  are sometimes called "1st order resources" or just "resources".  Pnp
  maintains a record of what it's done and allows device drivers to get
  this information.  If you don't understand what these 4 bus-resources
  are, read the following subsections of this HOWTO: I/O Addresses,
  IRQs, DMA Channels, Memory Regions.  An article in Linux Gazette
  regarding 3 of these bus-resources is Introduction to IRQs, DMAs and
  Base Addresses.  Once these bus-resources have been assigned (and if
  the correct driver is installed), the actual driver and the "files"
  for it in the /dev directory are ready to use.

  This PnP assignment of bus-resources is sometimes called "configuring"
  but it is only a low level type of configuring.  The /etc directory
  has many configuration files but most all of them are not for PnP
  configuring.  So most of the configuring of hardware devices has
  nothing to do with PnP or bus-resources.  For, example the
  initializing of a modem by an "init string" or setting it's speed is
  not PnP.  Thus when talking about PnP, "configuring" means only a
  certain type of configuring.  While other documentation (such as for
  MS Windows) simply calls bus-resources "resources",  I sometimes use
  the term "bus-resources" instead of just "resources" so as to
  distinguish it from the multitude of other kinds of resources.

  PnP is a process which is done by various software and hardware.  If
  there was just one program that handled PnP in Linux, it would be
  simple.  But with Linux each device driver does it's own PnP, using
  software supplied by the kernel.  The BIOS hardware of the PC does PnP
  when a PC is first powered up.  And there's a lot more to it than
  this.


  2.2.  Hardware Devices and Communication with them

  A computer consists of a CPU/processor to do the computing and RAM
  memory to store programs and data (for fast access).  In addition,
  there are a number of devices such as various kinds of disk-drives, a
  video card, a keyboard, network devices, modem cards, sound devices,
  the USB bus, serial and parallel ports, etc.  In olden days most
  devices were on cards inserted into slots in the PC.  Today, many
  devices that were formerly cards, are now on-board since they are
  contained in chips on the motherboard.  There is also a power supply
  to provide electric energy, various buses on a motherboard to connect
  the devices to the CPU, and a case to put all this into.

  Cards which plug into the motherboard may contain more than one
  device.  Memory chips are also sometimes considered to be devices but
  are not plug-and-play in the sense used in this HOWTO.

  For the computer system to work right, each device must be under the
  control of its "device driver".  This is software which is a part of
  the operating system (perhaps loaded as a module) and runs on the CPU.
  Device drivers are associated with "special files" in the /dev
  directory although they are not really files.  They have names such as
  hda3 (third partition on hard drive a), ttyS1 (the second serial
  port), eth0 (the first ethernet card), etc.

  The eth0 device is for an ethernet card (nic card).  Formerly it was
  /dev/eth0 but it's now just a virtual device in the kernel.  What eth0
  refers to depends on the type of ethernet card you have.  If the
  driver is a module, this assignment is likely in an internal kernel
  table but might be found in /etc/modules.conf (called "alias").  For
  example, if you have an ethernet card that uses the "tulip" chip you
  could put "alias eth0 tulip" into /etc/modules.conf so that when your
  computer asks for eth0 it finds the tulip driver.  However, modern
  kernels can usually find the right driver module so that you seldom
  need to specify it yourself.

  To control a device, the CPU (under the control of the device driver)
  sends commands and data to, and reads status and data from the various
  devices.  In order to do this each device driver must know the address
  of the device it controls.  Knowing such an address is equivalent to
  setting up a communication channel, even though the physical "channel"
  is actually the data bus inside the PC which is shared with many other
  devices.

  This communication channel is actually a little more complex than
  described above.  An "address" is actually a range of addresses so
  that sometimes the word "range" is used instead of "address".  There
  could even be more that one range (with no overlapping) for a single
  device.  Also, there is a reverse part of the channel (known as
  interrupts) which allows devices to send an urgent "help" request to
  their device driver.


  2.3.  Addresses

  The PCI bus has 3 address spaces: I/O, main memory (IO memory), and
  configuration.  The old ISA bus lacks a genuine "configuration"
  address space.  Only the I/0 and IO memory spaces are used for device
  IO.  Configuration addresses are fixed and can't be changed so they
  don't need to be allocated.  For more details see ``PCI Configuration
  Address Space''

  When the CPU wants to access a device, it puts the device's address on
  a major bus of the computer (for PCI: the address/data bus).  All
  types of addresses (such as both I/O and main memory) share the same
  bus inside the PC.  But the presence or absence of voltage on certain
  dedicated wires in the PC's bus tells which "space" an address is in:
  I/O, main memory, (see ``Memory Ranges''), or configuration (PCI
  only).  This is a little oversimplified since telling a PCI device
  that it's a configuration space access is actually more complex than
  described above.  See ``PCI Configuration Address Space'' for details.
  See ``Address Details'' for more details on addressing in general.

  The addresses of a device are stored in it's registers in the physical
  device.  They can be changed by software and they can be disabled so
  that the device has no address at all.  Except that the PCI
  configuration address can't be changed or disabled.


  2.4.  I/O Addresses (principles relevant to other resources too)

  Devices were originally located in I/O address space but today they
  may use space in main memory.  An I/0 address is sometimes just called
  "I/O", "IO", "i/o" or "io".  The terms "I/O port" or "I/O range" are
  also used.  Don't confuse these IO ports with "IO memory" located in
  main memory.  There are two main steps to allocate the I/O addresses
  (or some other bus-resources such as interrupts on the ISA bus):


  1. Set the I/O address, etc. in the hardware (in one of its registers)

  2. Let its device driver know what this I/O address, etc. is

  Often, the device driver does both of these (sort of).  The device
  driver doesn't actually need to set an I/O address if it finds out
  that the address has been previously set (perhaps by the BIOS) and is
  willing to accept that address.  Once the driver has either found out
  what address has been previously set or sets the address itself, then
  it obviously knows what the address is so there is no need to let the
  driver know the address --it already knows it.
  The two step process above (1. Set the address in the hardware.  2.
  Let the driver know it.) is something like the two part problem of
  finding someone's house number on a street.  Someone must install a
  number on the front of the house so that it may be found and then
  people who might want to go to this address must obtain (and write
  down) this house number so that they can find the house.  For
  computers, the device hardware must first get its address put into a
  special register in its hardware (put up the house number) and then
  the device driver must obtain this address (write the house number in
  its address book).  Both of these must be done, either automatically
  by software or by entering the data manually into configuration files.
  Problems may occur when only one of them gets done right.

  For manual PnP configuration some people make the mistake of doing
  only one of these two steps and then wonder why the computer can't
  find the device.  For example, they may use "setserial" to assign an
  address to a serial port without realizing that this only tells the
  driver an address.  It doesn't set the address in the serial port
  hardware itself.  If you told the driver wrong then you're in trouble.
  Another way to tell the driver is to give the address as an option to
  a kernel module (device driver).  If what you tell it is wrong, there
  could be problems.  A smart driver may detect how the hardware is
  actually set and reject the incorrect information supplied by the
  option (or at least issue an error message).

  An obvious requirement is that before the device driver can use an
  address it must be first set in the physical device (such as a card).
  Since device drivers often start up soon after you start the computer,
  they sometimes try to access a card (to see if it's there, etc.)
  before the address has been set in the card by a PnP configuration
  program.  Then you see an error message that they can't find the card
  even though it's there (but doesn't yet have an address yet).

  What was said in the last few paragraphs regarding I/O addresses
  applies with equal force to most other bus-resources: ``Memory
  Ranges'', ``IRQs --Overview'' and ``DMA Channels''.  What these are
  will be explained in the next 3 sections.  The exception is that
  interrupts on the PCI bus are not set by card registers but are
  instead routed (mapped) to IRQs by a chip on the motherboard.  Then
  the IRQ a PCI card is routed to is written into the card's register
  for information purposes only.

  To see what IO addresses are used on your PC, look at the
  /proc/ioports file.


  2.5.  Memory Ranges

  Many devices are assigned address space in main memory.  It's
  sometimes called "shared memory" or "memory-mapped IO" or "IO memory".
  This memory is physically located inside the physical device but the
  computer accesses it just like it would access memory on memory chips.
  When discussing bus-resources it's often just called "memory", "mem",
  or "iomem".  In addition to using such "memory", such a device might
  also use conventional IO address space.  To see what mem is in use on
  your computer, look at /proc/iomem.  This "file" includes the memory
  used by your ordinary RAM memory chips so it shows memory allocation
  in general and not just iomem allocation.  If you see a strange number
  instead of a name, it's likely the number of a PCI device which you
  can verify by typing "lspci".

  When you insert a card that uses iomem, you are in effect also
  inserting a memory module for main memory.  A high address is selected
  for it by PnP so that it doesn't conflict with the main memory modules
  (chips).  This memory can either be ROM (Read Only Memory) or shared
  memory.  Shared memory is shared between the device and the CPU
  (running the device driver) just as IO address space is shared between
  the device and the CPU.  This shared memory serves as a means of data
  "transfer" between the device and main memory. It's Input-Output (IO)
  but it's not done in IO space.  Both the card and the device driver
  need to know the memory range.

  ROM (Read Only Memory) on cards is a different kind of iomem.  It is
  likely a program (perhaps a device driver) which will be used with the
  device.  It could be initialization code so that a device driver is
  still required.  Hopefully, it will work with Linux and not just MS
  Windows.  It may need to be shadowed which means that it is copied to
  your main memory chips in order to run faster.  Once it's shadowed
  it's no longer "read only".


  2.6.  IRQs --Overview

  After reading this you may want to read ``Interrupts --Details'' for
  many more details.  The following is intentionally oversimplified:
  Besides the address, there is also an interrupt number to deal with
  (such as IRQ 5).  It's called an IRQ (Interrupt ReQuest) number or
  just an "irq" for short.  We already mentioned above that the device
  driver must know the address of a card in order to be able to
  communicate with it.

  But what about communication in the opposite direction?  Suppose the
  device needs to tell its device driver something immediately.  For
  example, the device may be receiving a lot of bytes destined for main
  memory and its buffer used to store these bytes is almost full.  Thus
  the device needs to tell its driver to fetch these bytes at once
  before the buffer overflows from the incoming flow of bytes.  Another
  example is to signal the driver that the device has finished sending
  out a bunch of bytes and is now waiting for some more bytes from the
  driver so that it can send them too.

  How should the device rapidly signal its driver?  It may not be able
  to use the main data bus since it's likely already in use.  Instead it
  puts a voltage on a dedicated interrupt wire (also called line or
  trace) which is often reserved for that device alone.  This voltage
  signal is called an Interrupt ReQuest (IRQ) or just an "interrupt" for
  short.  There are the equivalent of 16 (or 24, etc.) such wires in a
  PC and each wire leads (indirectly) to a certain device driver.  Each
  wire has a unique IRQ (Interrupt ReQuest) number.  The device must put
  its interrupt on the correct wire and the device driver must listen
  for the interrupt on the correct wire.  Which wire the device sends
  such "help requests" on is determined by the IRQ number stored in the
  device.  This same IRQ number must be known to the device driver so
  that the device driver knows which IRQ line to listen on.

  Once the device driver gets the interrupt from the device it must find
  out why the interrupt was issued and take appropriate action to
  service the interrupt.  On the ISA bus, each device usually needs its
  own unique IRQ number.  For the PCI bus and other special cases, the
  sharing of IRQs is allowed (two or more PCI devices may have the same
  IRQ number).  Also, for PCI, each PCI device has a fixed "PCI
  Interrupt" wire.  But a programmable routing chip maps the PCI wires
  to ISA-type interrupts.  See ``Interrupts --Details'' for details on
  how all the above works.


  2.7.  DMA (Direct Memory Access) or Bus Mastering

  For the PCI bus, DMA and Bus Mastering mean the same thing.  Prior to
  the PCI bus, Bus Mastering was rare and DMA worked differently and was
  slow.  Direct Memory Access (DMA) is where a device is allowed to take
  over the main computer bus from the CPU and transfer bytes directly to
  main memory or to some other device.  Normally the CPU would make a
  transfer from a device to main memory in a two step process:

  1. reading a chunk of bytes from the I/O memory space of the device
     and putting these bytes into CPU itself

  2. writing these bytes from the CPU to main memory

  With DMA it's a one step process of sending the bytes directly from
  the device to memory.  The device must have DMA capabilities built
  into its hardware and thus not all devices can do DMA.  While DMA is
  going on, the CPU can't do too much since the main bus is being used
  by the DMA transfer.

  The old ISA bus can do slow DMA while the PCI bus does "DMA" by Bus
  Mastering.  The LPC bus has both the old DMA and the new DMA (bus
  mastering).  On the PCI bus, what more precisely should be called "bus
  mastering" is often called "Ultra DMA", "BM-DNA", "udma", or just
  "DMA", Bus mastering allows devices to temporarily become bus masters
  and to transfer bytes almost like the bus master was the CPU.  It
  doesn't use any channel numbers since the organization of the PCI bus
  is such that the PCI hardware knows which device is currently the bus
  master and which device is requesting to become a bus master.  Thus
  there is no resource allocation of DMA channels for the PCI bus and no
  dma channel resources exist for this bus.  The LPC (Low Pin Count) bus
  is supposed to be configured by the BIOS so users shouldn't need to
  concern themselves with its DMA channels.


  2.8.  DMA Channels (not for PCI bus)

  This is only for the LPC bus and the old ISA bus.  When a device wants
  to do DMA it issues a DMA-request using dedicated DMA request wires
  much like an interrupt request.  DMA actually could have been handled
  by using interrupts but this would introduce some delays so it's
  faster to do it by having a special type of interrupt known as a DMA-
  request.  Like interrupts, DMA-requests are numbered so as to identify
  which device is making the request.  This number is called a DMA-
  channel.  Since DMA transfers all use the main bus (and only one can
  run at a time) they all actually use the same channel for data flow
  but the "DMA channel" number serves to identify who is using the
  "channel".  Hardware registers exist on the motherboard which store
  the current status of each "channel".  Thus in order to issue a DMA-
  request, the device must know its DMA-channel number which must be
  stored in a special register on the physical device.


  2.9.  "Resources" for both Device and Driver

  Thus device drivers must be "attached" in some way to the hardware
  they control.  This is done by allocating bus-resources (I/O, Memory,
  IRQ's, DMA's) to both the physical device and letting the device
  driver to find out about it.  For example, a serial port uses only 2
  resources: an IRQ and an I/O address.  Both of these values must be
  supplied to the device driver and the physical device.  The driver
  (and its device) is also given a name in the /dev directory (such as
  ttyS1).  The address and IRQ number is stored by the physical device
  in configuration registers on its card (or in a chip on the
  motherboard).  Old hardware (in the mid 1990's) used switches (or
  jumpers) to physically set the IRQ and address in the hardware.  This
  setting remained fixed until someone remover the computer's cover and
  moved the jumpers.

  But for the case of PnP (no jumpers), the configuration register data
  is usually lost when the PC is powered down (turned off) so that the
  bus-resource data must be supplied to each device anew each time the
  PC is powered on.


  2.10.  Resources are Limited

  2.10.1.  Ideal Computers

  The architecture of the PC provides only a limited number of
  resources: IRQ's, DMA channels, I/O address, and memory regions.  If
  there were only a limited number devices and they all used
  standardized bus-resources values (such as unique I/O addresses and
  IRQ numbers) there would be no problem of attaching device drivers to
  devices.  Each device would have a fixed resources which would not
  conflict with any other device on your computer.  No two devices would
  have the same addresses, there would be no IRQ conflicts on the ISA
  bus, etc.  Each driver would be programmed with the unique addresses,
  IRQ, etc. hard-coded into the program.  Life would be simple.

  Another way to prevent address conflicts would be to have each card's
  slot number included as part of the address.  Thus there could be no
  address conflict between two different cards (since they are in
  different slots).  Card design would not allow address conflicts
  between different functions of the card.  It turns out that the
  configuration address space (used for resource inquiry and assignment)
  actually does this.  But it's not done for I/O addresses nor memory
  regions.  Sharing IRQs as on the PCI bus also avoids conflicts but may
  cause other problems.


  2.10.2.  Real Computers

  But PC architecture has conflict problems.  The increase in the number
  of devices (including multiple devices of the same type) has tended to
  increase potential conflicts.  At the same time, the introduction of
  the PCI bus, where two or more devices can share the same interrupt
  and the introduction of more interrupts, has tended to reduce
  conflicts.  The overall result, due to going to PCI, has been a
  reduction in conflicts since the scarcest resource is IRQs.  However,
  even on the PCI bus it's more efficient to avoid IRQ sharing.  In some
  cases where interrupts happen in rapid succession and must be acted on
  fast (like audio) sharing can cause degradation in performance.  So
  it's not good to assign all PCI devices the same IRQ, the assignment
  needs to be balanced.  Yet some people find that all their PCI devices
  are on the same IRQ.

  So devices need to have some flexibility so that they can be set to
  whatever address, IRQ, etc. is needed to avoid any conflicts and
  achieve balancing.  But some IRQ's and addresses are pretty standard
  such as the ones for the clock and keyboard.  These don't need such
  flexibility.

  Besides the problem of conflicting allocation of bus-resources, there
  is a problem of making a mistake in telling the device driver what the
  bus-resources are.  This is more likely to happen for the case of old-
  fashioned manual configuration where the user types in the resources
  used into a configuration file stored on the harddrive.  This often
  worked OK when resources were set by jumpers on the cards (provided
  the user knew how they were set and made no mistakes in typing this
  data to configuration files).  But with resources being set by PnP
  software, they may not always get set the same and this may mean
  trouble for any manual configuration where the user types in the
  values of bus-resources that were set by PnP.

  The allocation of bus-resources, if done correctly, establishes non-
  conflicting channels of communication between physical hardware and
  their device drivers.  For example, if a certain I/O address range
  (resource) is allocated to both a device driver and a piece of
  hardware, then this has established a one-way communication channel
  between them.  The driver may send commands and other info to the
  device.  It's actually more than one-way communications since the
  driver may get information from the device by reading its registers.
  But the device can't initiate any communication this way.  To initiate
  communication the device needs an IRQ so it can send interrupts to its
  driver.  This creates a two-way communication channel where both the
  driver and the physical device can initiate communication.


  2.11.  Second Introduction to PnP

  The term Plug-and-Play (PnP) has various meanings.  In the broad sense
  it is just auto-configuration where one just plugs in a device and it
  configures itself.  In the sense used in this HOWTO, PnP means the
  configuring PnP bus-resources (setting them in the physical devices)
  and letting the device drivers know about it.  For the case of Linux,
  it is often just a driver determining how the BIOS has set bus-
  resources and if necessary, the driver giving a command to change
  (reset) the bus-resources.  "PnP" often just means PnP on the ISA bus
  so that the message from isapnp: "No Plug and Play device found" just
  means that no ISA PnP devices were found.  The standard PCI
  specifications (which were invented before coining the term "PnP")
  provide the equivalent of PnP for the PCI bus.

  PnP matches up devices with their device drivers and specifies their
  communication channels (by allocating bus-resources).  It
  electronically communicates with configuration registers located
  inside the physical devices using a standardized protocol.  On the ISA
  bus before Plug-and-Play, the bus-resources were formerly set in
  hardware devices by jumpers or switches.  Sometimes the bus-resources
  could be set into the hardware electronically by a driver (usually
  written only for a MS OS but in rare cases supported by a Linux
  driver).  This was something like PnP but there was no standardized
  protocol used so it wasn't really PnP.  Some cards had jumper setting
  which could be overridden by such software.  For Linux before PnP,
  most software drivers were assigned bus-resources by configuration
  files (or the like) or by probing the for the device at addresses
  where it was expected to reside.  But these methods are still in use
  today to allow Linux to use old non-PnP hardware.  And sometimes these
  old methods are still used today on PnP hardware (after say the BIOS
  has assigned resources to hardware by PnP methods).

  The PCI bus was PnP-like from the beginning, but it's not usually
  called PnP or "plug and play" with the result that PnP often means PnP
  on the ISA bus.  But PnP in this documents usually means PnP on either
  the ISA or PCI bus.


  2.12.  How Pnp Works (simplified)

  Here's how PnP should work in theory.  The hypothetical PnP
  configuration program finds all PnP devices and asks each what bus-
  resources it needs.  Then it checks what bus-resources (IRQs, etc.) it
  has to give away.  Of course, if it has reserved bus-resources used by
  non-PnP (legacy) devices (if it knows about them) it doesn't give
  these away.  Then it uses some criteria (not specified by PnP
  specifications) to give out the bus-resources so that there are no
  conflicts and so that all devices get what they need (if possible).
  It then indirectly tells each physical device what bus-resources are
  assigned to it and the devices set themselves up to use only the
  assigned bus-resources.  Then the device drivers somehow find out what
  bus-resources their devices use and are thus able to communicate
  effectively with the devices they control.

  For example, suppose a card needs one interrupt (IRQ number) and 1 MB
  of shared memory.  The PnP program reads this request from the
  configuration registers on the card.  It then assigns the card IRQ5
  and 1 MB of memory addresses space, starting at address 0xe9000000.
  The PnP program also reads identifying information from the card
  telling what type of device it is, its ID number, etc.  Then it
  directly or indirectly tells the appropriate device driver what it's
  done.  If it's the driver itself that is doing the PnP, then there's
  no need to find a driver for the device (since it's driver is already
  running).  Otherwise a suitable device driver needs to be found and
  sooner or later told how it's device is configured.

  It's not always this simple since the card (or routing table for PCI)
  may specify that it can only use certain IRQ numbers or that the 1 MB
  of memory must lie within a certain range of addresses.  The details
  are different for the PCI and ISA buses with more complexity on the
  ISA bus.

  One way commonly used to allocate resources is to start with one
  device and allocate it bus-resources.  Then do the same for the next
  device, etc.  Then if finally all devices get allocated resources
  without conflicts, then all is OK.  But if allocating a needed
  resource would create a conflict, then it's necessary to go back and
  try to make some changes in previous allocations so as to obtain the
  needed bus-resource.  This is called rebalancing.  Linux doesn't do
  rebalancing but MS Windows does in some cases.  For Linux, all this is
  done by the BIOS and/or kernel and/or device drivers.  In Linux, the
  device driver doesn't get it's final allocation of resources until the
  driver starts up, so one way to avoid conflicts is just not to start
  any device that might cause a conflict.  However, the BIOS often
  allocates resources to the physical device before Linux is even booted
  and the kernel checks PCI devices for addresses conflicts at boot-
  time.

  There are some shortcuts that PnP software may use.  One is to keep
  track of how it assigned bus-resources at the last configuration (when
  the computer was last used) and reuse this.  BIOSs do this as does MS
  Windows and this but standard Linux doesn't.  But in a way it does
  since it often uses what the BIOS has done.   Windows stores this info
  in its "Registry" on the hard disk and a PnP/PCI BIOS stores it in
  non-volatile memory in your PC (known as ESCD; see ``The BIOS's ESCD
  Database'').  Some say that not having a registry (like Linux) is
  better since with Windows, the registry may get corrupted and is
  difficult to edit.  But PnP in Linux has problems too.

  While MS Windows (except for Windows 3.x and NT4) were PnP, Linux was
  not originally a PnP OS but has been gradually becoming a PnP OS.  PnP
  originally worked for Linux because a PnP BIOS would configure the
  bus-resources and the device drivers would find out (using programs
  supplied by the Linux kernel) what the BIOS has done.  Today, most
  drivers can issue commands to do their own bus-resource configuring
  and don't need to always rely on the BIOS.  Unfortunately a driver
  could grab a bus-resource which another device will need later on.
  Some device drivers may store the last configuration they used in a
  configuration file and use it the next time the computer is powered
  on.

  If the device hardware remembered its previous configuration, then
  there wouldn't be any hardware to PnP configure at the next boot-time.
  But hardware seems to forget its configuration when the power is
  turned off.  Some devices contain a default configuration (but not
  necessarily the last one used).  Thus a PnP device needs to be re-
  configured each time the PC is powered on.  Also, if a new device has
  been added, then it too needs to be configured too.  Allocating bus-
  resources to this new device might involve taking some bus-resources
  away from an existing device and assigning the existing device
  alternative bus-resources that it can use instead.  At present, Linux
  can't allocate with this sophistication (and MS Windows XP may not be
  able to do it either).


  2.13.  Starting Up the PC

  When the PC is first turned on the BIOS chip runs its program to get
  the computer started (the first step is to check out the motherboard
  hardware).  If the operating system is stored on the hard-drive (as it
  normally is) then the BIOS must know about the hard-drive.  If the
  hard-drive is PnP then the BIOS may use PnP methods to find it.  Also,
  in order to permit the user to manually configure the BIOS's CMOS and
  respond to error messages when the computer starts up, a screen (video
  card) and keyboard are also required.  Thus the BIOS must always PnP-
  configure devices needed to load the operating system from the hard-
  drive.

  Once the BIOS has identified the hard-drive, the video card, and the
  keyboard it is ready to start booting (loading the operating system
  into memory from the hard-disk).  If you've told the BIOS that you
  have a PnP operating system (PnP OS), it should start booting the PC
  as above and let the operating system finish the PnP configuring.
  Otherwise, a PnP-BIOS will (prior to booting) likely try to do the
  rest of the PnP configuring of devices (but not inform the device
  drivers of what it did).  But the drivers can still find out this by
  utilizing functions available in the Linux kernel.


  2.14.  Buses

  To see what's on the PCI bus type lspci or lspci -vv.  Or type scanpci
  -v for the same information in the numeric code format where the
  device is shown by number (such as: "device 0x122d" instead of by
  name, etc.  In rare cases, scanpci will find a device that lspci can't
  find.

  The boot-time messages on your display show devices which have been
  found on various buses (use shift-PageUp to back up thru them).  See
  ``Boot-time Messages''

  ISA is the old bus of the old IBM-compatible PCs while PCI is a newer
  and faster bus from Intel.  The PCI bus was designed for what is today
  called PnP.  This makes it easy (as compared to the ISA bus) to find
  out how PnP bus-resources have been assigned to hardware devices.

  For the ISA bus there was a real problem with implementing PnP since
  no one had PnP in mind when the ISA bus was designed and there are
  almost no I/O addresses available for PnP to use for sending
  configuration info to a physical device.  As a result, the way PnP was
  shoehorned onto the ISA bus is very complicated.  Whole books have
  been written about it.  See ``PnP Book''.  Among other things, it
  requires that each PnP device be assigned a temporary "handle" by the
  PnP program so that one may address it for PnP configuring.  Assigning
  these "handles" is call "isolation".  See ``ISA Isolation'' for the
  complex details.

  As the ISA bus becomes extinct, PnP will be a little easier.  It will
  then not only be easier to find out how the BIOS has configured the
  hardware, but there will be less conflicts since PCI can share
  interrupts.  There will still be the need to match up device drivers
  with devices and also a need to configure devices that are added when
  the PC is up and running.  The serious problem of some devices not
  being supported by Linux will remain.


  2.15.  How Linux Does PnP

  Linux has had serious problems in the past in dealing with PnP but
  most of those problems have now been solved (as of mid 2004).  Linux
  has gone from a non-PnP system originally, to one that can be PnP if
  certain options are selected when compiling the kernel.  The BIOS may
  assign IRQs but Linux may also assign some of them or even reassign
  what the BIOS did.  The configuration part of ACPI (Advance
  Configuration and Power Interface) is designed to make it easy for
  operating systems to do their own configuring.  Linux can use ACPI if
  it's selected when the kernel is compiled.

  In Linux, it's traditional for each device driver to do it's own low
  level configuring.  This was difficult until Linux supplied software
  in the kernel that the drivers could use to make it easier on them.
  Today (2005) it has reached the point where the driver simply calls
  the kernel function: pci_enable_device() and the device gets
  configured by being enabled and having both an irq (if needed) and
  addresses assigned to the device.  This assignment could be what was
  previously assigned by the BIOS or what the kernel had previously
  reserved for it when the pci or isapnp device was detected by the
  kernel.  There's even an ACPI option for Linux to assign all devices
  IRQs at boot-time.

  So today, in a sense, the drivers are still doing the configuring but
  they can do it by just telling Linux to do it (and Linux may not need
  to do much since it sometimes is able to use what has already been set
  by the BIOS or Linux).  So it's really the non-device-driver part of
  the Linux kernel that is doing most of the configuring.  Thus, it may
  be correct to call Linux a PnP operating system, at least for common
  computer architectures.

  Then when a device driver finds its device, it asks to see what
  addresses and IRQ have been assigned (by the BIOS and/or Linux) and
  normally just accepts them.  But if the driver wants to do so, it can
  try to change the addresses, using functions supplied by the kernel.
  But the kernel will not accept addresses that conflict with other
  devices or ones that the hardware can't support.  When the PC starts
  up, you may note messages on the screen showing that some Linux device
  drivers have found their hardware devices and what the IRQ and address
  ranges are.

  Thus, the kernel provides the drivers with functions (program code)
  that the drivers may use to find out if their device exists, how it's
  been configured, and functions to modify the configuration if needed.
  Kernel 2.2 could do this only for the PCI bus but Kernel 2.4 had this
  feature for both the ISA and PCI buses (provided that the appropriate
  PNP and PCI options have been selected when compiling the kernel).
  Kernel 2.6 came out with better utilization of ACPI.  This by no means
  guarantees that all drivers will fully and correctly use these
  features.  And legacy devices that the BIOS doesn't know about, may
  not get configured until you (or some configuration utility) puts its
  address, irq, etc. into a configuration file.

  In addition, the kernel helps avoid resource conflicts by not allowing
  two devices that it knows about to use the same bus-resources at the
  same time.  Originally this was only for IRQs, and DMAs but now it's
  for address resources as well.

  If your have an old ISA bus, the program isapnp should run at boottime
  to find and configure pnp devices on the ISA bus.  Look at the
  messages with "dmesg".

  To see what help the kernel may provide to device drivers see the
  directory /usr/.../.../Documentation where one of the ... contains the
  word "kernel-doc" or the like.  Warning: documentation here tends to
  be out-of-date so to get the latest info you would need to read
  messages on mailing lists sent by kernel developers and possibly the
  computer code that they write including comments.  In this kernel
  documentation directory see pci.txt ("How to Write Linux PCI Drivers")
  and the file: /usr/include/linux/pci.h.  Unless you are a driver guru
  and know C Programming, these files are written so tersely that they
  will not actually enable you to write a driver.  But it will give you
  some idea of what PnP type functions are available for drivers to use.

  For kernel 2.4 see isapnp.txt.  For kernel 2.6, isapnp.txt is replaced
  by pnp.txt which is totally different than isapnp.txt and also deals
  with the PCI bus.  Also see the O'Reilly book: Linux Device Drivers,
  3rd ed., 2005.  The full text is on the Internet.


  2.16.  Problems with Linux PnP

  But there are a number of things that a real PnP operating system
  could handle better:


  �  Allocate bus-resources when they are in short supply by
     reallocation of resources if necessary

  �  Deal with choosing a driver when there is more than one driver for
     a physical device

  Since it's each driver for itself, a driver could grab bus-resources
  that are needed by other devices (but not yet allocated to them by the
  kernel).  Thus a more sophisticated PnP Linux kernel would be better,
  where the kernel did the allocation after all requests were in.
  Another alternative would be a try to reallocate resources already
  assigned if a devices couldn't get the resources it requested.

  The "shortage of bus-resources" problem is becoming less of a problem
  for two reasons:  One reason is that the PCI bus is replacing the ISA
  bus.  Under PCI there is no shortage of IRQs since IRQs may be shared
  (even though sharing is a little less efficient).  Also, PCI doesn't
  use DMA resources (although it does the equivalent of DMA without
  needing such resources).

  The second reason is that more address space is available for device
  I/0.  While the conventional I/O address space of the ISA bus was
  limited to 64KB, the PCI bus has 4GB of it.  Since more physical
  devices are using main memory addresses instead of IO address space,
  there is still more space available, even on the ISA bus.  On 32-bit
  PCs there is 4GB of main memory address space and much of this bus-
  resource is available for device IO (unless you have 4GB of main
  memory installed).

  There was at least one early attempt to make Linux a truly PnP
  operating system.  See  <http://www.astarte.free-online.co.uk>.  While
  developed around 1998 it never was put into the kernel (but probably
  should have been).


  3.  Setting up a PnP BIOS

  When the computer is first turned on, the BIOS program runs before the
  operating system is loaded.  Modern BIOSs are PnP and can configure
  most of the PnP devices.  Some old PCI BIOSs will only configure the
  PCI bus.  Here are some of the choices which may exist in your BIOS's
  CMOS menu:



  �  ``Do you have a PnP operating system?''

  �  ``How are bus-resources   to be controlled?''

  �  ``Reset the configuration?''


  3.1.  Do you have a PnP operating system?

  Regardless of how you answer this to the BIOS, the PnP BIOS will PnP-
  configure the hard-drive, floppy, video card, and keyboard to make the
  system bootable as well as configure the LPC bus (if you have one).
  If you said no PnP OS then the BIOS should configure everything.

  How should you answer this question to your BIOS?  If you have at at
  least the 2.4 kernel you could answer it either way and Linux will
  usually work fine.  Even if you have have Windows 2000 or XP on the
  same PC, it will usually work OK either way.  This is because both
  Windows and Linux are supposedly PnP OS's and if the OS is PnP it
  should be able to also handle the case where the BIOS has configured
  everything (if you said it wasn't PnP).  But I still suggest saying
  that it's not a PnP OS unless there is a known reason to say
  otherwise.


  3.1.1.  Linux prior to the 2.4 kernel

  It's not often clear whether to say yes or no.  If isapnp was used by
  Linux, then Linux does the configuring and it was claimed that it's
  best to say it's a PnP OS.  Why isapnp would have trouble when
  presented with devices already configured by the BIOS isn't clear, but
  such trouble sometimes happened and was fixed by stopping the BIOS
  from configuring (saying yes, it's a PnP OS).  There were a few cases
  where saying no fixed a problem.  So if isapnp is doing it's job OK,
  you should probably say it's PnP.  If isapnp isn't used, no is usually
  best.  The Linux device drivers for PCI devices should configure PCI
  devices OK.  But for the case of PCI devices driven by non-PCI
  drivers, then you may say it's not PnP to get the BIOS to configure
  them.


  3.1.2.  Windows 2000 and XP

  If you also run these Windows OS's on the same PC, you should say that
  you don't have a PnP OS.  That's what MS suggests you do.  Perhaps MS
  hopes that the BIOS will do a better job at configuring than Windows
  will.  That makes sense because the BIOS should be designed for the
  particular idiosyncrasies of the motherboard, especially today when
  many devices are built into the motherboard.  PnP OS = no should also
  be OK for Linux kernels 2.4 and higher.  But for Linux kernel prior to
  2.4, it's not clear which is best.  (see the above subsection).  So if
  you have problems with Linux you might try saying you have a PnP OS to
  satisfy Linux but this is going against what MS suggest (but will
  probably work OK anyway).

  When the BIOS configures a device different from what Windows has in
  it's registry, Windows will tell you that it's finding new hardware.
  What it's really doing is finding old hardware that has been
  configured differently so it thinks it's new hardware.  At any rate,
  it records the configuration that the BIOS has used in its registry
  and the device should work OK from now on.



  3.1.3.  MS Windows 95, 98 (and Me ?)

  For Windows9x, MS suggest that you tell the BIOS that you have a PnP
  OS (the exact opposite of the case for Windows 2000 and XP).  This
  should also be OK for Linux if you have kernel 2.4 or later.  But if
  you have a Linux kernel prior to 2.4 then it's best for Linux to say
  that it's not a PnP OS.  One way to resolve this dilemma is to set it
  up for the OS you use more frequently.  Then when you boot the other
  OS, manually go into the BIOS and change the setting.  This is a lot
  of bother but it's feasible if you almost never use one of the OS's.
  Otherwise there are better ways to resolved this dilemma.

  The second way to resolve this dilemma is to get Linux to resource-
  configure everything.  See ``Linux prior to the 2.4 kernel''.  Then
  you tell the BIOS it's a PnP OS.

  The third way to resolve this dilemma is to tell the BIOS it's not a
  PnP OS.  This is going against what MS says you should do, but it's
  possible to get MS Windows9x to work OK if you understand what to do
  (and why).  If you tell the BIOS it's not a PnP OS, shouldn't MS
  Windows detect how the BIOS has configured things and change it if it
  doesn't like what the BIOS has done?  It should, but unfortunately, it
  doesn't seem to work this way.

  What Windows9x seems to do when it finds hardware that is already
  configured by the BIOS is to just leave it alone and not reconfigure
  it.  Now Windows9x keeps a record of the bus-resource configuration in
  its registry.  If the BIOS configuration is different, it should
  either correct what's in its registry to conform to what the BIOS has
  set or reconfigure everything per what's in the registry.  Bad news.
  It seems to do neither and thinks the actual configuration is the same
  as in the registry when in fact it's different.

  But if the registry happens to contain a bus-resource configuration
  that is exactly the same as how the BIOS configures things, then
  everything will obviously work OK.  A device will thus work fine if
  the BIOS has configured it the same as recorded in the registry.  So
  the way to get MS Windows to work OK is to get the registry in sync
  with how the BIOS configures.  As mentioned previously, the BIOS
  configures things per its ESCD (which is something like the registry
  for the BIOS).  See ``The BIOS's ESCD Database''.  So we need to get
  the registry in sync with the BIOS's ESCD so that the registry and the
  ESCD contain the same configuration.  In some cases, these two just
  happen to be in sync and you don't need to do anything.

  One question you may think of is: how did the BIOS's ESCD and Windows
  registry ever get out of sync in the first place?  Here's one
  scenario.  You install Windows with the BIOS set to a PnP OS.  Then
  Windows configures most everything and saves that configuration in its
  registry.  Then later on you change the BIOS setting to not a PnP OS.
  Then upon booting, the BIOS configures everything and it doesn't do it
  exactly like Windows did it.  Thus the actual configuration of the
  hardware and what Windows has in its registry are now different.

  One way to try to get the Registry and the ESCD the same is to install
  (or reinstall) Windows when the BIOS is set for "not a PnP OS".  This
  should present Windows with hardware configured by the BIOS.  If this
  configuration is without conflicts, Windows will hopefully leave it
  alone and save it in it's Registry.  Then the ESCD and the registry
  are in sync.

  Another method is to remove devices that are causing problems in
  Windows by clicking on "remove" in the Device Manager.  Then reboot
  with "Not a PnP OS" (set it in the BIOS's CMOS as you start to boot).
  Windows will then reinstall the devices, hopefully using the bus-
  resource settings as configured by the BIOS.  Be warned that Windows
  will likely ask you to insert the Window installation CD since it
  sometimes can't find the driver files (and the like) even though they
  are still there.  A workaround for this is to select "skip file" which
  will avoid installing the file from a CD.  If the file is still on the
  HD, then the driver will hopefully find it OK even though the Window's
  install program requested you install it from a CD (which you skipped
  doing).

  As a test I "removed" a NIC card which used a Novell compatible
  driver.  Upon rebooting, Windows reinstalled it with Microsoft
  Networking instead of Novell.  This meant that the Novell Client
  needed to be reinstalled --a lot of unnecessary work.  So in a case
  like this it may be better to not fib to Windows95/98 but instead to
  get Linux to configure bus-resources.

  When using a Window-Linux PC (dual boot) you might notice a change in
  the way the BIOS configures due to Windows9x (and other versions of
  Windows ??) modifying the ESCD.  It supposedly does this only if you
  "force" a configuration or install a legacy device.  See ``Using
  Windows to set ESCD''.  Device drivers that do configuring may modify
  what the BIOS has done as will the isapnp or PCI Utilities programs if
  you run them.


  3.2.  Assigning Resources by the BIOS

  Modern BIOSs allow you to manually allocate resources, primarily IRQs.
  There is usually an option to set the an allocation to "auto" so that
  the BIOS decides how to allocate the resource.  "Auto" is often a good
  choice unless you have old legacy non-pnp ISA cards.

  If you have such non-PnP cards, then it may be important to reserve
  resources (such as IRQ's) for these in the BIOS.  Otherwise the BIOS
  may use these resources for some other device and create conflicts.
  An exception is that for some common legacy devices (such as parallel
  and serial ports, disk drives), the BIOS may find them (look at the
  screen at boot-time) so you don't need to reserve resources for them.
  If you've used Windows on your PC, it might be true that Windows has
  already told the BIOS about them by running the ICU utility (or the
  like) under Windows.

  For PCI, the BIOS may let you assign IRQs to card slots 1, 2, 3, 4,
  etc.  If you do this, you should know what card is in what slot.
  Actually, each slot has 4 PCI IRQs: A, B, C, and D.  If the BIOS menu
  doesn't say which of these (A, B, C, D) is being assigned to an IRQ
  number, it's likely that it's only assigning the IRQ number to PCI IRQ
  A.  But many PCI cards only use IRQ A so it's then just like assigning
  an IRQ to a slot.   See ``PCI Interrupts''


  3.3.  Reset the configuration?

  This is a little risky to do.  It will erase the BIOSs ESCD data-base
  of how your PnP devices should be configured as well as the list of
  how legacy (non-PnP) devices are configured.  Never do this unless you
  are convinced that this data-base is wrong and needs to be remade.  It
  was stated somewhere that you should do this only if you can't get
  your computer to boot.  If the BIOS loses the data on legacy ISA
  devices, then you'll need to run ICA again under DOS/Windows to
  reestablish this data.


  4.  How to Deal with PnP Cards



  4.1.  Introduction to Dealing with PnP Devices

  Today almost all new internal boards (cards) are Plug-and-Play (PnP).
  Thus, the configuring of bus-resources should, in almost all cases be
  entirely automatic.  If a device is not working, see if it was
  detected, possibly by rebooting.  If the device driver can't resource-
  configure it, then hopefully one or more of methods 2-6 will:


  1. ``Device Driver Configures''

  2. ``/sys User Interface Configures'' kernel 2.6 + (not for PCI yet,
     other severe limitations)

  3. ``BIOS Configures'' (For the PCI bus you only need a PCI BIOS,
     otherwise you need a PnP BIOS)

  4. ``ISA cards only: Disable PnP''  by jumpers or DOS/Windows software
     (but many cards can't do this)

  5. ``ISA Bus: Isapnp'' is a program you can always use to configure
     ISA PnP devices

  6. ``PCI Utilities'' is for configuring the PCI bus but the device
     driver should handle it

  7. ``Windows Configures'' and then you boot Linux from within
     Windows/DOS.  Use as a last resort

  Any of the above will set the bus-resources in the hardware but only
  the first one (and possibly the second) tells the driver what has been
  done.  How the driver gets informed depends on the driver.  You may
  need to do something to inform it.  See ``Tell the Driver the
  Configuration''


  4.2.  Device Driver Configures, Reserving Resources

  Device drivers (with the help of code provided by the kernel) can be
  written to use PnP methods to set the bus-resources in the hardware
  but only for the device that they control.  But many device drivers
  just accept what the BIOS or Linux has configured and use code
  provided by the kernel to find out how this device has been
  configured.  Since the driver has checked the configuration and
  possibly reconfigured it, it obviously knows the configuration and
  there is no need for you to tell it this info.  This is obviously the
  easiest way to do it since you don't have to do anything if the driver
  does it all.

  If you have old pre-PnP ISA hardware, the Linux PnP software may not
  know about it and the bus-resources it requires.  So it might
  erroneously allocate the resources that this old hardware needs to
  some other device.  The result is a resource conflict but there's a
  way to try to avoid it.  You can reserve the resources that the old
  ISA card needs by configuring the BIOS at boot-time (usually), the
  isa-pnp module or to the kernel (if the PnP is built into the kernel).
  For example, to reserve IRQ 5 give this argument to the isa-pnp module
  (or to the kernel): isapnp_reserve_irq=5.  See BootPrompt-HOWTO.
  Instead of ..._irq there are also _io, _dma, and _mem.

  For PCI devices, most drivers will configure PnP.  Unfortunately, a
  driver could grab bus-resources that are needed by other devices (but
  not yet allocated to them by the kernel).  Thus a more sophisticated
  PnP Linux kernel would be better, where the kernel did the allocation
  after all requests were in.  See ``How Linux Does PnP''.

  4.3.  /sys User Interface Configures

  Starting with kernel 2.6 there's supposedly a new way for the user to
  resource configure using the /sys directory tree.  But as of Aug.
  2004, it can't be used for configuring in most cases.  See ``The /sys
  Directory Tree''.


  4.4.  BIOS Configures

  4.4.1.  Intro to Using the BIOS to Configure PnP

  If you have a PnP BIOS, it can configure the hardware.  If the driver
  can't do it, the BIOS probably can.  This means that your BIOS reads
  the resource requirements of all devices and configures them
  (allocates bus-resources to them).  It is a substitute for a PnP OS
  except that the BIOS doesn't match up the drivers with their devices
  nor tell the drivers how it has done the configuring.  It should
  normally use the configuration it has stored in its non-volatile
  memory (ESCD).  If it finds a new device or if there's a conflict, the
  BIOS should make the necessary changes to the configuration and may
  not use the same configuration as was in the ESCD.  In this case it
  should update the ESCD to reflect the new situation.

  Your BIOS needs to support such configuring and there have been cases
  where it doesn't do it correctly or completely.  The BIOS may need to
  be told via the CMOS menu that it's not a PnP OS.  While many device
  drivers will be able to automatically detect what the BIOS has done,
  in some cases you may need to determine it (not always easy).  See
  ``What Is My Current Configuration?''  A possible advantage to letting
  the BIOS do it is that it does its work before Linux starts so it all
  gets done early in the boot process.

  Most BIOS made after about 1996 ?? can resource-configure both the PCI
  and ISA buses.  But it's been claimed that some older BIOSs can only
  do the PCI.  And of course, for PCs with only the PCI bus, the BIOS
  only needs to do PCI.  To try to find out more about your BIOS, look
  on the Web.  Please don't ask me as I don't have data on this.  The
  details of the BIOS that you would like to know about may be hard to
  find (or not available).  Some old BIOS's may have minimal PnP
  capabilities and seemingly expect the operating system to do it right.
  If this happens you'll either have to find another method or try to
  set up the ESCD database if the BIOS has one.  See the next section.


  4.4.2.  The BIOS's ESCD Database

  The BIOS maintains a non-volatile database containing a PnP-
  configuration that it will try to use (if you claim that it's not a
  PnP OS).  It's called the ESCD (Extended System Configuration Data).
  Again, the provision of ESCD is optional but most PnP-BIOSs have it.
  The ESCD not only stores the resource-configuration of PnP devices but
  also stores configuration information of non-PnP devices (and marks
  them as such) so as to avoid conflicts.  The ESCD data is usually
  saved on a chip and remains intact when the power is off, but
  sometimes it's kept on a hard-drive??

  The ESCD is intended to hold the last used configuration.  But since
  Linux can change how devices are configured (including the user using
  isapnp or pci utilities) then the ESCD will not know about this and
  will not save this configuration in the ESCD.  A good PnP OS might
  update the ESCD so you can use it later on for a non-PnP OS (like
  standard Linux).  MS Windows9x does this only in special cases.  See
  ``Using Windows to set ESCD''.  Starting with kernel 2.6, Linux is
  capable of modifying the ESCD but it's not used yet (as of Aug. 2004).

  To use what's set in ESCD be sure you've set "Not a PnP OS" or the
  like in the BIOS's CMOS.  Then each time the BIOS starts up (before
  the Linux OS is loaded) it should configure things this way.  If the
  BIOS detects a new PnP card which is not in the ESCD, then it must
  allocate bus-resources to the card and update the ESCD.  It may even
  have to change the bus-resources assigned to existing PnP cards and
  modify the ESCD accordingly.

  There's a program that you may use to view the contents of the ESCD.
  It shows IRQs and IO addresses etc., but device names are missing
  (only EISA device-ID numbers).  It's at: Index of
  /home/gunther.mayer/lsescd <http://home.t-
  online.de/home/gunther.mayer/lsescd/>

  If each device saved its last configuration in its hardware, hardware
  configuring wouldn't be needed each time you start your PC.  But it
  doesn't work this way.  So all the ESCD data needs to be kept correct
  if you use the BIOS for PnP.  There are some BIOSs that don't have an
  ESCD but do have some non-volatile memory to store info regarding
  which bus-resources have been reserved for use by non-PnP cards.  Many
  BIOSs have both.


  4.4.3.  Using Windows to set the ESCD

  Eventually the Linux kernel may set the ESCD.  Starting with kernel
  2.6, a function in the new code could do it provided the kernel has
  been compiled with PNPBIOS.  But it currently sits in the code unused.

  If the BIOS doesn't set up the ESCD the way you want it (or the way it
  should be) then it would be nice to have a Linux utility to set the
  ESCD.  One may resort to attempting to use Windows for this (if you
  have it on the same PC) to do this.

  There are three ways to use Windows to try to set/modify the ESCD.
  One way is to use the ICU utility designed for DOS or Windows 3.x.  It
  should also work OK for Windows 9x/2k ??  Another way is to set up
  devices manually ("forced") under Windows 9x/2k so that Windows will
  put this info into the ESCD when Windows is shut down normally.  The
  third way is only for legacy devices that are not plug-and-play.  If
  Windows knows about them and what bus-resources they use, then Windows
  should put this info into the ESCD.

  If PnP devices are configured automatically by Windows without the
  user "forcing" it to change settings, then such settings probably will
  not make it into the ESCD.  Of course Windows may well decide on its
  own to configure the same as what is set in the ESCD so they could
  wind up being the same by coincidence.

  Windows 9x are PnP operating systems and automatically PnP-configure
  devices.  They maintain their own PnP-database deep down in the
  Registry (stored in binary Windows files).  There is also a lot of
  other configuration stuff in the Registry besides PnP-bus-resources.
  There is both a current PnP resource configuration in memory and
  another (perhaps about the same) stored on the hard disk.  To look at
  this in Windows98 or to force changes to it you use the Device
  Manager.

  In Windows98 there are 2 ways to get to the Device Manager: 1. My
  Computer --> Control Panel --> System Properties --> Device Manager.
  2. (right-click) My Computer --> Properties --> Device Manager.  Then
  in Device Manager you select a device (sometimes a multi-step process
  if there are a few devices of the same class).  Then click on
  "Properties" and then on "Resources".  To attempt to change the
  resource configuration manually, uncheck "Use automatic settings" and
  then click on  "Change Settings".  Now try to change the setting, but
  it may not let you change it.  If it does let you, you have "forced" a
  change.  A message should inform you that it's being forced.  If you
  want to keep the existing setting shown by Windows but make it
  "forced" then you will have to force a change to something else and
  then force it back to its original setting.

  To see what has been "forced" under Windows98 look at the "forced
  hardware" list: Start --> Programs --> Accessories --> System Tools
  --> System Information --> Hardware Resources --> Forced Hardware.
  When you "force" a change of bus-resources in Windows, it should put
  your change into the ESCD (provided you exit Windows normally).  From
  the "System Information" window you may also inspect how IRQs and IO
  ports have been allocated under Windows.

  Even if Windows shows no conflict of bus-resources, there may be a
  conflict under Linux.  That's because Windows may assign bus-resources
  differently than the ESCD does.  In the rare case where all devices
  under Windows are either legacy devices or have been "forced", then
  Windows and the ESCD configurations should be identical.


  4.4.4.  Adding a New Device (under Linux or Windows)

  If you add a new PnP device and have the BIOS set to "not a PnP OS",
  then the BIOS should automatically configure it and store the
  configuration in ESCD.  If it's a non-PnP legacy device (or one made
  that way by jumpers, etc.) then here are a few options to handle it:

  You may be able to tell the BIOS directly (via the CMOS setup menus)
  that certain bus-resources it uses (such as IRQs) are reserved and are
  not to be allocated by PnP.  This does not put this info into the
  ESCD.  But there may be a BIOS menu selection as to whether or not to
  have these CMOS choices override what may be in the ESCD in case of
  conflict.  Another method is to run ICU under DOS/Windows.  Still
  another is to install it manually under Windows 9x/2k and then make
  sure its configuration is "forced" (see the previous section).  If
  it's "forced" Windows should update the ESCD when you shut down the
  PC.


  4.5.  ISA cards only: Disable PnP ?

  PCI devices are inherently PnP so it can't be disabled.  But a few ISA
  devices once had options for disabling PnP by jumpers or by running a
  Windows program that comes with the device (jumperless configuration).
  If the device driver can't configure it, this will avoid the possibly
  complicated task of doing PnP configuring.   Don't forget to tell the
  BIOS that these bus-resources are reserved.  But since Linux support
  for PnP has improved, you usually don't want to disable PnP.  Here's
  some more arguments in favor of PnP:


  1. If you have MS Windows on the same machine, then you may want to
     allow PnP to configure devices differently under Windows from what
     it does under Linux.

  2. The range of selection for IRQ numbers (or port addresses) etc.
     may be too limited unless you use PnP.

  3. You might have a Linux device driver that uses PnP methods to
     search for the device it controls.

  4. If you need to change the configuration in the future, it may be
     easier to do this if it's PnP (no setting of jumpers or running a
     Dos/Windows program).

  Once configured as non-PnP devices, they can't be configured by PnP
  software or a PnP-BIOS (until you move jumpers and/or use the
  Dos/Windows configuration software again).


  4.6.  ISA Bus: Isapnp (part of isapnptools)

  The isapnp standalone program is only for PnP devices on the ISA bus
  (non-PCI).  It was much needed prior to the 2.4 kernels.  After the
  2.4 kernel, which provided functionality to allow drivers deal with
  ISA PnP, the isapnp standalone program is less significant.  Also, the
  BIOS may configure ISA PnP satisfactory.  But the isa-pnp module (or
  the equivalent built into the kernel) is now very significant since
  various ISA device drivers call on it to configure bus-resources.
  Prior to kernel 2.6 it resulted a /proc/isapnp "file" which may be
  used to manually configure (see isapnp.txt in the kernel
  documentation).

  In some cases Linux distributions have been set up to run isapnp
  automatically at startup.  It's still done in 2004 but it isn't really
  needed if the device drivers work well.  If you need to set it up
  yourself much of the documentation for isapnp is difficult to
  understand unless you know the basics of PnP.  This HOWTO should help
  you understand it as well the FAQ that comes with isapnp.  Running the
  Linux program "isapnp" at boot-time will configure such devices to the
  resource values specified in /etc/isapnp.conf.  Its possible to create
  this configuration file automatically but you then should edit it
  manually to choose between various options.  Then to let the driver
  know the resources, you often need to specify them as parameters to
  the appropriate modules (drivers).  This is done with configuration
  files, often in the /etc directory.  Look there for files named mod*,
  etc.  If the driver is built into the kernel, then they may sometimes
  be given as a parameter to the kernel.  See BootPrompt-HOWTO.

  With isapnp there once was a problem where a device driver which is
  built into the kernel may run too early before isapnp has set the
  address, etc.  in the hardware.  This resulted in the device driver
  not being able to find the device.  The driver tries the right address
  but the address hasn't been set yet in the hardware.  Is this still a
  problem ??

  If your Linux distribution automatically installed isapnptools, isapnp
  may already be running at startup.  In this case, all you need to do
  is to edit /etc/isapnp.conf per "man isapnp.conf".  Note that this is
  like manually configuring PnP since you make the decisions as to how
  to configure as you edit the configuration file.

  If the configuration file is wrong or doesn't exist, you can use the
  program "pnpdump" to help create the configuration file.  It almost
  creates a configuration file for you but you must skillfully edit it a
  little before using it.  It contains some comments to help you edit
  it.  While the BIOS may also configure the ISA devices (if you've told
  it that you don't have a PnP OS), isapnp will redo it.

  The terminology used in the /etc/isapnp.conf file may seem odd at
  first.  For example for an IO address of 0x3e8 you might see "(IO 0
  (BASE 0x3e8))" instead.  The "IO 0" means this is the first (0th) IO
  address-range that this device uses.   Another way to express all this
  would be: "IO[0] = 0x3e8" but isapnp doesn't do it this way.  "IO 1"
  would mean that this is the second IO address range used by this
  device, etc.  "INT 0" has a similar meaning but for IRQs (interrupts).
  A single card may contain several physical devices but the above
  explanation was for just one of these devices.



  4.7.  PCI Utilities

  The package PCI Utilities (= pciutils, sometimes called "pcitools"),
  allows one to manually PnP-configure the PCI bus (with difficulty).
  "lspci" or "scanpci" lists bus-resources while "setpci" sets resource
  allocations (except IRQs) in the hardware devices.  It appears that
  setpci is mainly intended for use in scripts and one needs to
  understand the details of the PCI configuration registers in order to
  use it.  That's a topic not explained here nor in the manual page for
  setpci.

  People have used this to configure PCI devices where the driver failed
  to do it.  An example is found in my Modem-HOWTO and Serial-HOWTO in
  the subsection "PCI: Enabling a disabled port".  However, enabling a
  device is of no use unless you have a working driver for the device.


  4.8.  Windows Configures

  This method uses MS Windows to configure and should be used only if
  all else fails.  If you have Windows9x (or 2k) on the same PC, then
  just start Windows and let it configure PnP.  Then start Linux from
  Windows (or DOS) using, for example, loadlin.exe.  But there may be a
  problem with IRQs for PCI devices.  As Windows shuts down (without any
  messages) to make way for Linux, it may erase (zero) the IRQ which is
  stored in one of the PCI device's configuration registers.  Linux will
  complain that it has found an IRQ of zero.

  The above is reported to happen if you start Linux using a shortcut
  (PIF file).  But a workaround is reported where you still use the
  shortcut PIF.  A shortcut is something like a symbolic link in Linux
  but it's more than that since it may be "configured".  To start Linux
  from DOS you create a batch file (script) which starts Linux.  (The
  program that starts Linux is in the package called "loadlin").  Then
  create a PIF shortcut to that batch file and get to the "Properties"
  dialog box for the shortcut.  Select "Advanced" and then check "MS-DOS
  mode" to get it to start in genuine MS-DOS.

  Now here's the trick to prevent zeroing the PCI IRQs.  Click "Specify
  a new MS-DOS configuration".  Then either accept the default
  configuration presented to you or click on "Configuration" to change
  it.  Now when you start Linux by clicking on the shortcut, new
  configuration files (Config.sys and Autoexec.bat) will be created per
  your new configuration.

  The old files are stored as "Config.wos and Autoexec.wos".  After you
  are done using Linux and shut down your PC then you'll need these
  files again so that you can run DOS the next time you start your PC.
  You need to ensure that the names get restored to *.sys and *.bat.
  When you leave Windows/DOS to enter Linux, Windows is expecting that
  when you are done using Linux you will return to Windows so that
  Windows can automatically restore these files to their original names.
  But this doesn't happen since when you exit Linux you shut down your
  PC and don't get back to Windows.  So how do you get these files
  renamed?  It's easy, just put commands into your "start-Linux" batch
  file to rename these files to their *.bat and *.sys names.  Put these
  renaming commands into your batch file just before the line that loads
  Linux.

  Also it's reported that you should click on the "General" tab (of the
  "Properties" dialog of your shortcut) and check "Read-only".
  Otherwise Windows may reset the "Advanced Settings" to "Use current
  MS-DOS configuration" and PCI IRQs get zeroed.  Thus Windows erases
  the IRQs when you use the current MS-DOS configuration but doesn't
  erase when you use a new configuration (which may actually configure
  things identical to the old configuration).  Windows does not seem to
  be very consistent.


  4.9.  PnP Software/Documents


  �  Isapnptools homepage <http://www.roestock.demon.co.uk/isapnptools/>

  �  Proposal for a Configuration Manager for Linux
     <http://www.astarte.free-online.co.uk> 1999 (Never got into kernel
     but Linux is slowly "evolving" in this direction).

  �  PnP Specs. from Microsoft
     <http://www.microsoft.com/hwdev/tech/pnp/default.asp>

  �  Book: PCI System Architecture, 4th ed.  by Tom Shanley +, MindShare
     1999.  Covers PnP-like features on the PCI bus.

  �  Book: Plug and Play System Architecture, by Tom Shanley, Mind Share
     1999.  Details of PnP on the ISA bus.  Only a terse overview of PnP
     on the PCI bus.

  �  Book: Programming Plug and Play, by James Kelsey, Sams 1995.
     Details of programming to communicate with a PnP BIOS.  Covers ISA,
     PCI, and PCMCIA buses.


  5.  Tell the Driver the Configuration ??

  5.1.  Introduction

  A modern driver for a device will find out the bus-resource
  configuration without you having to tell it anything.  It may even set
  the bus-resources in the hardware using PnP methods.  Some drivers
  have more than one way to find out how their physical device is
  configured.  In the worst case you must hard-code the bus-resources
  into the kernel (or a module) and recompile.

  In the middle are cases such as where you run a program to give the
  bus-resource info to the driver or put the info in a configuration
  file.  In some cases the driver may probe for the device at addresses
  where it suspects the device resides (but it will never find a PnP
  device if it hasn't been enabled by PnP methods).  It may then try to
  test various IRQs to see which one works.   It may or may not
  automatically do this.

  In the modern case the driver should use PnP methods to find the
  device and how the bus-resources have been set by the BIOS, etc. but
  will not actually set them.  It may also look at some of the "files"
  in the /proc directory.

  One may need to "manually" tell a driver what bus-resources it should
  use.  You give such bus-resources as a parameter to the kernel or to a
  loadable module.  If the driver is built into the kernel, you pass the
  parameters to the kernel via the "boot-prompt".   See The Boot-Prompt-
  HOWTO which describes some of the bus-resource and other parameters.
  Once you know what parameters to give to the kernel, one may put them
  into a boot loader configuration file.  For example, put append="...".
  into the lilo.conf file and then use the lilo command to get this info
  into the lilo kernel loader.

  If the driver is loaded as a module, in many cases the module will
  find the bus-resources needed and then set them in the device.  In
  other cases (mostly for older PCs) you may need to give bus-resources
  as parameters to the module.  Parameters to a module (including ones
  that automatically load) may be specified in /etc/modules.conf.  There
  are usually tools used to modify this file which are distribution-
  dependent.  Comments in this file should help regarding how to modify
  it.  Also, any module your put in /etc/modules will get loaded along
  with its parameters.

  While there is much non-uniformity about how drivers find out about
  bus-resources, the end goal is the same.  If you're having problems
  with a driver you may need to look at the driver documentation (check
  the kernel documentation tree).  Some brief examples of a few drivers
  is presented in the following sections:


  5.2.  Serial Port Driver Example

  For PCI serial ports (and for ISA PnP serial ports after 2.4 kernels)
  the serial driver detects the type of serial port and PnP configures
  it.  Unfortunately, there may be some PCI serial ports that are not
  supported yet.

  For the standard ISA serial port with very old older versions of the
  kernel and serial driver (not for multiport cards) the driver probes
  two standard addresses for serial ports.  It doesn't probe for IRQs
  but it just assigns the "standard" IRQ to the first two serial ports.
  This could be wrong.

  For anything else the configuration file for the setserial program
  must be manually modified.  See Serial-HOWTO for more details.  You
  use setserial to inform the driver of the IO address and Setserial is
  often run from a start-up file.  In newer versions there is a
  /etc/serial.conf file (or /var/lib/setserial/autoconfig that you
  "edit" by simply using the setserial command in the normal way and
  what you set using setserial is saved in the serial.conf configuration
  file.  The serial.conf file should be consulted when the setserial
  command runs from a start-up file.  Your distribution may or may not
  set this up for you.

  There are two different ways to use setserial depending on the options
  you give it.  One use is used to manually tell the driver the
  configuration.  The other use is to probe at a given address and
  report if a serial port exists there.  It can also probe this address
  and try to detect what IRQ is used for this port.

  Even with modern kernels, setserial is sometimes needed if the driver
  fails to detect the serial port, or if you have very old hardware.


  6.  How Do I Find Devices and How Are They Configured?

  6.1.  Finding and How-Configured Are Related

  Once you find your hardware, the same program that found it usually
  tells you how it's configured.  So finding out how it's configured is
  usually the same procedure as finding the hardware.


  6.2.  Devices May Have Two "Configurations"

  Here "configuration" means the assignment of PnP bus-resources
  (addresses, IRQs, and DMAs).  For each device, there are two parts to
  the configuration question:

  1. What does the driver think the hardware configuration is?

  2. What configuration (if any) is actually set in the device hardware?

     Each part should have the same answer (the same configuration).
     The configuration of the device hardware and its driver should
     obviously be the same (and usually is).  But if things are not
     working right, it could be because there's a difference.  This
     means that the driver has incorrect information about the actual
     configuration of the hardware.  This spells trouble.  If the
     software you use doesn't adequately tell you what's wrong (or
     automatically configure it correctly) then you need to investigate
     how your hardware devices and their drivers are configured.  While
     Linux device drivers should "tell all", in some cases it may not be
     easy to determine what has been set in the hardware.

  Another problem is that when you view configuration messages on the
  screen you need to know whether the reported configuration is that of
  the device driver, the device hardware, or both.  If the device driver
  has either set the configuration in the hardware or has otherwise
  checked the hardware then the driver should have the correct
  information.

  But sometimes the driver has been provided incorrect resources by a
  script, configuration file, by incorrect resource parameters given to
  a module, or perhaps just hasn't been told what the resources are and
  tries to use incorrect default resources.  For example, one can uses
  "setserial" to tell the serial port driver an incorrect resource
  configuration and the driver accepts it without question.  But the
  serial port doesn't work right (if at all).


  6.3.  Finding Hardware

  A common problem is that the software doesn't detect your device
  and/or determine the right driver for it.  For PnP devices, detecting
  them is easy via PnP software except for the unusual case where the
  hardware has been disabled.  The BIOS can sometimes be set to disable
  PnP devices or a jumper/switch on the physical device itself could
  disable it.  In such a cases, the hardware can't be detected at all
  until you either reconfigure the BIOS or change a jumper/switch.

  Since the PCI bus is inherently PnP, there are no hidden devices.
  Even though PnP devices are easy to find by PnP methods, if the driver
  doesn't use PnP methods but uses the old method of probing for them at
  likely address, they may not be found.  This is because that, until
  the resources are set in a PnP device (by the BIOS or Linux), the
  device may have no address at all, so probing at likely address yields
  nothing.  For the old ISA bus, some of the devices may be non-PnP and
  thus the old probing methods may find them.  So many drivers still
  probe at likely address, in addition to using PnP methods (= PnP
  probing which is sometimes also just called "probing").

  Ways to Find Hardware Devices (and their configurations): (follow link
  to more details)

  �  Check the BIOS to make sure they are not disabled

  �  Watch the ``Boot-time Messages'' on the screen

  �  Look in ``The /proc Directory Tree''

  �  ``Tools for Detecting and/or Configuring   all Hardware'' lsdev,
     hwinfo, discover, kudzu

  �  ``Tools for Detecting and/or   Configuring One Type of Hardware''

  �  PCI: ``PCI Bus Inspection''

  �  ISA Bus: ``ISA Bus Introduction''

  �  ISA Bus: ``PnP cards''

  �  ISA Bus: For ``Non-PnP Cards''

  �  ISA Bus: For ``Cards with jumpers''

  �  ISA Bus: If ``Neither PnP nor jumpers''

  �  ``Use MS Windows''


  6.4.  Boot-time Messages

  Significant info on the configuration may be obtained by reading the
  messages from the BIOS and from Linux that appear on the screen when
  you first start the computer.  These messages often flash by too fast
  to read but once they stop type Shift-PageUp a few times to scroll
  back to them.  To scroll forward thru them type Shift-PageDown.
  Typing "dmesg" at any time to the shell prompt will show only the
  Linux kernel messages and may miss some of the most important ones
  (including ones from the BIOS).  The messages from Linux may sometimes
  only show what the device driver thinks the configuration is, perhaps
  as told it via an incorrect configuration file.  Checking log files in
  /var/log may also be useful.

  For the PCI bus, the notation: 00:1a:0 means the PCI bus 00 (the main
  PCI bus), PCI card (or chip) 1a, and function 0 (the first device) on
  the card or chip.  The 2nd device on the card (or chip) 08 would be:
  00:08:1.

  The BIOS messages display first and will show the actual hardware
  configuration at that time, but isapnp, or pci utilities, or device
  drivers may change it later.  In some cases it doesn't show devices
  that the BIOS didn't configure.

  If the BIOS messages don't show as you back up to the start of the
  BIOS messages using Shift-PageUp, try freezing them as they flash by,
  by hitting the "Pause" key as soon as the first words flash on the
  screen.  Press any key to resume.  It's often tricky to hit Pause
  exactly at the right time.  Be sure to hold down the "Shift" key
  before hitting "Pause" since "Pause" is a shifted key.  If you miss,
  hit Ctrl-Alt-Del when Linux starts booting to reboot and try again.
  Once the messages from Linux start to appear, it's too late to use
  "Pause" since it will not freeze the messages from Linux.

  To set things in the BIOS such as IRQs reserved for legacy hardware,
  serial port addresses, etc. you need to get into the BIOS (CMOS) setup
  menus at boot time.  Each BIOS brand has different keys you need to
  hold down to do this.  There are lists on the Internet.  Sometimes by
  freezing the BIOS messages or watching the screen, the key you need to
  press will be indicated in a message such as "Press DEL for setup".
  But it may flash by so fast that you miss it.  Of course, you don't
  set stuff in the BIOS that you don't understand, or your PC may become
  disabled.

  Messages from the BIOS at boot-time tell you how the hardware
  configuration was then.  The current configuration may still be the
  same since Linux should hopefully accept what the BIOS has done if
  it's OK.  Messages from Linux may be from drivers that used kernel PnP
  functions to inspect and/or set bus-resources.  These should be
  correct, but beware of messages that only show what the driver was
  told from a configuration file.  It could be wrong.  Of course, if the
  device works fine, then it's likely configured the same as the driver.



  6.5.  The /proc Tree

  Starting with Kernel 2.6, in addition to the /proc directory tree,
  there's also a /sys  tree See ``The /sys Tree''.   These trees are
  useful for finding resource configurations and devices.  The "files"
  in them represent data in the kernel memory and don't exist at all on
  you harddrive.  Programs such as lspci get their info from the /proc
  tree so such programs should display the results in more readable form
  than directly inspecting the "files" in /proc.  Here are 4 /proc
  "files" that show resources which have been registered in the kernel
  by device drivers.

  Since Linux's plug-and-play works by letting device drivers allocate
  resources for their device, there may be no listing of resources used
  by some of your hardware if the driver hasn't yet requested that such
  resources be reserved.  For the case of kernel modules (loadable
  device drivers), if the module hasn't loaded yet, the kernel doesn't
  know about any resources it needs.  Sometimes, the module only loads
  when you start an application that needs it.  So if certain hardware
  is missing from these "files" in /proc, it may mean that the hardware
  hasn't yet been used.  For example, even though your floppy drive has
  a floppy disk in it and is ready to use, the interrupt for it will not
  show up unless its in use.

  /pts shows I/O addresses.  If there's a mistake (wrong address) it
  means trouble since the device will not get bytes sent to it.
  /proc/iomem shows registered IO memory addresses.
  /proc/interrupts shows the interrupts currently in use.
  /proc/dma shows the dma (Direct Memory Access) ISA dma channel
  allocations.

  In the past, the author observed the listing of interrupts that didn't
  exist.  In some cases it showed that a few such interrupts were
  actually sent.  This could be due to the issuing of erroneous
  interrupts due to hardware defects.

  /proc/bus/ has subdirectories (subfolders) input/, pci/, and isapnp/.
  The format of most of the files in this directory is very cryptic,
  often just a copy of the bytes in the configuration space.  So, use
  them only as a last resort.  The input/ subdirectory has information
  on input devices such as the keyboard and mouse.  It's not as cryptic
  as the other directories under /proc/bus/ and might yield some useful
  information about input devices that are PS2 or on the LPC bus (See
  ``LPC Bus'').  Unfortunately, what I've seen doesn't say that it's on
  the LPC bus when it likely is.  In /pci/00/ there is one binary file
  for each pci device where the file names are the pci-slot-numbers
  (also called pci-slot-names).  The 00 means pci bus 0.


  6.6.  The /sys Tree

  Starting with kernel 2.6 there's a new /sys directory for PnP
  configuration.  It's a sysfs type of file system and it's something
  like the /proc filesystem since the "files" represent information in
  the kernel memory and are not on your harddrive.  But it's not as
  useful as the /proc filesystem.  Originally (in the 2.5 kernels) it
  was called "driver file system" of type "driverfs".

  In the sysfs, each device which exists on your system has it's own
  directory which contains files showing the resources allocated to it.
  Such device directories have names like 0000:00:12.0@ or 00:06@.  What
  devices are these?  The first is a PCI card in "slot" 12 of your PC.
  The slot may actually be labeled PCI2 inside your PC (2 instead of
  12).  That's because low numbered "slots" are used for built-in
  devices on the motherboard that don't use any physical slots.  In this
  example, "slots" 1-10 would be built-in and actual slots 11-14 are
  labeled 1-4.  By typing "lspci" you'll be able to match the numbers
  (like 0000:00:12.0) to names (like IDE interface).  Type "lspci -v" or
  "lspci -vv" to see more.

  Well then, what is 00:06 ?  It's an ISA card (or built-in device) but
  it's not ISA slot 6 (like the PCI numbering).  When a search was made
  for ISA-PNP devices, it was the 6th one found.  More precisely, it was
  the 7th one found since there's a device numbered: 00:00.  So how does
  one identify them?  Well, you could type: "cat */*" and display all
  the files for all the devices, but even then you don't see the device
  names (but do see info from which you can identify them).  This
  inconvenience will hopefully be fixed in the future.

  Not only do these files supply information on the bus-resource
  configuration (in somewhat cryptic format) and drivers (in "driver"
  directories), but in the future, you should be able to use them to
  change the resource configuration.  Right now (Aug 2004) you can't
  configure the PCI bus with it.  A serious limitation is that per the
  present "driver model" you can't change the resource of a device that
  has been assigned to a driver which likely means that you'll need to
  unload the driver module in order to use it.  If the driver is built
  in, there's no hope.  These serious limitations will hopefully be
  eliminated in the future.  In the kernel documentation is a file:
  "pnp.txt" telling how to configure.  As of Aug. 2004, it was much out-
  of-date but the author is working on an update.  Using the /sys tree
  to configure resources is known as the "Linux Plug and Play User
  Interface".

  The other part of "Linux Plug and Play" is the kernel interface used
  by device drivers.  This has changed a lot starting with kernel 2.6
  but most drivers are still using the old interface (as of Aug. 2004).
  It's possible also for drivers (or you) to use the "user interface"
  which needs improvement.


  6.7.  PCI Bus Inspection

  It's easy to find out what bus-resources have been assigned to devices
  on the PCI bus with the "lspci" and/or "scanpci" commands The options
  -v or -vv will show more detail.  In some cases, "scanpci" will find a
  device that "lspci" can't find.  That's because "scanpci" directly
  searches for devices on the pci bus (via the configuration space) and
  doesn't use data obtained by the kernel (where it could be wrong due
  to a kernel bug --I've just found such a case).

  This info in more cryptic format is found in "files" located in the
  /sys and /proc trees. In /sys/bus/pci/devices the file vendor will
  contain the vendor id number such as 0x4B8C, etc.  In still more
  cryptic format it's in /proc/bus/pci.  Such information in older
  kernels prior to kernel 2.6, was in /proc/pci (non-cryptic but IRQs in
  hexadecimal) or in /proc/buspci/devices (cryptic display).

  In most cases for PCI you will only see how the hardware is now
  configured and not what resources are required.  In some cases you
  only see the base addresses (the starting addresses of the range) but
  not the ending addresses.  If you see the entire range then you can
  determine how many bytes of address resources are needed.


  6.8.  ISA Bus Introduction

  For cards on the ISA bus, it's not as simple as for the PCI bus which
  is inherently PnP.  Later ISA cards were PnP but older ones were not.
  Also, some cards that are PnP had their PnP disabled by special
  software which runs only on MS.  The non PnP cards are configured by
  jumpers on the card or by MS software.
  6.9.  ISA PnP cards

  If it's a PnP card you may try running pnpdump --dumpregs but it's not
  a sure thing.  The results may be seem cryptic but they can be
  deciphered.  Don't confuse the read-port address which pnpdump uses
  for communication with PnP cards with the I/O address of the found
  device.  They are not the same.


  6.10.  LPC Bus

  LPC (Low Pin Count) is a bus-like interface often used on laptops and
  increasingly used on desktops too.  To find out if you have LPC type
  "lspci" and look for "LPC".  There are other words next to "LPC" such
  as "ISA Bridge ... LPC Interface Controller" or "LPC Bridge", etc.
  LPC is not really ISA but it substitutes for an ISA bus.

  The old ISA bus was slow and devices that needed more speed were put
  on the newer PCI but.  But devices that didn't need high speed were
  often implemented by chips on the motherboard and remained on the ISA
  bus even though there were no slots for any ISA cards.  Then the LPC
  bus came along to replace what remained of the ISA bus.  LPC is much
  smaller than ISA and just as fast since it runs at 4 times the clock
  speed of ISA.  Its multiplexed bus for data/address and control is
  only 4 bits wide.  To send a byte requires splitting the byte into 2
  half-bytes and then putting them back together.  So its clear why it's
  "Low Pin Count" = LPC.  There's also a few other lines in the bus.

  This small LPC interface is used for slow "legacy" devices such as
  serial ports, parallel ports, and floppy drives.  So a computer using
  LPC will have all fast devices on the PCI bus, etc. and slow (legacy)
  devices on the LPC bus interface.  All LPC devices will be on-board;
  there are no LPC slots.

  LPC has no standards for Plug-and-Play configuring but says that the
  BIOS or ACPI should do the configuring.  Devices on this bus sometimes
  use isapnp.  Linux support for LPC as of late 2004 was very much
  incomplete but Linux has some support for the configuring aspects of
  ACPI.  Sometimes a BIOS menu lets one manually PnP-configure devices
  on the LPC bus but it may not tell you that the device resides on LPC.

  A major chip on the LPC bus is the superio chip which contains legacy
  IO devices: serial and parallel ports, floppy controller, keyboard
  controller, mice, etc.  BIOS data may also reside on the LPC bus.  The
  keyboard and mouse (input devices) should be listed in
  /proc/bus/input/devices but instead of seeing "lpc" it seems to show
  "isa0060/serio0, etc. even though it's on the lpc bus and not the isa
  bus.


  6.11.  X-bus

  Before the LPC bus became popular, there was an "X-bus" (not covered
  in this HOWTO) which served the same purpose as the LPC bus but wasn't
  so compact as LPC.  Some PCs have both LPC and an X-bus.


  6.12.  Non-PnP Cards

  In contrast to PnP cards, non-PnP cards always have their resources
  set in the hardware.  That is they always have an address and IRQ
  unless there is a jumper setting, etc. for disabling the device.
  Sometimes the resources used can be found by probing done by the
  device driver or by other software that does probing.  For example
  "scanport" (Debian only ??) probes most IO port address and may find
  ISA devices.  But be warned that it might hang your PC.  Sometimes it
  will fail to find hardware that's actually there (since the hardware
  has the default 0xff in it's registers).  Even if It finds the
  hardware it will not show the IRQ nor will it positively identify the
  hardware.

  So one way to try to find such hardware is to start a driver, which
  may probe for such hardware.  By looking at the boot-time messages,
  you might see a driver start and find the hardware.  Otherwise, you
  may need to find a driver and start it (for example, by having it load
  as a module).

  Finding the right driver may be difficult.  Sometimes there just isn't
  any driver since some devices aren't (yet ?) supported by Linux.  To
  determine which driver you need, look at any documentation which might
  identify the card.  If this fails, look on the card itself, including
  important names/numbers on the chips.  But the identification of the
  driver module you need may not be anywhere on the card.  You could
  find the FCC id on the card and then search the Internet with the FCC
  id number to try to find more information about the card (or the chips
  on it).


  6.13.  Non-PnP Cards with jumpers

  If the card has jumpers to set the resources (configuration) then one
  may look at how the jumpers are set.  There are some cards that had
  both PnP and jumpers.   They worked like jumper cards if PnP was
  somehow disabled.  Sometimes a card has labels on it showing how to
  set the jumpers (or at least gives some clue).  You may need the
  documentation that came with the card (either printed or on a floppy
  disk).  Perhaps you can find it on the Internet.


  6.14.  Neither PnP nor jumpers

  One the most difficult cases is where software running under MS has
  been used to configure either a non-PnP card or a PnP card where PnP
  has been disabled by the same MS software.  So you can't configure it
  by PnP nor by jumpers.  In this case your only hope is to probe for
  addresses as described in ``Non-PnP Cards''.  Or try to find the MS
  software that configured it.


  6.15.  Tools for Detecting and/or Configuring all Hardware

  In a duplication of effort, various major distributions of Linux
  developed their own tools for detection and/or configuration of
  hardware.  This configuring is usually a lot more than just the
  resource type configuring of Plug-and-Play.  It's configuring in
  general which is mostly beyond the scope of this howto.

  Then other distributions, such as Debian, might obtain copies of the
  tool and offer it to their users as an option, or as a troubleshooting
  tool.  These tools likely make use of the standard Linux tools for
  detecting hardware such as "lspci".  In the following list of tools,
  the name of the distribution that developed it is in parentheses, but
  the tool is likely available also in other distributions.


  �  hardinfo

  �  hwinfo (SuSE) detects move stuff than discover

  �  discover (Progeny, used by Debian)


  �  Kudzu (RedHat) detects and configures

  �  lsdev (standard Linux command)

  �  hwsetup-knoppix (Knoppix, based on Kudzu)


  6.16.  Tools for Detecting and Configuring One Type of Hardware

  There are various tools available to find and possibly configure
  various type of devices.  This configuring is configuring in general
  which is not covered by this howto.


  �  read-edid (get-edid): gets parameters of VESA monitors (except very
     old ones)

  �  sndconfig: for soundcards

  �  printtool: printers, must have X-window running

  �  pconf-detect: parallel ports

  �  gpm-mouse-test:detects and tests mice

  �  mdetect: detects and configures mice Does it know about the mice
     devices in /dev/input/?

  �

  �  nictools-pci (and nictools-nopci)  for ethernet cards

  �  hdparm: configure hard drive hardware

  �  hotplug: used by kernel

  �  xvidtune: tune video for use with Xwindows (See XFree86-Video-
     Timings-HOWTO)


  6.17.  Use MS Windows

  Some people have attempted to use Windows to see how bus-resources
  have been set up.  Unfortunately, since PnP hardware forgets its bus-
  resource configuration when powered down, the configuration may not be
  the same under Linux.  For non PnP hardware (or where someone has
  disabled PnP inside the device by jumpers or Windows software), then
  using Windows should work OK.  Even for PnP, it often turns out to be
  the same because in many cases both Windows and Linux simply accept
  what the BIOS has set.  But where Windows and/or Linux do the
  configuring, they may do it differently.  So don't count on PnP
  devices being configured the same.


  7.  PCI Interrupts

  7.1.  Introduction

  Each PCI device that needs an interrupt comes with a fixed PCI
  interrupt that can't be changed.  It's designated by a slot number and
  a letter A, B, C, or D.  Example 3:B.  But this PCI interrupt is
  mapped (routed or redirected) to an interrupt number like say 21 by a
  chip on the motherboard.

  This routing is done by  a "programmable interrupt router" = PIR.
  Alternatively, an interrupt line may just routed directly (without any
  PIR).  If there's a PIR (router) it can be programmed by the BIOS or
  by Linux.  Thus a PCI device's interrupt may be sometimes be changed,
  not by sending the interrupt on a different wire but by changing the
  routing of the pulse on that wire by programming the PIR.  When the
  routing changes, the interrupt provide by this new routing is written
  in a configuration register located in the device chip.


  7.2.  History: From ISA to PCI Interrupts

  Before the PCI bus, PCs used the ISA bus and then during the
  transition to the newer PCI bus, most PC computers used both the PCI
  and ISA busses.  The ISA bus had all interrupt lines going to every
  card so any card could change its irq number just by sending out its
  interrupt signal on a different line (on a different pin).  All the
  interrupt signals were sent to the in interrupt controller which then
  signalled the CPU to temporarily stop whatever it was doing and run
  driver code to service the interrupt.

  When PCI first appeared, the simple solution was just to map the PCI
  interrupts to available ISA interrupts that weren't being used.  This
  required the use of "programmable interrupt router" = PIR (hardware)
  to do this mapping.  But since there were only 15 such interrupts, it
  was common to put many PCI devices on just the few available
  interrupts.  To solve this problem is simple: provide new hardware to
  increase the number of interrupts.  The result was the APIC.  But it
  was slow to be adopted since the ability of the PCI bus to share
  interrupts eased the interrupt shortage problem.  So APIC was mostly
  used where it was needed for dual processors.


  7.3.  Advanced Programmable Interrupt Controller (APIC)

  This can provide (depending on the model) 16, 24, 32, or 64
  interrupts, etc.  It also can handle the routing of interrupts from
  one CPU to another for cases of multiple CPUs.  See the file "IO-APIC"
  in the i386 directory of the kernel documentation and the ACPI-HOWTO.
  Don't confuse APIC with ACPI (Advanced Configuration and Power
  Interface) which may be used by the kernel to configure the APIC.

  The actual APIC controller that is connected to the interrupt lines is
  an I/O APIC (or IO-APIC or IOAPIC).  By using more than one IO-APIC
  one may obtain more interrupts and they are numbered so as to be
  unique.  For example, the first controller could have input pins 0-23
  and the second would call its input pins 24-47, resulting in 48
  interrupts numbered 0-47.  But a few people find they have high
  interrupt numbers.  Could it be that the second IO-APIC is starting
  with a higher base number than it should, leaving a long gap of non-
  existent irqs?

  Besides IO-APICs there are local APICs (LAPIC) which are part of each
  CPU.  The IO-APIC does it work by communicating with the LAPICs inside
  the CPUs.

  When APIC was introduced, the old ISA PICs were also retained giving
  one a choice of whether or not to use APIC or ISA's PIC (which is
  sometimes just called PIC or XT-PIC in /proc/interrupts; the "XT"
  comes from IBM's XT PC which was IBM's second model PC in 1983).  It's
  possible to tell the kernel (on the kernel command line) to not use
  APIC in which case it will use the old XT-PIC if its available.  But
  since APIC can have more interrupts than the 15 provided by XT-PIC,
  there could be problems ??

  To see if you have PIC or APIC look at /proc/interrupts.  If you see
  XT-PIC for just irq 2 but IO-APIC for the others, it may mean that you
  have the old XT-PIC but it isn't being currently used.  Well, irq 2 is
  available for communication between two old XT-PICs just in case you
  might need to use them if you were to disable APIC.  There are two XT-
  PICs since each only supports 8 interrupts.


  7.4.  Message Signalled Interrupts (MSI)

  Another development is Message Signalled Interrupts (MSI) where the
  interrupt is just a message sent to a special address over the main
  computer bus (no interrupt lines needed).  But the device that sends
  such a message must first gain control of the main bus so that it can
  send the interrupt message.  Such a message contains more info than
  just "I'm sending an interrupt".  It contains an index for the address
  of program that needs to be run to service the IRQ.  That index, such
  as 3, would mean the the cpu find the address it must jump to in the
  3rd element of a special table that the cpu knows about.

  Since cards must support MSI and many cards don't, it seems that the
  conventional methods of interrupt hardware support (called INTx) will
  be around for a long time.


  7.5.  Sharing PCI Interrupts

  PCI interrupts may be shared, meaning that two or more PCI devices
  will generate the same IRQ.  If feasible, it's usually better not to
  share.  Sharing doesn't work right for: 1. very old PCI hardware
  (before 1995 ??) 2. defective PCI hardware which may have a factory
  defect (it was made that way).  For example, if a PCI device on IRQ9
  falsely claims that any IRQ9 was intended for it, then other devices
  using IRQ9 may wind up having all IRQs they issue ignored since the
  bad device is falsely claiming their IRQs.  With no sharing, this
  problem is avoided.

  For an example of sharing the same IRQ between two PCI devices.  see
  ``PCI interrupt sharing'' This sharing ability is built into the
  hardware and all device drivers are supposed to support it.  Note that
  you usually can't share the same interrupt between the PCI and ISA
  bus.


  7.6.  Looking at Routing Tables

  Some info is provided by the boot-time messages which may be viewed by
  typing dmesg.  The following ways of looking at tables involve
  software which you may not have (or doesn't exist yet).  To check
  routing where PCI routes to the 16 ISA interrupts use pirtool which
  shows the $PIR routing table.  If you have APIC with hard-wired
  routing (no PIR), use mptable to look at the MP table.  For routeable
  APIC, a table is accessed by ACPI _PRT methods (but is there a
  command-line command to do this?)


  7.7.  For More Information

  Detailed technical information about interrupts is at PCI Interrupts
  for x86 Machines under FreeBSD
  <http://people.freebsd.org/~jhb/papers/bsdcan/2007/article/article.html>.
  Microsoft has The Importance of Implementing APIC-Based Interrupt
  Subsystems on Uniprocessor PCs
  <http://www.microsoft.com/whdc/system/sysperf/apic.mspx>



  7.8.  PCI Interrupt Linking

  Here are some of the details of the PCI interrupt system.  Each PCI
  card (and device mounted on the motherboard) has 4 possible
  interrupts: INTA#, INTB#, INTC#, INTD#.  From now on we will call them
  just A, B, C, and D.  Each has its own pin on the edge connector of a
  PCI card.  Thus for a 7-slot system (for 7 cards) there could be 7 x 4
  = 28  different interrupt lines for these cards.  Devices built into
  the motherboard also have additional interrupts.  But the specs permit
  a fewer number of interrupt lines, so some PCI buses seem to be made
  with only 4 or 8 interrupt lines.  This is not too restrictive since
  interrupts may be shared.  For 4 interrupt line (wires, traces, or
  links) LNKA, LNKB, LNKC, LNKD there is a programmable "interrupt
  router" chip that routes LNKA, LNKB, LNKC, LNKD to selected IRQs.
  This routing can be changed by the BIOS or Linux.  For example, LNKA
  may be routed to IRQ5.  Suppose we designate the B interrupt from slot
  3 as interrupt 3B.  Then interrupts 3B and 2A could both be
  permanently connected to LNKA which is routed to IRQ5.  These 2
  interrupts: 3B and 2A are permanently shared by hardwiring on the
  motherboard.

  One may type "dmesg" on the command line to see how interrupt lines
  like LNKA are routed (or linked) to IRQs (*5 means that it's linked to
  IRQ 5).  Look for "PCI Interrupt Link".   Note that "link" is used
  here with two meanings: 1. the linking (routing) of PCI interrupt
  lines to IRQs.  2. the label of an interrupt line such as LNKB (link
  B).  The interrupt line labels seem to be provided by the Bios ?? and
  they may have many different names like: LNKC, LNK2, APCF, LUBA, LIDE,
  etc.  Question: When a large number of interrupt lines are shown
  disabled, do they all physically exist on the motherboard?  Or do they
  just exist only in the ACPI BIOS software so that the BIOS can work
  with motherboards which have more interrupt lines?

  One simple method of connecting (hard-wiring) these lines from PCI
  devices (such as 3B) to the interrupts LNKA, etc. would be to connect
  all A interrupts (INTA#) to line LNKA, all B's to LNKB, etc.  This
  method was once used many years ago but it is not a good solution.
  Here's why.  If a card only needs one interrupt, it's required that it
  use A.  If it needs two interrupts, it must use both A and B, etc.
  Thus INTA# is used much more often than INTD#.  So one winds up with
  an excessive number of interrupts sharing the first line (LNKA
  connected to all the INTA#).  To overcome this problem one may connect
  them in a more random way so that each of the 4 interrupt lines (LNKA,
  LNKB, LNKC, LNKD) will share about the same number of actual PCI
  interrupts.

  One method of doing this would be to have wire LNKA share interrupts
  1A, 2B, 3C, 4D, 5A, 6B, 7C.  This is done by physically connecting
  wire W to wires 1A, 2B, etc.  Likewise wire LNKB could be connected to
  wires 1B, 2C, 3D, 4A, 5B, 6C, 7D, etc.  Then on startup, the BIOS maps
  the LNKB, LNKA, LNKC, LNKD to IRQs.  After that, it writes the IRQ
  that each device uses into a hardware configuration register in each
  device.  From then on, any program interrogating this register can
  find out what IRQ the device uses.  Note that just writing the IRQ in
  a register on a PCI card doesn't in any way set the IRQ for that
  device.

  A practical use for this info is that, as a last resort, one may
  change the IRQs of a PCI card by inserting it in a different slot.  In
  the above example, INTA# of a PCI card will be connected to wire LNKA
  if the card is inserted into slot 1 (1A maps to LNKA) but INTA# will
  be connected to wire LNKB when it's inserted into slot 4 (4A maps to
  LNKB).

  A card in a slot may have up to 8 devices on it but there are only 4
  PCI interrupts for it (A, B, C, D).  This is OK since interrupts may
  be shared so that each of the 8 devices (if they exist) can have an
  shared interrupt.  The PCI interrupt letter of a device is often fixed
  and hardwired into the device.  The assignment of interrupts is done
  by either the BIOS or Linux mapping the PCI interrupts to the ISA-like
  interrupts as mentioned above.

  If there are only 4 lines (LNKA, LNKB, LNKC, and LNKD) as in the above
  example, the mapping choices that the PCI BIOS has are limited.  Some
  motherboards may use more lines and thus have more choices.  For
  example LNKA-LNKH (8 lines).  The boot-time messages (and dmesg) may
  display them and how they are mapped.  The BIOS knows about how they
  are wired.

  On the PCI bus, the BIOS (or Linux) assigns IRQs (interrupts) so as to
  avoid conflicts with the IRQs it knows about on the ISA bus.
  Sometimes the CMOS BIOS menu may allow one to assign IRQs to PCI cards
  or to tell the BIOS what IRQs are to be reserved for ISA devices.  The
  assignments are known as a "routing table".  In MS Windows it's called
  "IRQ steering" but this also covers the case of dynamic IRQ routing
  after boot-time.  The BIOS may support it's own IRQ steering.

  If your PC uses PCI interrupts which are mapped to ISA interrupts, you
  right think that interrupts might be slow since the ISA bus was slow.
  Not really.  The ISA Interrupt Controller Chip(s) has a direct
  interrupt wire going to the CPU so it can get immediate attention.
  While signals on the old ISA address and data buses are slow to get to
  the CPU, the IRQ interrupt signals get there very fast.


  8.  PnP for External and Plug-in Devices

  8.1.  USB Bus

  The USB (Universal Serial Bus) is a high speed bus on an external
  cable that plugs into a PC.  The external bus cable has its own
  communication protocols and doesn't use any IRQs, I/0 addresses (or
  other bus-resources).  Communication is by packets, something like the
  Internet, only there are time-slice allocations which prevent any one
  device from hogging the bus if other devices need it.  There are free
  time slots which allow every device to send a short message to the bus
  controller without any need for IRQs on the bus.

  However, the USB bus controller inside the PC does have an IRQ and an
  address on the PCI bus (or ISA) which are used for communication
  between the CPU and all devices on the USB.  Thus there's no resource
  allocations needed for the individual devices on the USB.  One could
  also think of this as all devices on the USB sharing the one interrupt
  and address.  If a device is on the USB it needs a driver that
  understands the USB.

  But each device on the USB does have an IDs, just like cards do on the
  PCI bus.  So Linux likely maintains a table of these IDs so that
  device drivers can check them to find their device.  The USB also
  support "hot plug".  To find out what is on the USB bus, you could use
  a general hardware detection tool like "discover" or "hwinfo".


  8.2.  Hot Plug

  "Hot plug" is where you plug something into a PC (usually via a cable
  connection) and it is instantly detected.   If required, it is
  configured with bus-resources.  The driver for it is also started,
  perhaps by loading a module for it.  For this to work the hardware
  used must be designed for hot plugging.  One can hot plug certain PCI
  cards (Cardbus), USB devices, and IEEE 1394 devices (Firewire).

  When a new device is detected, it's registers are read so as to get
  the ID number of the device.  Then to find a driver, Linux must
  maintain a table mapping ID numbers to drivers.  It wasn't until
  kernel 2.4 that such a table existed since Linux once shunned
  centralized PnP.  It's named: MODULE_DEVICE_TABLE.


  8.3.  Hot Swap

  "Hot Swap" is where you remove an old device and then plug in a new
  device to replace the old one.  You have thus "swapped" devices.  Now
  in addition to being able to detect that a new device has been plugged
  in, the removal of the old device needs to be detected too.


  8.4.  PnP Finds Devices Plugged Into Serial Ports

  External devices that connect to the serial port via a cable (such as
  external modems) can also be called Plug-and-Play.  Since only the
  serial port itself needs bus-resources (an IRQ and I/O address) there
  are no bus-resources to allocate to such plug-in devices.  In this
  case, PnP is used only to identify the modem (read it's model code
  number).  This could be important if the modem is a software modem
  (linmodem) and requires a special driver.  There is a special PnP
  specification for such external serial devices (something connected to
  the serial port).

  Linux doesn't support this yet ??  For a hardware modem, the ordinary
  serial driver will work OK so there's little need for using the
  special serial PnP to find a driver.  You still need to tell the
  communications program what port (such as /dev/ttyS1) the modem is on.
  With PnP you wouldn't need to even do this.  With the advent of
  software modems that have Linux drivers (linmodems), it would be nice
  to have the appropriate driver install itself automatically via PnP.


  9.  Error Messages

  9.1.  Unexpected Interrupt

  This means that an interrupt happened that no driver expected.  It's
  unlikely that the hardware issued an interrupt by mistake.  It's more
  likely that the software has a minor bug and doesn't realize that some
  software did something to cause the interrupt.  In many cases you can
  safely ignore this error message, especially if it only happens once
  or twice at boot-time.  For boot-time messages, look at the messages
  which are nearby for a clue as to what is going on.  For example, if
  probing is going on, perhaps a probe for a physical device caused that
  device to issue an interrupt that the driver didn't expect.  Perhaps
  the driver wasn't listening for the correct IRQ number.


  9.2.  Plug and Play Configuration Error (Dell BIOS)

  The BIOS was unable to configure bus-resource.  There may be an
  interrupt conflict which can't be avoided.  Dell suggests that you
  remove some of your non-essential cards and see if it goes away.  In
  one case this problem was due to a defective motherboard.


  9.3.  isapnp: Write Data Register 0xa79 already used (from logs)

  If you use isa-pnp, the IO address 0xa79 must not ever be used by any
  device.  So if other hardware is using 0xa79 when you try to load the
  isa-pnp module, you'll get this message in your logs and the isa-pnp
  will exit.  One way to try to fix this is to load the isa-pnp module
  early before other hardware is initialized.  For PCMCIA this means to
  load isa-pnp before running cb modules and service.


  9.4.  Can't allocate region (PCI)

  Here "region" means address range.  A PCI device that needs two
  addresses will have region 0 for the first address and region 1 for
  the second address needed.  Use the command: lspci -vv to see the
  various resource regions (often just called regions) and whether the
  address is of type IO or memory.  In PCI jargon region 2 is "base
  address 2" (or "base address register 2"), etc.


  10.  Interrupt Sharing and Interrupt Conflicts

  10.1.  Introduction

  When two or more devices use the same interrupt line (and the same IRQ
  number) it's either "Interrupt Sharing" or an "Interrupt Conflict".
  The PCI bus allows all PCI devices to share interrupts with each other
  so this is called "sharing".  But if an ISA device (or a LPC device
  ??) uses the same interrupt (IRQ) as some other device (either PCI,
  ISA, or LPC ??) there is usually an interrupt conflict.

  There are exceptions to what's stated above.  Some very old PCI
  devices (pre 1995 ??) do not allow interrupt sharing.  Conversely, a
  few ISA devices have been designed to share interrupts (between two
  ISA devices ??) but both ISA devices must be designed this way and be
  driven by software that knows about sharing interrupts.  The
  motherboard must support it too.  The following discussion pertains to
  PCs that have an ISA bus.

  A conflict means that when an interrupt happens, no device driver (or
  the wrong one) may be called and bad things happen like buffer
  overruns (loss of data).  A device may nearly ground its interrupt
  line when it's not sending its interrupt, thus preventing any other
  device from using that interrupt wire.  That's OK if only that device
  uses that interrupt.  But if a second device tries to use the same
  interrupt line it can't do so.  If this second device also nearly
  grounds the line when not sending an interrupt, then neither device
  can use the interrupt.  But both Linux and the two devices are unaware
  of this conflict and merrily send out interrupts anyway that mostly go
  nowhere and are thus lost.

  Interrupt conflicts were common when the IRQs were set by jumpers on
  cards (ISA bus) because the kernel often didn't know how these jumpers
  were set.  ISA plug-and-play (no jumpers on the cards) helped since
  the software could change IRQs.  The demise of ISA in favor of PCI has
  nearly eliminated IRQ conflicts.  Still, your PC likely has devices on
  the motherboard (not on a plug-in card) on an ISA bus, a LPC bus, or
  an X-bus.  But the BIOS and the kernel should know how these are set
  and thus avoid using them for PCI devices, thereby avoiding interrupt
  conflicts.  But there is still a possible interrupt problem with PCI
  since it could run out of available interrupts, especially on older
  PCs that only have 16 interrupts.

  But IRQ sharing on the PCI bus, while eliminating the conflict
  problem, has introduced another problem which is less serious: the IRQ
  balancing problem.  If too many high-irq-issuing devices share the
  same IRQ, it may cause delays in the IRQs getting serviced and can't
  even result in buffer overruns and other errors.  This is not due to
  congestion on the interrupt line, but it's due to the way that the
  software determines which device issued the interrupt.  ``PCI
  interrupt sharing''

  There are two types of interrupt conflicts.  One is a real conflict as
  described above.  In this case interrupts don't work and the device
  driver keeps trying to control its device and is not aware that
  interrupts are not working.  The second type of interrupt conflict is
  where a device driver is started but discovers that the interrupt it
  needs is already in use so it issues an error message and exits.  The
  message could say something like "resource busy", and not clearly
  state that it was an interrupt problem.


  10.2.  Real Interrupt Conflict

  Both the BIOS and the the kernel will not knowingly allow any
  interrupt conflict, so how can they happen?   One way is if someone
  has put an incorrect IRQ into a configuration file, such as giving a
  parameter to a module like: irq=9.  In this example, suppose the irq
  of the device is really irq5.  Then when another device driver starts
  up where its device is set to irq5, you have two real devices using
  irq5 and a real conflict.  The kernel approved of letting the second
  device use irq5 since it erroneously thought that the first device was
  using irq9 and that irq5 was free.

  There are other cases like this where the kernel fails to know that an
  irq is in use.  One is when an old ISA card with an irq set by a
  jumper is present, but it's driver hasn't started yet (or it may not
  even have a driver).  Another case is where the BIOS set an irq in the
  hardware but no linux driver for that hardware ever started and Linux
  doesn't know about that irq.  This can happen even for a PCI card and
  the irq will show up in lspci -v but will not be in the
  /proc/interrupts directory and thus not known by the kernel.  Is this
  a bug in the kernel?

  What are the symptoms of an interrupt conflict.  One might think that
  the devices will not work at all, but since the addresses are known,
  the driver does communicate.  Interrupts are often used to control the
  flow of data to and from the device and without interrupts, flow is
  not controlled.  This may mean buffer overruns or even no flow at all
  since interrupt are used to initiate flow.  For a serial modem, the
  result is extremely slow flow with long pauses and frequent errors.
  For a sound card it may mean that a word or two is heard and then
  nothing more.


  10.3.  No Interrupt Available

  This is when a device driver starts but immediately exits in order to
  avoid an interrupt conflict.  It should display or log an error
  message something like "resource busy".

  One case when an ISA device is activated but can't be assigned an
  interrupt (IRQ) since none are available.  Or an interrupt may be
  available, but it can't be used since the hardware of the device that
  needs the interrupt doesn't support the interrupt number available (or
  the motherboard doesn't support it due to "routing" problems, see
  ``PCI Interrupts'').  If the ISA devices use up all the interrupts,
  then one or more PCI cards may be in conflict since they can't get any
  IRQs.

  Normally, the BIOS will assign interrupts and will not create
  conflicts.  But it may be forced to create conflicts if it runs out of
  IRQs.  This can happen if someone has set up the BIOS to reserve
  certain IRQs for legacy ISA devices that are not PnP.  These settings
  may be wrong and should be checked out, especially if you're having
  problems.  For example, someone may have reserved an IRQ for an ISA
  card that has long since been removed from the PC.  If you unreserved
  this IRQ then this IRQ is available and and conflict disappears.
  Sometimes the BIOS will solve the problem of an IRQ shortage by using
  what it calls IRQ 0.  There is no such IRQ available since the real
  IRQ 0 is permanently assigned to the computer's timer.  But IRQ 0 here
  means that the driver should use polling instead of IRQs.  This means
  that the driver frequently checks the device (polls it) to see if the
  device needs servicing by the interrupt service routine.  Of course,
  this wastes computer time and there's more likelihood of a buffer
  overrun inside a device since it might not get serviced by the driver
  promptly enough.



  11.  Appendix

  11.1.  Universal Plug and Play (UPnP)

  This is actually a sort of network plug-and-play developed by
  Microsoft but usable by Linux.  You plug something into a network and
  that something doesn't need to be configured provided it will only
  communicate with other UPnP enabled devices on the network.  Here
  "configure" is used in the broad sense and doesn't mean just
  configuring bus-resources.  One objective is to allow people who know
  little about networks or configuring to install routers, gateways,
  network printers, etc.  A major use for UPnP would be in wireless
  networking.

  UPnP uses:

  �  Simple Service Discovery Protocol to find devices

  �  General Event Notification Architecture

  �  Simple Object Access Protocol for controlling devices

  This HOWTO doesn't cover UPnP.  UPnP for Linux is supported by Intel
  which has developed software for it.  There are other programs which
  do about the same thing as UPnP.  A comparison of some of them is at
  <http://www.cs.umbc.edu/~dchakr1/papers/mcommerce.html> A UPnP project
  for Linux is at SourceForge: UPnP SDK for Linux
  <http://sourceforge.net/projects/upnp/>


  11.2.  Address Details

  There are three types of addresses: main memory addresses, I/O
  addresses (ports) and configuration addresses.  On the PCI bus,
  configuration addresses constitute a separate address space just like
  I/O addresses do.  Except for the complicated case of ISA
  configuration addresses, whether or not an address on the bus is a
  memory address, I/O address, or configuration address depends only on
  the voltage on other wires (traces) of the bus.  For the ISA
  configuration addresses see ``ISA Bus Configuration Addresses (Read-
  Port etc.)'' for details


  11.2.1.  Address ranges

  The term "address" is sometimes used in this document to mean a
  contiguous range of addresses.  Addresses are in units of bytes, So
  for example, a serial port at I/O address range 3F8-3FF will often
  just be referred to by its base address, 3F8.  The 3F8 is the location
  of the first byte in the range (address range).  To see the address
  ranges for various devices, look at /proc/iomem and /proc/ioports.



  11.2.2.  Address space

  To access both I/O and (main) memory address "spaces" the same address
  bus is used (the wires used for the address are shared).  How does the
  device know whether or not an address which appears on the address bus
  is a memory address or I/O address?  Well, for ISA (for PCI read this
  too), there are 4 dedicated wires on the bus that convey this sort of
  information.  If a certain one of these 4 wires is asserted, it says
  that the CPU wants to read from an I/O address, and the main memory
  ignores the address on the bus.  In all, read and write wires exist
  for both main memory and I/O addresses (4 wires in all).

  For the PCI bus it's the same basic idea (also using 4 wires) but it's
  done a little differently.  Instead of only one of the four wires
  being asserted, a binary number is put on the wires (16 different
  possibilities).  Thus, more info may be conveyed by these 4 wires..
  Four of these 16 numbers serve the I/O and memory spaces as in the
  above paragraph.  In addition there is also configuration address
  space which uses up two more numbers.  This leaves 10 more numbers
  left over for other purposes.


  11.2.3.  PCI Configuration Address Space

  This is different from the IO and memory address spaces because
  configuration address space is "geographic".  Each slot for a card has
  the slot number as part of the address.  This way, Linux (or the BIOS)
  can address a certain slot and find out what type of card is in that
  slot.  Each device has 64 standard byte-size registers and some of
  these hold numbers which can unambiguously identify the device.  Since
  the number of slots is limited as are the number of PCI devices built
  into motherboard, Linux (or the BIOS) only needs to check a limited
  number of addresses to find all the PCI devices.  If it reads all ones
  (0xFF in hexadecimal) from the first register of a device, then that
  means that no device is present.  Since there is no card or device to
  supply all these ones (0xFF) number, the PCI "host bridge" on the
  motherboard supplies (spoofs) this number for all non-existent device.

  The PCI slot number is called (in PCI lingo) the "Device Number" and
  since a card may have up to 8 devices on it, a "Function Number" from
  0-7 identifies which device it is on a PCI card.  These numbers are
  part of the geographic address.  Linux programmers call it "pci-slot-
  name".  Thus what Linux calls a "device" is actually a "function" in
  PCI lingo.  The PCI bus number (often 00) also becomes part of the
  geographic address.  For example, 0000:00:0d.2 is PCI bus 0, slot 0d,
  function 2.  For the full geographic address, one must include the
  double-word number of the device's configuration registers which one
  wants to access.  The leading 0000 (in 1999) were reserved for future
  use.

  How does the CPU designate that a read or write is to a PCI
  configuration space?  It doesn't, at least not directly.  Instead when
  access to configuration space is desired it does a 32-bit (double-
  word) write to 0cf8-0cfb in IO space and writes the full geographic
  address there.  The PCI host bridge is listening at this address and
  insures that any next write of data to 0cfc-0cff is put into the
  specified configuration registers of the specified device.  The bridge
  does this both by sending a special signal to the specified PCI card
  (or the like) on a dedicated wire that goes only to the slot where the
  card is plugged in.  It also puts bits on the control bus saying that
  what's on the address bus now is a geographic configuration space
  address.

  Why not make it simple and just have the CPU put bits on the control
  bus to say that the address on the main bus is a geographic one for
  PCI configuration?  Well, most CPU's are not capable of doing this so
  the PCI host bridge gets to do it instead.


  11.2.4.  Range Check (ISA Testing for IO Address Conflicts)

  On the ISA bus, there's a method built into each PnP card for checking
  that there are no other cards that use the same I/O address.  If two
  or more cards use the same IO address, neither card is likely to work
  right (if at all).  Good PnP software should assign bus-resources so
  as to avoid this conflict, but even in this case a legacy card might
  be lurking somewhere with the same address.

  The test works by a card putting a known test number in its own IO
  registers.  Then the PnP software reads it and verifies that what it
  reads is the same as the known test number.  If not, something is
  wrong (such as another card with the same address).  It repeats the
  same test with another test number.  Since it actually checks the
  range of IO addresses assigned to the card, it's called a "range
  check".  It could be better called an address-conflict test.  If there
  is an address conflict you get an error message.


  11.2.5.  Communicating Directly via Memory

  Traditionally, most I/O devices used only I/O memory to communicate
  with the CPU.  The device driver, running on the CPU would read and
  write data to/from the I/O address space and main memory.
  Unfortunately, this requires two steps.  For example, 1. read data
  from a device (in IO address space) and temporarily store in in the
  CPU; 2.  write this data to main memory.  A faster way would be for
  the device itself to put the data directly into main memory.  One way
  to do this is by using ISA ``DMA Channels'' or PCI bus mastering.
  Another way is for the physical device to actually contain some main
  memory (at high addresses so as not to conflict with main memory chip
  addresses).  This way the device reads and writes directly to it's
  self-contained main memory without having to bother with DMA or bus
  mastering.  Such a device may also use IO addresses.


  11.3.  ISA Bus Configuration Addresses (Read-Port etc.)

  These addresses are also known as the "Auto-configuration Ports".  For
  the ISA bus, there is technically no configuration address space, but
  there is a special way for the CPU to access PnP configuration
  registers on the PnP cards.  For this purpose 3 @ I/O addresses are
  allocated and each addresses only a single byte (there is no "range").
  This is not 3 addresses for each card but 3 addresses shared by all
  ISA-PnP cards.

  These 3 addresses are named read-port, write-port, and address-port.
  Each port is just one byte in size.  Each PnP card has many
  configuration registers so that just 3 addresses are not even
  sufficient for the configuration registers on a single card.  To solve
  this problem, each card is assigned a card number (handle) using a
  technique called "isolation".  See ``ISA Isolation'' for the complex
  details.

  Then to configure a certain card, its card number (handle) is sent out
  via the write-port address to tell that card that it is to listen at
  its address port.  All other cards note that this isn't their card
  number and thus don't listen.  Then the address of a configuration
  register (for that card) is sent to the address-port (for all cards
  --but only one is listening).  Next, data transfer takes place with
  that configuration register on that card by either doing a read on the
  read-port or a write on the write-port.

  The write-port is always at A79 and the address-port is always at 279
  (hex).  The read-port is not fixed but is set by the configuration
  software at some address (in the range 203-3FF) that will hopefully
  not conflict with any other ISA card.  If there is a conflict, it will
  change the address.  All PnP cards get "programmed" with this address.
  Thus if you use say isapnp to set or check configuration data it must
  determine this read-port address.


  11.4.  Interrupts --Details

  11.4.1.  Serialized Interrupts

  It was previously stated that there was a wire for each interrupt.
  But the serialized interrupt (or serial interrupt) is an exception.  A
  single wire is used for all interrupt which are multiplexed on that
  wire.  Each interrupt has a time slot on the interrupt line.  It's
  used on the LPC bus and is also for the PCI bus, but it's seldom used
  for PCI ??


  11.4.2.  DMA

  Before going into interrupt details, there is another way for some
  devices to initiate communication besides sending out an interrupt.
  This method is a DMA (Direct Memory Access) request to take control of
  the computer from the CPU for a limited amount of time.  On the PCI
  bus, it uses no "resources".  Not all devices are capable of doing
  DMA.  See ``DMA Channels''.


  11.4.3.  Soft interrupts

  There's also another type of interrupt known as a "soft interrupt"
  which is not covered in this HOWTO and doesn't use any "resources".
  While a hardware interrupt is generated by hardware, a soft interrupt
  is initiated by software.  There are a couple of ways to do this.  One
  way is for software to tell the CPU to issue an interrupt (an
  interrupt instruction).  Another way is for the software to send
  messages to other processes so as to interrupt them although it's not
  clear that this should be called an interrupt.  The ksoftirq process,
  which you may find running on a Linux PC, is a program which does this
  kind of interrupt for dealing with device drivers.  The device driver
  starts running due to a hardware interrupt but later on, software
  interrupts are used for the "bottom half" of the driver's interrupt
  service routine.  Thus, the ksoftirq process is also known as "bottom-
  half".  For more details see the kernel documentation.


  11.4.4.  Hardware interrupts

  Interrupts convey a lot of information but only indirectly.  The
  interrupt request signal (a voltage on a wire) sent by a device just
  tells a chip called the interrupt controller that a certain device
  needs attention.  The interrupt controller then signals the CPU.  The
  CPU then interrupts whatever it was doing, finds the driver code for
  this device and runs a part of it known as an "interrupt service
  routine" (or "interrupt handler").  This "routine" tries to find out
  what has happened and then deals with the problem.  For example, bytes
  may need to be transferred from/to the device.   This program
  (routine) can easily find out what has happened since the device has
  registers at addresses known to the driver software (provided the IRQ
  number and the I/O address of the device has been set correctly).
  These registers contain status information about the device .  The
  software reads the contents of these registers and by inspecting the
  contents, finds out what happened and takes appropriate action.
  Thus each device driver needs to know what interrupt number (IRQ) to
  listen for.  On the PCI bus (and for some special cases on the ISA
  bus) it's possible for two (or more) devices to share the same IRQ
  number.  Note that you can't share a PCI interrupt with an ISA
  interrupt (are there any exceptions ??).  When a shared interrupt is
  issued, the CPU runs all interrupt service routines sequentially for
  all devices using that interrupt.  The first thing such a service
  routine does is to check its device's registers to see if an interrupt
  actually happened for its device.  If it finds that its device didn't
  issue an interrupt (a false alarm) then it likely will immediately
  exit and the next service routine begins for the second device which
  uses that same interrupt.  It checks out the device like described
  above.  This sequence is repeated until the device is found that
  actually issued the interrupt.  All the interrupt routines for an
  interrupt are said to be constitute a chain.  So the chain is
  traversed until a routine on the chain claims the interrupt by saying
  in effect: this interrupt was for me.  After it handles the interrupt,
  the interrupt service routines further out on the chain don't run.

  The putting of a voltage on the IRQ line is only a request that the
  CPU be interrupted so it can run a device driver.  In almost all cases
  the CPU is interrupted per the request.  But interrupts may be
  temporarily disabled or prioritized so that in rare cases the actual
  interrupt of the CPU doesn't happen (or gets delayed).  Thus what was
  above called an "interrupt" is more precisely only an interrupt
  request and explains why IRQ stands for Interrupt ReQuest.


  11.5.  How the Device Driver Catches its Interrupt

  The previous statement, that device drivers listen for their
  interrupt, was an oversimplification.   Actually it's a chip (or part
  of a chip) on the motherboard called the "interrupt controller" that
  listens for all interrupts.  When the interrupt controller catches an
  interrupt, it sends a signal to the CPU to start the appropriate
  device driver's "interrupt service routine" to handle the interrupt.

  There are various types of interrupt controllers.  One type is the
  APIC = Advanced Programmable Interrupt Controller which usually has
  input pins for many interrupts, including PCI interrupts.  Older
  controllers only have pins for ISA interrupts but they can still
  handle PCI interrupts since there is a "programmable interrupt router"
  that converts PCI interrupts to ISA interrupts and routes them to
  certain pins (= certain IRQs) on the ISA interrupt controller.


  11.6.  ISA Isolation

  This is only for the old ISA bus.  Isolation is a complex method of
  assigning a temporary handle (id number or Card Select Number = CSN)
  to each PnP device on the ISA bus.  Since there are more efficient
  (but more complex) ways to do this, some might claim that it's a
  simple method.  Only one write address is used for PnP writes to all
  PnP devices so that writing to this address goes to all PnP device
  that are listening.  This write address is used to send (assign) a
  unique handle to each PnP device.  To assign this handle requires that
  only one device be listening when the handle is sent (written) to this
  common address.  All PnP devices have a unique serial number which
  they use for the process of isolation.  Doing isolation is something
  like a game.  It's done using the equivalent of just one common bus
  wire connecting all PnP devices to the isolation program.

  For the first round of the "game" all PnP devices listen on this wire
  and send out simultaneously a sequence of bits to the wire.  The
  allowed bits are either a 1 (positive voltage) or an "open 0" of no
  voltage (open circuit or tri-state).  To do this, each PnP device just
  starts to sequentially send out its serial number on this wire,
  voltage (open circuit or tri-state).  To do this, each PnP device just
  starts to sequentially send out its serial number on this wire, bit-
  by-bit, starting with the high-order bit.  If any device sends a 1, a
  1 will be heard on the wire by all other devices.  If all devices send
  an "open 0" nothing will be heard on the wire.  The object is to
  eliminate (by the end of this first round) all but highest serial
  number device.  "Eliminate" means to drop out of this round of the
  game and thus temporarily cease to listen anymore to the wire.  (Note
  that all serial numbers are of the same length.)  When there remains
  only one device still listening, it will be given a handle (card
  number).

  First consider only the high-order bit of the serial number which is
  put on the wire first by all devices which have no handle yet.  If any
  PnP device sends out a 0 (open 0) but hears a 1, this means that some
  other PnP device has a higher serial number, so it temporarily drops
  out of this round.  Now the devices remaining in the game (for this
  round) all have the same leading digit (a 1) so we may strip off this
  digit and consider only the resulting "stripped serial number" for
  future participation in this round.  Then go to the start of this
  paragraph and repeat until the entire serial number has been examined
  for each device (see below for the all-0 case).

  Thus it's clear that only cards with the lower serial number get
  eliminated during a round.  But what happens if all devices in the
  game all send out a 0 as their high-order bit?  In this case an "open
  0" is sent on the line and all participants stay in the game.  If they
  all have a leading 0 then this is a tie and the 0's are stripped off
  just like the 1's were in the above paragraph.  The game then
  continues as the next digit (of the serial number) is sent out.

  At the end of the round (after the low-order bit of the serial number
  has been sent out) only one PnP device with the highest serial number
  remains in the game.  It then gets assigned a handle and drops out of
  the game permanently.  Then all the dropouts from the previous round
  (that don't have a handle yet) reenter the game and a new round begins
  with one less participant.  Eventually, all PnP devices are assigned
  handles.  It's easy to prove that this algorithm works.  The actual
  algorithm is a little more complex than that presented above since
  each step is repeated twice to ensure reliability and the repeats are
  done somewhat differently (but use the same basic idea).

  Once all handles are assigned, they are used to address each PnP
  device for sending/reading configuration data.  Note that these
  handles are only used for PnP configuration and are not used for
  normal communication with the PnP device.  When the computer starts up
  a PnP BIOS will often do such an isolation and then a PnP
  configuration.  After that, all the handles are "lost" so that if one
  wants to change (or inspect) the configuration again, the isolation
  must be done over again.


  11.7.  Bus Mastering and DMA resources

  If a bus has bus mastering available, it's unlikely that any resources
  will be needed for DMA on that bus.  For example, the PCI bus doesn't
  need DMA resources since it has "bus mastering".  However, "bus
  mastering" is often called DMA.   But since it's not strictly DMA it
  needs no DMA resources.  The ISA and VESA local bus had no bus
  mastering.  The old MCA and EISA buses did have bus mastering.



  11.8.  Historical and Obsolete

  11.8.1.  OSS-Lite Sound Driver

  You must give the IO, IRQ, and DMA as parameters to a module or
  compile them into the kernel.  But some PCI cards will get
  automatically detected.  RedHat supplies a program "sndconfig" which
  detects ISA PnP sound cards and automatically sets up the modules for
  loading with the detected bus-resources.


  11.8.2.  ALSA (Advanced Linux Sound Architecture) as of 2000

  This will detect the card by PnP methods and then select the
  appropriate driver and load it.  It will also set the bus-resources on
  an ISA-PnP cards or PCI cards.  OSS (Open Sound System) was formerly
  popular.


  11.8.3.  MS Windows Notes

  Windows NT4 didn't support ISAPNP but had a PNPISA program which one
  could "use at your own risk".  For NT4 users were advised to set "not
  a PnP OS" in the BIOS so that the BIOS would do the resource
  configuring.  Thus both MS Windows and Linux were in olden days
  dependent on the BIOS doing the configuring (and still are).

  END OF Plug-and-Play-HOWTO







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