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  Linux 2.4.x Initialization for IA-32 HOWTO
  Randy Dunlap, rddunlap@ieee.org
  v1.0, 2001-05-17

  This document contains a description of the Linux 2.4 kernel initial�
  ization sequence on IA-32 processors.
  ______________________________________________________________________

  Table of Contents



   Introduction

  1. Overview

  2. This document

  3. Contributions

  4. Trademarks

  5. License

  5. Linux init ("ASCII art")

  5. Linux early setup

  6. IA-32 Kernel Setup

     6.1 start_of_setup:
        6.1.1 Read second hard drive DASD type
        6.1.2 Check that LILO loaded us right
        6.1.3 Check old loader trying to load a big kernel
        6.1.4 Determine system memory size
        6.1.5 Video adapter modes
        6.1.6 Get Hard Disk parameters
        6.1.7 Get Micro Channel bus information
        6.1.8 Check for mouse
        6.1.9 Check for APM BIOS support
        6.1.10 Prepare to move to protected mode
        6.1.11 Enable address line A20
        6.1.12 Make sure any possible coprocessor is properly reset
        6.1.13 Mask all interrupts
        6.1.14 Move to Protected Mode
        6.1.15 Jump to startup_32 code

  7. Video Setup

     7.1 video:
        7.1.1 basic_detect:
        7.1.2 mode_params:
        7.1.3 mopar_gr:
        7.1.4 mode_menu:
        7.1.5 mode_set:
        7.1.6 store_screen:
        7.1.7 restore_screen:
        7.1.8 mode_table:
        7.1.9 mode_scan:
        7.1.10 svga_modes:
        7.1.10 Linux architecture-specific initialization

  8. startup_32:

  9. Set segment registers to known values

  10. SMP BSP (Bootstrap Processor) check

  11. Initialize page tables

  12. Enable paging

  13. Clear BSS

  14. 32-bit setup

  15. Copy boot parameters and command line out of the way

  16. checkCPUtype

  17. Count this processor

  18. Load descriptor table pointer registers

  19. Start other processors

  19. Linux architecture-independent initialization

  20. start_kernel:

     20.1 More architecture-specific init
     20.2 Continue architecture-independent init
     20.3 Parsing command line options
     20.4 trap_init
     20.5 init_IRQ
     20.6 sched_init
     20.7 time_init
     20.8 softirq_init
     20.9 console_init
     20.10 init_modules
     20.11 Profiling setup
     20.12 kmem_cache_init
     20.13 sti
     20.14 calibrate_delay
     20.15 INITRD setup
     20.16 mem_init
     20.17 kmem_cache_sizes_init
     20.18 proc_root_init
     20.19 mempages = num_physpages;
     20.20 fork_init(mempages)
     20.21 proc_caches_init()
     20.22 vfs_caches_init(mempages)
     20.23 buffer_init(mempages)
     20.24 page_cache_init(mempages)
     20.25 kiobuf_setup()
     20.26 signals_init()
     20.27 bdev_init()
     20.28 inode_init(mempages)
     20.29 ipc_init()
     20.30 dquot_init_hash()
     20.31 check_bugs()
     20.32 Start other SMP processors (as applicable)
     20.33 Start init thread
     20.34 unlock_kernel()
     20.35 current->need_resched = 1;
     20.36 cpu_idle()

  21. setup_arch

     21.1 Copy and convert system parameter data
     21.2 For RAMdisk-enabled configs (CONFIG_BLK_DEV_RAM)
     21.3 setup_memory_region
     21.4 Set memory limits
     21.5 parse_mem_cmdline
     21.6 Setup Page Frames
     21.7 Handle SMP and IO APIC Configurations
     21.8 paging_init()
     21.9 Save the boot-time SMP configuration
     21.10 Reserve INITRD memory
     21.11 Scan for option ROMs
     21.12 Reserve system resources

  22. init thread

  23. do_basic_setup {part of the init thread}

     23.1 Be the reaper of orphaned children
     23.2 MTRRs
     23.3 SYSCTLs
     23.4 Init Many Devices
     23.5 PCI
     23.6 Micro Channel
     23.7 ISA PnP
     23.8 Networking Init
     23.9 Initial RamDisk
     23.10 Start the kernel "context" thread (keventd)
     23.11 Initcalls
     23.12 Filesystems
     23.13 IRDA
     23.14 PCMCIA
     23.15 Mount the root filesystem
     23.16 Mount the dev (device) filesystem
     23.17 Switch to the Initial RamDisk
     23.17 Glossary
     23.17 References


  ______________________________________________________________________



  1.  Introduction


  Portions of this text come from comments in the kernel source files
  (obviously).  I have added annotations in many places.  I hope that
  this will be useful to kernel developers -- either new ones or
  experienced ones who need more of this type of information.  However,
  if there's not enough detail here for you, "Use the Source."


  1.1.  Overview


  This description is organized as a brief overview which lists the
  sections that are described later in more detail.

  The description is in three main sections.  The first section covers
  early kernel initialization on IA-32 (but only after your boot loader
  of choice and other intermediate loaders have run; i.e., this
  description does not cover loading the kernel).  This section is based
  on the code in "linux/arch/i386/boot/setup.S" and
  "linux/arch/i386/boot/video.S".

  The second major section covers Linux initialization that is x86- (or
  i386- or IA-32-) specific.  This section is based on the source files
  "linux/arch/i386/kernel/head.S" and "linux/arch/i386/kernel/setup.c".

  The third major section covers Linux initialization that is
  architecture-independent.  This section is based on the flow in the
  source file "linux/init/main.c".

  See the References section for other valuable documents about booting,
  loading, and initialization.


  1.2.  This document


  This document describes Linux 2.4.x initialization on IA-32 (or i386
  or x86) processors -- after one or more kernel boot loaders (if any)
  have done their job.

  You can format it using the commands (for example):



       % sgml2txt ia32_init_240.sgml



  or


       % sgml2html ia32_init_240.sgml



  This will produce plain ASCII or HTML files respectively.  You can
  also produce LaTeX, GNU, and RTF info by using the proper sgmltool
  (man sgmltools).



  1.3.  Contributions


  Additions and corrections are welcome.  Please send them to me
  (rddunlap@ieee.org).  Contributions of section descriptions that are
  used will be credited to their author(s).


  1.4.  Trademarks


  All trademarks are the property of their respective owners.


  1.5.  License


  Copyright (C) 2001 Randy Dunlap.

  This document may be distributed only subject to the terms and
  conditions set forth in the LDP (Linux Documentation Project) License
  at "http://www.linuxdoc.org/COPYRIGHT.html".



  2.  Linux init ("ASCII art")


  Pictorially (loosely speaking :), Linux initialization looks like
  this, where "[...]" means optional (depends on the kernel's
  configuration) and "{...}" is a comment.



    +-------------------------------+
    | arch/i386/boot/setup.S:: +    |
    | arch/i386/boot/video.S::      |
    |-------------------------------|
    | start_of_setup:               |
    |   check that loaded OK        |
    |   get system memory size      |
    |   get video mode(s)           |
    |   get hard disk parameters    |
    |   get MC bus information      |
    |   get mouse information       |
    |   get APM BIOS information    |
    |   enable address line A20     |
    |   reset coprocessor           |
    |   mask all interrupts         |
    |   move to protected mode      |
    |   jmp to startup_32           |
    +-------------------------------+
                    |
                    v
    +-------------------------------+
    | arch/i386/kernel/head.S::     |
    |-------------------------------|
    | startup_32:                   |
    |   set segment registers to    |
    |     known values              |
    |   init basic page tables      |
    |   setup the stack pointer     |
    |   clear kernel BSS            |
    |   setup the IDT               |
    |   checkCPUtype                |
    |   load GDT, IDT, and LDT      |
    |     pointer registers         |
    |   start_kernel                |
    |     {it does not return}      |
    +-------------------------------+
                    |
                    v
    +-------------------------------+     +-------------------------------+
    | init/main.c::                 |  +->| arch/i386/kernel/setup.c::    |
    |-------------------------------|  |  |-------------------------------|
    | start_kernel():               |  |  | setup_arch():                 |
    |   lock_kernel                 |  |  |   copy boot parameters        |
    |   setup_arch                  |--+  |   init ramdisk                |
    |   parse_options               |<-+  |   setup_memory_region         |
    |   trap_init                   |  |  |   parse_cmd_line              |
    |     cpu_init                  |  |  |   use the BIOS memory map to  |
    |   init_IRQ                    |  |  |     setup page frame info.    |
    |   sched_init                  |  |  |   reserve physical page 0     |
    |     init_timervecs            |  |  |  [find_smp_config]            |
    |   time_init                   |  |  |   paging_init                 |
    |   softirq_init                |  |  |  [get_smp_config]             |
    |   console_init                |  |  |  [init_apic_mappings]         |
    |  [init_modules]               |  |  |  [reserve INITRD memory]      |
    |  [profiling setup]            |  |  |   probe_roms to search        |
    |   kmem_cache_init             |  |  |     for option ROMs           |
    |   sti                         |  |  |   request_resource to         |
    |   calibrate_delay             |  |  |     reserve video RAM memory  |
    |  [INITRD setup]               |  |  |   request_resource to         |
    |   mem_init                    |  |  |     reserve all standard PC   |
    |     free_all_bootmem          |  +--|     I/O system board resources|
    |   kmem_cache_sizes_init       |     +-------------------------------+
    |  [proc_root_init]             |
    |   fork_init                   |
    |   proc_caches_init            |
    |   vfs_caches_init             |
    |   buffer_init                 |
    |   page_cache_init             |
    |   kiobuf_setup                |
    |   signals_init                |     +-------------------------------+
    |   bdev_init                   |     | init/main.c::                 |
    |   inode_init                  |     | init(): {...init thread...}   |
    |  [ipc_init]                   |     |   do_basic_setup              |
    |  [dquot_init_hash]            |     |     {bus/dev init & initcalls}|
    |   check_bugs                  |     |   free_initmem                |
    |  [smp_init] {*below}          |     |   open /dev/console           |
    |   start init thread {---->}   |.....| exec init script or shell     |
    |   unlock_kernel               |     |   or panic                    |
    |   cpu_idle                    |     +-------------------------------+
    +-------------------------------+


    +-------------------------------+
    | smpboot.c::smp_init           |
    |-------------------------------|
    | arch/i386/kernel/smpboot.c::  |
    | smp_boot_cpus():              |
    |  [mtrr_init_boot_cpu]         |
    |   smp_store_cpu_info          |
    |   print_cpu_info              |
    |   save CPU ID/APIC ID mappings|
    |   verify_local_APIC           |
    |   connect_bsp_APIC            |
    |   setup_local_APIC            |
    |   foreach valid APIC ID       |
    |     do_boot_cpu(apicid)       |
    |   setup_IO_APIC               |
    |   setup_APIC_clocks           |
    |   synchronize_tsc_bp          |
    +-------------------------------+



  3.  Linux early setup


  (from linux/arch/i386/boot/setup.S and linux/arch/i386/boot/video.S)

  NOTE:  Register notation is %regname and constant notation is a
  number, with or without a leading '$' sign.


  3.1.  IA-32 Kernel Setup


  "setup.S" is responsible for getting the system data from the BIOS and
  putting them into the appropriate places in system memory.

  Both "setup.S" and the kernel have been loaded by the boot block.

  "setup.S" is assembled as 16-bit real-mode code.  It switches the
  processor to 32-bit protected mode and jumps to the 32-bit kernel
  code.

  This code asks the BIOS for memory/disk/other parameters, and puts
  them in a "safe" place: 0x90000-0x901FF, that is, where the boot block
  used to be.  It is then up to the protected mode system to read them
  from there before the area is overwritten for buffer-blocks.

  The "setup.S" code begins with a jmp instruction around the "setup
  header", which must begin at location %cs:2.

  This is the setup header:



  ______________________________________________________________________
                  .ascii        "HdrS"              # header signature
                  .word 0x0202          # header version number
  realmode_swtch: .word 0, 0            # default_switch, SETUPSEG
  start_sys_seg:  .word SYSSEG
                  .word kernel_version  # pointer to kernel version string
  type_of_loader: .byte 0
  loadflags:
  LOADED_HIGH     = 1             # If set, the kernel is loaded high
  #ifndef __BIG_KERNEL__
                  .byte 0
  #else
                  .byte LOADED_HIGH
  #endif
  setup_move_size: .word  0x8000  # size to move, when setup is not
                                  # loaded at 0x90000.
  code32_start:                   # here loaders can put a different
                                  # start address for 32-bit code.
  #ifndef __BIG_KERNEL__
                  .long 0x1000  # default for zImage
  #else
                  .long 0x100000# default for big kernel
  #endif
  ramdisk_image:  .long 0       # address of loaded ramdisk image
  ramdisk_size:   .long 0       # its size in bytes
  bootsect_kludge: .word  bootsect_helper, SETUPSEG
  heap_end_ptr:   .word modelist+1024   # (Header version 0x0201 or later)
                                          # space from here (exclusive) down to
                                          # end of setup code can be used by setup
                                          # for local heap purposes.
  pad1:           .word 0
  cmd_line_ptr:   .long 0       # (Header version 0x0202 or later)
                                  # If nonzero, a 32-bit pointer
                                  # to the kernel command line.
  trampoline:     call    start_of_setup  # no return from start_of_setup
                  .space        1024
  # End of setup header #####################################################
  ______________________________________________________________________



  3.1.1.  start_of_setup:



  3.1.1.1.  Read second hard drive DASD type


  Read the DASD type of the second hard drive (BIOS int. 0x13,
  %ax=0x1500, %dl=0x81).

  # Bootlin depends on this being done early. [TBD:why?]


  3.1.1.2.  Check that LILO loaded us right


  Check the signature words at the end of setup.  Signature words are
  used to ensure that LILO loaded us right.  If the two words are not
  found correctly, copy the setup sectors and check for the signature
  words again.  If they still aren't found, panic("No setup signature
  found ...").


  3.1.1.3.  Check old loader trying to load a big kernel


  If the kernel image is "big" (and hence is "loaded high"), then if the
  loader cannot handle "loaded high" images, then panic ("Wrong loader,
  giving up...").



  3.1.1.4.  Determine system memory size


  Get the extended memory size {above 1 MB} in KB.  First clear the
  extended memory size to 0.

  #ifndef STANDARD_MEMORY_BIOS_CALL

  Clear the E820 memory area counter.

  Try three different memory detection schemes.
  First, try E820h, which lets us assemble a memory map, then try E801h,
  which returns a 32-bit memory size, and finally 88h, which returns
  0-64 MB.

  Method E820H populates a table in the empty_zero_block that contains a
  list of usable address/size/type tuples.  In
  "linux/arch/i386/kernel/setup.c", this information is transferred into
  the e820map, and in "linux/arch/i386/mm/init.c", that new information
  is used to mark pages reserved or not.

  Method E820H:
  Get the BIOS memory map.  E820h returns memory classified into
  different types and allows memory holes.  We scan through this memory
  map and build a list of the first 32 memory areas {up to 32 entries or
  BIOS says that there are no more entries}, which we return at
  "E820MAP".  [See URL:
  http://www.teleport.com/ acpi/acpihtml/topic245.htm]

  Method E801H:
  We store the 0xe801 memory size in a completely different place,
  because it will most likely be longer than 16 bits.

  This is the sum of 2 registers, normalized to 1 KB chunk sizes: %ecx =
  memory size from 1 MB to 16 MB range, in 1 KB chunks + %edx = memory
  size above 16 MB, in 64 KB chunks.

  Ye Olde Traditional Methode:
  BIOS int. 0x15/AH=0x88 returns the memory size (up to 16 MB or 64 MB,
  depending on the BIOS).  We always use this method, regardless of the
  results of the other two methods.

  #endif

  Set the keyboard repeat rate to the maximum rate using using BIOS int.
  0x16.


  3.1.1.5.  Video adapter modes


  Find the video adapter and its supported modes and allow the user to
  browse video modes.

  call    video           # {see Video section below}


  3.1.1.6.  Get Hard Disk parameters


  Get hd0 data: Save the hd0 descriptor (from int. vector 0x41) at
  INITSEG:0x80 length 0x10.

  Get hd1 data: Save the hd1 descriptor (from int. vector 0x46) at
  INITSEG:0x90 length 0x10.

  Check that there IS an hd1, using BIOS int. 0x13.  If not, clear its
  descriptor.


  3.1.1.7.  Get Micro Channel bus information


  Check for Micro Channel (MCA) bus:

  �  Set MCA feature table length to 0 in case not found.

  �  Get System Configuration Parameters (BIOS int. 0x15/%ah=0xc0).
     This sets %es:%bx to point to the system feature table.

  �  We keep only the first 16 bytes of the system feature table if
     found: Structure size, Model byte, Submodel byte, BIOS revision,
     and Feature information bytes 1-5.  Bit 0 or 1 (either one) of
     Feature byte 1 indicates that the system contains a Micro Channel
     bus.


  3.1.1.8.  Check for mouse


  Check for PS/2 pointing device by using BIOS int. 0x11 {get equipment
  list}.

  �  Clear the pointing device flag (default).

  �  BIOS int. 0x11: get equipment list.

  �  If bit 2 (value 0x04) is set, then a mouse is installed and the
     pointing device flag is set to indicate that the device is present.


  3.1.1.9.  Check for APM BIOS support


  Check for an APM BIOS (if kernel is configured for APM support):

  �  start: clear version field to 0, which means no APM BIOS present.

  �  Check for APM BIOS installation using BIOS int. 0x15.

  �  If not present, done.

  �  Check for "PM" signature returned in %bx.

  �  If no signature, then no APM BIOS: done.

  �  Check for 32-bit support in %cx.

  �  If no 32-bit support, no (good) APM BIOS: done.  Must have 32-bit
     APM BIOS support to be used by Linux.

  �  Save the BIOS code segment, BIOS entry point offset, BIOS 16-bit
     code segment, BIOS data segment, BIOS code segment length, and BIOS
     data segment length.

  �  Record the APM BIOS version and flags.


  3.1.1.10.  Prepare to move to protected mode


  We build a jump instruction to the kernel's code32_start address.
  (The loader may have changed it.)

  Move the kernel to its correct place if necessary.

  Load the segment descriptors (load %ds = %cs).

  Make sure that we are at the right position in memory, to accommodate
  the command line and boot parameters at their fixed locations.

  Load the IDT pointer register with 0,0.

  Calculate the linear base address of the kernel GDT (table) and load
  the GDT pointer register with its base address and limit.  This early
  kernel GDT describes kernel code as 4 GB, with base address 0,
  code/readable/executable, with granularity of 4 KB.  The kernel data
  segment is described as 4 GB, with base address 0,
  data/readable/writable, with granularity of 4 KB.


  3.1.1.11.  Enable address line A20



  �  Empty the 8042 (keyboard controller) of any queued keys.

  �  Write 0xd1 (Write Output Port) to Command Register port 0x64.

  �  Empty the 8042 (keyboard controller) of any queued keys.

  �  Write 0xdf (Gate A20 + more) to Output port 0x60.

  �  Empty the 8042 (keyboard controller) of any queued keys.

  �  Set bit number 1 (value 0x02: FAST_A20) in the "port 0x92" system
     control register.  This enables A20 on some systems, depending on
     the chipset used in them.

  �  Wait until A20 really *is* enabled; it can take a fair amount of
     time on certain systems.  The memory location used here (0x200) is
     the int 0x80 vector, which should be safe to use.  When A20 is
     disabled, the test memory locations are an alias of each other
     (segment 0:offset 0x200 and segment 0xffff:offset 0x210).  {0xffff0
     + 0x210 = 0x100200, but if A20 is disabled, this becomes 0x000200.}
     We just wait (busy wait/loop) until these memory locations are no
     longer aliased.


  3.1.1.12.  Make sure any possible coprocessor is properly reset



  �  Write 0 to port 0xf0 to clear the Math Coprocessor '-busy' signal.

  �  Write 0 to port 0xf1 to reset the Math Coprocessor.



  3.1.1.13.  Mask all interrupts


  Now we mask all interrupts; the rest is done in init_IRQ().


  �  Mask off all interrupts on the slave PIC: write 0xff to port 0xa1.

  �  Mask off all interrupts on the master PIC except for IRQ2, which is
     the cascaded IRQ input from the slave PIC: write 0xfb to port 0x21.


  3.1.1.14.  Move to Protected Mode


  Now is the time to actually move into protected mode.  To make things
  as simple as possible, we do no register setup or anything, we let the
  GNU-compiled 32-bit programs do that.  We just jump to absolute
  address 0x1000 (or the loader supplied one), in 32-bit protected mode.

  Note that the short jump isn't strictly needed, although there are
  reasons why it might be a good idea.  It won't hurt in any case.

  Set the PE (Protected mode Enable) bit in the MSW and jump to the
  following instruction to flush the instruction fetch queue.

  Clear %bx to indicate that this is the BSP (first CPU only).


  3.1.1.15.  Jump to startup_32 code


  Jump to the 32-bit kernel code (startup_32).

  NOTE:  For high-loaded big kernels we need:


               jmpi    0x100000,__KERNEL_CS



  but we yet haven't reloaded the %cs register, so the default size of
  the target offset still is 16 bit.  However, using an operand prefix
  (0x66), the CPU will properly take our 48-bit far pointer. (INTeL
  80386 Programmer's Reference Manual, Mixing 16-bit and 32-bit code,
  page 16-6).



               .byte 0x66, 0xea              # prefix + jmpi-opcode
       code32: .long 0x1000                  # or 0x100000 for big kernels
               .word __KERNEL_CS



  This jumps to "startup_32" in "linux/arch/i386/kernel/head.S".


  3.2.  Video Setup


  "linux/arch/i386/boot/video.S" is included into
  "linux/arch/i386/boot/setup.S", so they are assembled together.  The
  file separation is a logical module separation even though the two
  modules aren't built separately.

  "video.S" handles Linux/i386 display adapter and video mode setup.
  For more information about Linux/i386 video modes, see
  "linux/Documentation/svga.txt" by Martin Mares [mj@ucw.cz].

  Video mode selection is a kernel build option.  When it is enabled,
  You can select a specific (fixed) video mode to be used during kernel
  booting or you can ask to view a selection menu and then choose a
  video mode from that menu.

  There are a few esoteric (!) "video.S" build options that not covered
  here.  See "linux/Documentation/svga.txt" for all of them.

  CONFIG_VIDEO_SVGA (for automatic detection of SVGA adapters and modes)
  is normally #undefined.  The normal method of video adapter detection
  on Linux/i386 is VESA (CONFIG_VIDEO_VESA, for autodetection of VESA
  modes).

  "video:" is the main entry point called by "setup.S".  The %ds
  register *must* be pointing to the bootsector.  The "video.S" code
  uses different segments from the main "setup.S" code.

  This is a simplified description of the code flow in "video.S".  It
  does not address the CONFIG_VIDEO_LOCAL, CONFIG_VIDEO_400_HACK, and
  CONFIG_VIDEO_GFX_HACK build options and it does not dive deep into
  video BIOS calls or video register accesses.


  3.2.1.  video:



  �  %fs is set to the original %ds value

  �  %ds and %es are set to %cs

  �  %gs is set to zero

  �  Detect the video adapter type and supported modes. (call
     basic_detect)

  �  #ifdef CONFIG_VIDEO_SELECT

  �  If the user wants to see a list of the supported VGA adapter modes,
     list them. (call mode_menu)

  �  Set the selected video mode. (call mode_set)

  �  #ifdef CONFIG_VIDEO_RETAIN

  �  Restore the screen contents. (call restore_screen)

  �  #endif /* CONFIG_VIDEO_RETAIN */

  �  #endif /* CONFIG_VIDEO_SELECT */

  �  Store mode parameters for kernel. (call mode_params)

  �  Restore original DS register value.


  3.2.1.1.  basic_detect:



  �  Detect if we have CGA, MDA, HGA, EGA, or VGA and pass it to the
     kernel.

  �  Check for EGA/VGA using BIOS int. 0x10 calls.  This also tells
     whether the video adapter is CGA/MDA/HGA.

  �  The "adapter" variable is returned as 0 for CGA/MDA/HGA, 1 for EGA,
     and 2 for VGA.


  3.2.1.2.  mode_params:



  �  Store the video mode parameters for later use by the kernel.  This
     is done by asking the BIOS for mode parameters except for the
     rows/columns parameters in the default 80x25 mode -- these are set
     directly, because some very obscure BIOSes supply insane values.

  �  #ifdef CONFIG_VIDEO_SELECT

  �  For graphics mode with a linear frame buffer, goto mopar_gr.

  �  #endif /* CONFIG_VIDEO_SELECT */

  �  For MDA/CGA/HGA/EGA/VGA:

  �  Read and save cursor position.

  �  Read and save video page/mode/width.

  �  For MDA/HGA, change the video_segment to $0xb000.  (Leave it at its
     initial value of $0xb800 for all other adapters.)

  �  Get the Font size (valid only on EGA/VGA).

  �  Save the number of video columns and lines.

  #ifdef CONFIG_VIDEO_SELECT


  3.2.1.3.  mopar_gr:



  �  Get VESA frame buffer parameters.

  �  Get video mem size and protected mode interface information using
     BIOS int. 0x10 calls.


  3.2.1.4.  mode_menu:


  Build the mode list table and display the mode menu.


  3.2.1.5.  mode_set:


  For the selected video mode, use BIOS int. 0x10 calls or register
  writes as needed to set some or all of:

  �  Reset the video mode


  �  Number of scan lines

  �  Font pixel size

  �  Save the screen size in force_size.  "force_size" is used to
     override possibly broken video BIOS interfaces and is used instead
     of the BIOS variables.

  Some video modes require register writes to set:

  �  Location of the cursor scan lines

  �  Vertical sync start

  �  Vertical sync end

  �  Vertical display end

  �  Vertical blank start

  �  Vertical blank end

  �  Vertical total

  �  (Vertical) overflow

  �  Correct sync polarity

  �  Preserve clock select bits and color bit

  {end of mode_set}

  #ifdef CONFIG_VIDEO_RETAIN      /* Normally _IS_ #defined */


  3.2.1.6.  store_screen:


  CONFIG_VIDEO_RETAIN is used to retain screen contents when switching
  modes.  This option stores the screen contents to a temporary memory
  buffer (if there is enough memory) so that they can be restored later.


  �  Save the current number of video lines and columns, cursor
     position, and video mode.

  �  Calculate the image size.

  �  Save the screen image.

  �  Set the "do_restore" flag so that the screen contents will be
     restored at the end of video mode detection/selection.


  3.2.1.7.  restore_screen:


  Restores screen contents from temporary buffer (if already saved).


  �  Get parameters of current mode.

  �  Set cursor position.

  �  Restore the screen contents.

  #endif /* CONFIG_VIDEO_RETAIN */


  3.2.1.8.  mode_table:


  Build the table of video modes at `modelist'.


  �  Store standard modes.

  �  Add modes for standard VGA.

  �  #ifdef CONFIG_VIDEO_LOCAL

  �  Add locally-defined video modes. (call local_modes)

  �  #endif /* CONFIG_VIDEO_LOCAL */

  �  #ifdef CONFIG_VIDEO_VESA

  �  Auto-detect VESA VGA modes. (call vesa_modes)

  �  #endif /* CONFIG_VIDEO_VESA */

  �  #ifdef CONFIG_VIDEO_SVGA

  �  Detect SVGA cards & modes. (call svga_modes)

  �  #endif /* CONFIG_VIDEO_SVGA */

  �  #ifdef CONFIG_VIDEO_COMPACT

  �  Compact the video modes list, removing duplicate entries.

  �  #endif /* CONFIG_VIDEO_COMPACT */


  3.2.1.9.  mode_scan:


  Scans for video modes.


  �  Start with mode 0.

  �  Test the mode.

  �  Test if it's a text mode.

  �  OK, store the mode.

  �  Restore back to mode 3.

  #ifdef CONFIG_VIDEO_SVGA


  3.2.1.10.  svga_modes:


  Try to detect the type of SVGA card and supply (usually approximate)
  video mode table for it.


  �  Test all known SVGA adapters.

  �  Call the test routine for each adapter.

  �  If adapter is found, copy the video modes.

  �  Store pointer to card name.

  #endif  /* CONFIG_VIDEO_SVGA */

  #endif /* CONFIG_VIDEO_SELECT */



  4.  Linux architecture-specific initialization


  (from "linux/arch/i386/kernel/head.S")

  The boot code in "linux/arch/i386/boot/setup.S" transfers execution to
  the beginning code in "linux/arch/i386/kernel/head.S" (labeled
  "startup_32:").

  To get to this point, a small uncompressed kernel function
  decompresses the remaining compressed kernel image and then it jumps
  to the new kernel code.

  This is a description of what the "head.S" code does.


  4.1.  startup_32:


  swapper_pg_dir is the top-level page directory, address 0x00101000.

  On entry, %esi points to the real-mode code as a 32-bit pointer.


  4.2.  Set segment registers to known values


  Set the %ds, %es, %fs, and %gs registers to __KERNEL_DS.


  4.3.  SMP BSP (Bootstrap Processor) check


  #ifdef CONFIG_SMP

  If %bx is zero, this is a boot on the Bootstrap Processor (BSP), so
  skip this.  Otherwise, for an AP (Application Processor):

  If the desired %cr4 setting is non-zero, turn on the paging options
  (PSE, PAE, ...) and skip "Initialize page tables" (jump to "Enable
  paging").

  #endif /* CONFIG_SMP */


  4.4.  Initialize page tables


  Begin at pg0 (page 0) and init all pages to 007 (PRESENT + RW + USER).


  4.5.  Enable paging


  Set %cr3 (page table pointer) to swapper_pg_dir.

  Set the paging ("PG") bit of %cr0 to
  ********** enable paging **********.

  Jump $ to flush the prefetch queue.

  Jump *[$] to make sure that %eip is relocated.

  Setup the stack pointer (lss stack_start, %esp).


  #ifdef CONFIG_SMP

  If this is not the BSP (Bootstrap Processor), clear all flags bits and
  jump to checkCPUtype.

  #endif /* CONFIG_SMP */


  4.6.  Clear BSS


  The BSP clears all of BSS (area between __bss_start and _end) for the
  kernel.


  4.7.  32-bit setup


  Setup the IDT for 32-bit mode (call setup_idt).  setup_idt sets up an
  IDT with 256 entries pointing to the default interrupt handler
  "ignore_int" as interrupt gates.  It doesn't actually load the IDT;
  that can be done only after paging has been enabled and the kernel
  moved to PAGE_OFFSET.  Interrupts are enabled elsewhere, when we can
  be relatively sure everything is OK.

  Clear the eflags register (before switching to protected mode).


  4.8.  Copy boot parameters and command line out of the way


  First 2 KB of _empty_zero_page is for boot parameters, second 2 KB is
  for the command line.


  4.9.  checkCPUtype


  Initialize X86_CPUID to -1.

  Use Flags register, push/pop results, and CPUID instruction(s) to
  determine CPU type and vendor: Sets X86, X86_CPUID, X86_MODEL,
  X86_MASK, and X86_CAPABILITY.  Sets bits in %cr0 accordingly.

  Also checks for presence of an 80287 or 80387 coprocessor.  Sets
  X86_HARD_MATH if a math coprocessor or floating point unit is found.


  4.10.  Count this processor


  For CONFIG_SMP builds, increment the "ready" counter to keep a tally
  of the number of CPUs that have been initialized.


  4.11.  Load descriptor table pointer registers


  Load GDT with gdt_descr and IDT with idt_descr.  The GDT contains 2
  entries for the kernel (4 GB each for code and data, beginning at 0)
  and 2 userspace entries (4 GB each for code and data, beginning at 0).
  There are 2 null descriptors between the userspace descriptors and the
  APM descriptors.

  The GDT also contains 4 entries for APM segments.  The APM segments
  have byte granularity and their bases and limits are set at runtime.
  The rest of the gdt_table (after the APM segments) is space for TSSes
  and LDTs.

  Jump to __KERNEL_CS:%eip to cause the GDT to be used.  Now in
  ********** protected mode **********.

  Reload all of the segment registers: Set the %ds, %es, %fs, and %gs
  registers to __KERNEL_DS.

  #ifdef CONFIG_SMP

  Reload the stack pointer segment only (%ss) with __KERNEL_DS.

  #else /* not CONFIG_SMP */

  Reload the stack pointer (%ss:%esp) with stack_start.

  #endif /* CONFIG_SMP */

  Clear the LDT pointer to 0.

  Clear the processor's Direction Flag (DF) to 0 for gcc.


  4.12.  Start other processors


  For CONFIG_SMP builds, if this is not the first (Bootstrap) CPU, call
  initialize_secondary(), which does not return.  The secondary (AP)
  processor(s) are initialized and then enter idle state until processes
  are scheduled on them.

  If this is the first or only CPU, call start_kernel(). (see below)

  /* the calls above should never return, but in case they do: */

  L6:  jmp L6



  5.  Linux architecture-independent initialization


  (from "linux/init/main.c")

  "linux/init/main.c" begins execution with the start_kernel() function,
  which is called from "linux/arch/i386/kernel/head.S".  start_kernel()
  never returns to its caller.  It ends by calling the cpu_idle()
  function.


  5.1.  start_kernel:


  Interrupts are still disabled.  Do necessary setups, then enable them.

  Lock the kernel (BKL: big kernel lock).

  Print the linux_banner string (this string resides in
  "linux/init/version.c") using printk().  NOTE:  printk() doesn't
  actually print this to the console yet; it just buffers the string
  until a console device registers itself with the kernel, then the
  kernel passes the buffered console log contents to the registered
  console device(s).  There can be multiple registered console devices.

  ********** printk() can be called very early because it doesn't
  actually print to anywhere.  It just logs the message to "log_buf",
  which is allocated statically in "linux/kernel/printk.c".  The
  messages that are saved in "log_buf" are passed to registered console
  devices as they register. **********


  5.1.1.  More architecture-specific init


  Call setup_arch(&command_line):

  This performs architecture-specific initializations (details below).
  Then back to architecture-independent initialization....

  The remainder of start_kernel() is done as follows for all processor
  architecures, although several of these function calls are to
  architecture-specific setup/init functions.


  5.1.2.  Continue architecture-independent init

  Print the kernel command line.


  5.1.3.  Parsing command line options


  parse_options(command_line): Parse the kernel options on the command
  line.  This is a simple kernel command line parsing function.  It
  parses the command line and fills in the arguments and environment to
  init (thread) as appropriate.  Any command-line option is taken to be
  an environment variable if it contains the character '='.  It also
  checks for options meant for the kernel by calling checksetup(), which
  checks the command line for kernel parameters, these being specified
  by declaring them using "__setup", as in:



  ______________________________________________________________________
  __setup("debug", debug_kernel);
  ______________________________________________________________________



  This declaration causes the debug_kernel() function to be called when
  the string "debug" is scanned.  See "linux/Documentation/kernel-
  parameters.txt" for the list of kernel parameters.

  These options are not given to init -- they are for internal kernel
  use only.  The default argument list for the init thread is {"init",
  NULL}, with a maximum of 8 command-line arguments.  The default
  environment list for the init thread is {"HOME=/", "TERM=linux",
  NULL}, with a maximum of 8 command-line environment variable settings.
  In case LILO is going to boot us with default command line, it
  prepends "auto" before the whole cmdline which makes the shell think
  it should execute a script with such name.  So we ignore all arguments
  entered _before_ init=... [MJ]


  5.1.4.  trap_init


  (in linux/arch/i386/kernel/traps.c)

  Install exception handlers for basic processor exceptions, i.e., not
  hardware device interrupt handlers.

  Install the handler for the system call software interrupt.

  Install handlers for lcall7 (for iBCS) and lcall27 (for Solaris/x86
  binaries).

  Call cpu_init() to do:

  �  initialize per-CPU state

  �  reload the GDT and IDT

  �  mask off the eflags NT (Nested Task) bit

  �  set up and load the per-CPU TSS and LDT

  �  clear 6 debug registers (0, 1, 2, 3, 6, and 7)

  �  stts(): set the 0x08 bit (TS: Task Switched) in CR0 to enable lazy
     register saves on context switches


  5.1.5.  init_IRQ


  (in linux/arch/i386/kernel/i8259.c)

  Call init_ISA_irqs() to initialize the two 8259A interrupt controllers
  and install default interrupt handlers for the ISA IRQs.

  Set an interrupt gate for all unused interrupt vectors.

  For CONFIG_SMP configurations, set up IRQ 0 early, since it's used
  before the IO APIC is set up.

  For CONFIG_SMP, install the interrupt handler for CPU-to-CPU IPIs that
  are used for the "reschedule helper."
  For CONFIG_SMP, install the interrupt handler for the IPI that is used
  to invalidate TLBs.

  For CONFIG_SMP, install the interrupt handler for the IPI that is used
  for generic function calls.

  For CONFIG_X86_LOCAL_APIC configurations, install the interrupt
  handler for the self-generated local APIC timer IPI.

  For CONFIG_X86_LOCAL_APIC configurations, install interrupt handlers
  for spurious and error interrupts.

  Set the system's clock chip to generate a timer tick interrupt every
  HZ Hz.

  If the system has an external FPU, set up IRQ 13 to handle floating
  point exceptions.


  5.1.6.  sched_init


  (in linux/kernel/sched.c)


  �  Set the init_task's processor ID.

  �  Clear the pidhash table. TBD: Why? isn't it in BSS?

  �  call init_timervecs()

  �  call init_bh() to init "bottom half" queues for timer_bh,
     tqueue_bh, and immediate_bh.


  5.1.7.  time_init


  (in linux/arch/i386/kernel/time.c)

  Initialize the system's current time of day (xtime) from CMOS.

  Install the irq0 timer tick interrupt handler.


  5.1.8.  softirq_init


  (in linux/kernel/softirq.c)


  5.1.9.  console_init


  (in linux/drivers/char/tty_io.c)

  HACK ALERT! This is early. We're enabling the console before we've
  done PCI setups etc., and console_init() must be aware of this.  But
  we do want output early, in case something goes wrong.


  5.1.10.  init_modules


  (in linux/kernel/module.c)

  For CONFIG_MODULES configurations, call init_modules().  This
  initializes the size (or number of symbols) of the kernel symbol
  table.


  5.1.11.  Profiling setup


  if profiling ("profile=#" on the kernel command line): calculate the
  kernel text (code) profile "segment" size; calculate the profile
  buffer size in pages (round up); allocate the profile buffer:
  prof_buffer = alloc_bootmem(size);


  5.1.12.  kmem_cache_init


  (in linux/mm/slab.c)


  5.1.13.  sti


  ********** Interrupts are now enabled. **********
  This allows "calibrate_delay()" (below) to work.


  5.1.14.  calibrate_delay


  Calculate the "loops_per_jiffy" delay loop value and print it in
  BogoMIPS.


  5.1.15.  INITRD setup



       #ifdef CONFIG_BLK_DEV_INITRD

               if (initrd_start && !initrd_below_start_ok &&
                               initrd_start < (min_low_pfn << PAGE_SHIFT)) {
                       printk("initrd overwritten (initrd_start < (min_low_pfn << PAGE_SHIFT)) - disabling it.\n");
                       initrd_start = 0;       // mark initrd as disabled
               }

       #endif /* CONFIG_BLK_DEV_INITRD */



  5.1.16.  mem_init


  (in linux/arch/i386/mm/init.c)


  �  Clear the empty_zero_page.

  �  Call free_all_bootmem() and add that released memory to
     totalram_pages.

  �  Count the number of reserved RAM pages.

  �  Print the system memory sizes (free/total), kernel code size,
     reserved memory size, kernel data size, kernel "init" size, and the
     highmem size.

  �  For CONFIG_SMP, call zap_low_mappings().

  ********** get_free_pages() can be used after mem_init(). **********


  5.1.17.  kmem_cache_sizes_init


  (in linux/mm/slab.c)

  Set up remaining internal and general caches.  Called after the
  "get_free_page()" functions have been enabled and before smp_init().

  ********** kmalloc() can be used after kmem_cache_sizes_init().
  **********


  5.1.18.  proc_root_init


  (in linux/fs/proc/root.c)

  For CONFIG_PROC_FS configurations:

  �  call proc_misc_init()

  �  mkdir /proc/net

  �  for CONFIG_SYSVIPC, mkdir /proc/sysvipc

  �  for CONFIG_SYSCTL, mkdir /proc/sys

  �  mkdir /proc/fs

  �  mkdir /proc/driver

  �  call proc_tty_init()

  �  mkdir /proc/bus


  5.1.19.  mempages = num_physpages;



  5.1.20.  fork_init(mempages)


  (in linux/kernel/fork.c)

  The default maximum number of threads is set to a safe value: the
  thread structures can take up at most half of memory.


  5.1.21.  proc_caches_init()


  (in linux/kernel/fork.c)

  Call kmem_cache_create() to create slab caches for signal_act (signal
  action), files_cache (files_struct), fs_cache (fs_struct),
  vm_area_struct, and mm_struct.
  5.1.22.  vfs_caches_init(mempages)


  (in linux/fs/dcache.c)

  Call kmem_cache_create() to create slab caches for buffer_head,
  names_cache, filp, and for CONFIG_QUOTA, dquot.

  Call dcache_init() to create the dentry_cache and dentry_hashtable.


  5.1.23.  buffer_init(mempages)


  (in linux/fs/buffer.c)

  Allocate the buffer cache hash table and init the free list.
  Use get_free_pages() for the hash table to decrease TLB misses; use
  SLAB cache for buffer heads.
  Setup the hash chains, free lists, and LRU lists.


  5.1.24.  page_cache_init(mempages)


  (in linux/mm/filemap.c)

  Allocate and clear the page-cache hash table.


  5.1.25.  kiobuf_setup()


  (in linux/fs/iobuf.c)

  Call kmem_cache_create() to create the kernel iobuf cache.


  5.1.26.  signals_init()


  (in linux/kernel/signal.c)

  Call kmem_cache_create() to create the "sigqueue" SLAB cache.


  5.1.27.  bdev_init()


  (in linux/fs/block_dev.c)

  Initialize the bdev_hashtable list heads.

  Call kmem_cache_create() to create the "bdev_cache" SLAB cache.


  5.1.28.  inode_init(mempages)


  (in linux/fs/inode.c)


  �  Allocate memory for the inode_hashtable.

  �  Intialize the inode_hashtable list heads.

  �  Call kmem_cache_create() to create the inode SLAB cache.


  5.1.29.  ipc_init()


  (in linux/ipc/util.c)

  For CONFIG_SYSVIPC configurations, call ipc_init().

  The various System V IPC resources (semaphores, messages, and shared
  memory) are initialized.


  5.1.30.  dquot_init_hash()


  (in linux/fs/dquot.c)

  For CONFIG_QUOTA configurations, call dquot_init_hash().


  �  Clear dquot_hash.  TBD: Why? Is it in BSS? Yes.

  �  Clear dqstats.     TBD: Why? Is it in BSS? Yes.


  5.1.31.  check_bugs()


  (in linux/include/asm-i386/bugs.h)


  �  identify_cpu()

  �  For non-CONFIG_SMP configurations, print_cpu_info()

  �  check_config()

  �  check_fpu()

  �  check_hlt()

  �  check_popad()

  �  Update system_utsname.machine{byte 1} with boot_cpu_data.x86


  5.1.32.  Start other SMP processors (as applicable)


  smp_init() works in one of three ways, depending upon the kernel
  configuration.

  For a uniprocessor (UP) system without an IO APIC (CONFIG_X86_IO_APIC
  is not defined), smp_init() is empty -- it has nothing to do.

  For a UP system with (an) IO APIC for interrupt routing, it calls
  IO_APIC_init_uniprocessor().

  For an SMP system, its main job is to call the architecture-specific
  function "smp_boot_cpus()", which does the following.


  �  For CONFIG_MTRR kernels, calls mtrr_init_boot_cpu(), which must be
     done before the other processors are booted.
  �  Stores and prints the BSP CPU information.

  �  Saves the BSP APIC ID and BSP logical CPU ID (latter is 0).

  �  If an MP BIOS interrupt routing table was not found, revert to
     using only one CPU and exit.

  �  Verify existence of a local APIC for the BSP.

  �  If the "maxcpus" boot option was used to limit the number of CPUs
     actually used to 1 (not SMP), then ignore the MP BIOS interrupt
     routing table.

  �  Switch the system from PIC mode to symmetric I/O interrupt mode.

  �  Setup the BSP's local APIC.

  �  Use the CPU present map to boot the APs serially.  Wait for each AP
     to finish booting before starting the next one.

  �  If using (an) IO APIC {which is True unless the "noapic" boot
     option was used}, setup the IO APIC(s).


  5.1.33.  Start init thread


  We count on the initial thread going OK.

  Like idlers, init is an unlocked kernel thread, which will make
  syscalls (and thus be locked).



       kernel_thread(init, NULL, CLONE_FS | CLONE_FILES | CLONE_SIGNAL);



  {details below}


  5.1.34.  unlock_kernel()


  Release the BKL.


  5.1.35.  current->need_resched = 1;



  5.1.36.  cpu_idle()


  This function remains as process number 0.  Its purpose is to use up
  idle CPU cycles.  If the kernel is configured for APM support or ACPI
  support, cpu_idle() invokes the supported power-saving features of
  these specifications.  Otherwise it nominally executes a "hlt"
  instruction.

  {end of start_kernel()}



  5.2.  setup_arch


  (in "linux/arch/i386/kernel/setup.c")


  5.2.1.  Copy and convert system parameter data


  Copy and convert parameter data passed from 16-bit real mode to the
  32-bit startup code.


  5.2.2.  For RAMdisk-enabled configs (CONFIG_BLK_DEV_RAM)


  Initialize rd_image_start, rd_prompt, and rd_doload from the real-mode
  parameter data.


  5.2.3.  setup_memory_region


  Use the BIOS-supplied memory map to setup memory regions.


  5.2.4.  Set memory limits


  Set values for the start of kernel code, end of kernel code, end of
  kernel data, and "_end" (end of kernel code = the "brk" address).

  Set values for code_resource start and end and data_resource start and
  end.


  5.2.5.  parse_mem_cmdline


  Parse any "mem=" parameters on the kernel command line and remember
  them.


  5.2.6.  Setup Page Frames


  Use the BIOS-supplied memory map to setup page frames.

  Register available low RAM pages with the bootmem allocator.

  Reserve physical page 0: "it's a special BIOS page on many boxes,
  enabling clean reboots, SMP operation, laptop functions."


  5.2.7.  Handle SMP and IO APIC Configurations


  For CONFIG_SMP, reserve the page immediately above page 0 for stack
  and trampoline usage, then call smp_alloc_memory() to allocate low
  memory for AP processor(s) real mode trampoline code.

  For CONFIG_X86_IO_APIC configurations, call find_smp_config() to find
  and reserve any boot-time SMP configuration information memory, such
  as MP (Multi Processor) table data from the BIOS.


  5.2.8.  paging_init()


  paging_init() sets up the page tables - note that the first 8 MB are
  already mapped by head.S.

  This routine also unmaps the page at virtual kernel address 0, so that
  we can trap those pesky NULL-reference errors in the kernel.


  5.2.9.  Save the boot-time SMP configuration


  For CONFIG_X86_IO_APIC configurations, call get_smp_config() to read
  and save the MP table IO APIC interrupt routing configuration data.

  For CONFIG_X86_LOCAL_APIC configurations, call init_apic_mappings().


  5.2.10.  Reserve INITRD memory


  For CONFIG_BLK_DEV_INITRD configurations, if there is enough memory
  for the initial RamDisk, call reserve_bootmem() to reserve RAM for the
  initial RamDisk.


  5.2.11.  Scan for option ROMs


  Call probe_roms() and reserve their memory space resource(s) if found
  and valid.  This is done for the standard video BIOS ROM image, any
  option ROMs found, and for the system board extension ROM (space).


  5.2.12.  Reserve system resources


  Call request_resource() to reserve video RAM memory.

  Call request_resource() to reserve all standard PC I/O system board
  resources.

  {end of setup_arch()}


  5.3.  init thread


  The init thread begins at the init() function in "linux/init/main.c".
  This is always expected to be process number 1.

  init() first locks the kernel and then calls do_basic_setup() to
  perform lots of bus and/or device initialization {more detail below}.
  After do_basic_setup(), most kernel initialization has been completed.
  init() then frees any memory that was specified as being for
  initialization only [marked with "__init", "__initdata",
  "__init_call", or "__initsetup"] and unlocks the kernel (BKL).

  init() next opens /dev/console and duplicates that file descriptor two
  times to create stdin, stdout, and stderr files for init and all of
  its children.

  Finally init() tries to execute the command specified on the kernel
  parameters command line if there was one, or an init program or script
  if it can find one in {/sbin/init, /etc/init, /bin/init}, and lastly
  /bin/sh.  If init() cannot execute any of these, it panics ("No init
  found.  Try passing init= option to kernel.")


  5.4.  do_basic_setup {part of the init thread}


  The machine is now initialized.  None of the devices have been touched
  yet, but the CPU subsystem is up and running, and memory and process
  management works.


  5.4.1.  Be the reaper of orphaned children


  The init process handles all orphaned tasks.


  5.4.2.  MTRRs


  // SMP init is completed before this.
  For CONFIG_MTRR, call mtrr_init() [in linux/arch/i386/kernel/mtrr.c].


  5.4.3.  SYSCTLs


  For CONFIG_SYSCTL configurations, call sysctl_init() [in
  linux/kernel/sysctl.c].


  5.4.4.  Init Many Devices



       /*
        * Ok, at this point all CPU's should be initialized, so
        * we can start looking into devices..
        */



  5.4.5.  PCI


  For CONFIG_PCI configurations, call pci_init() [in
  linux/drivers/pci/pci.c].


  5.4.6.  Micro Channel


  For CONFIG_MCA configurations, call mca_init() [in
  linux/arch/i386/kernel/mca.c].


  5.4.7.  ISA PnP


  For CONFIG_ISAPNP configurations, call isapnp_init() [in
  linux/drivers/pnp/isapnp.c].

  5.4.8.  Networking Init



               /* Networking initialization needs a process context */
               sock_init();



  [in linux/net/socket.c]


  5.4.9.  Initial RamDisk



       #ifdef CONFIG_BLK_DEV_INITRD

               real_root_dev = ROOT_DEV;
               real_root_mountflags = root_mountflags;
               if (initrd_start && mount_initrd)
                       root_mountflags &= ~MS_RDONLY;  // change to read/write
               else
                       mount_initrd =0;

       #endif /* CONFIG_BLK_DEV_INITRD */



  5.4.10.  Start the kernel "context" thread (keventd)


  [in linux/kernel/context.c]


  5.4.11.  Initcalls


  Call all functions marked as "__initcall":


               do_initcalls();



  [in linux/init/main.c]

  This initializes many functions and some subsystems --- in no specific
  or guaranteed order unless fixed in their Makefiles --- if they were
  built into the kernel, such as:


  �  APM: apm_init() {in linux/arch/i386/kernel/apm.c}

  �  cpuid: cpuid_init() {in linux/arch/i386/kernel/cpuid.c}

  �  DMI: dmi_scan_machine() {in linux/arch/i386/kernel/dmi_scan.c}

  �  microcode: microcode_init() {in linux/arch/i386/kernel/microcode.c}

  �  MSR: msr_init() {in linux/arch/i386/kernel/msr.c}

  �  partitions: partition_setup() {in linux/fs/partitions/check.s}

  �  file systems, pipes, buffer and cache management, various binary
     format loaders, NLS character sets: too numerous to list {in
     linux/fs/*}

  �  user cache (for limits): uid_cache_init() {in linux/kernel/user.c}

  �  kmem_cpu_cache: kmem_cpucache_init() {in linux/mm/slab.c}

  �  shmem: init_shmem_fs() {in linux/mm/shmem.c}

  �  kswapd: kswapd_init() {in linux/mm/vmscan.c}

  �  networking, TCP/IP, IPv6, sockets, 802.2, SNAP, LLC, X.25, AX.25,
     IPX, kHTTPd, ATM LAN emulation (LANE), IP chains/forwarding,
     NAT/masquerading, packet matching/filtering/logging, firewalling,
     DECnet, bridging, and other networking protocols too numerous to
     list {in linux/net/*}

  �  drivers, some of which are not exactly device drivers, but help out
     with bus/device enumeration and initialization, such as:

  �  ACPI: acpi_init() {in linux/drivers/acpi/*}

  �  PCI: pci_proc_init() {in linux/drivers/pci/*}

  �  PCMCIA controllers {in linux/drivers/pcmcia/*}

  �  and...

  �  atm drivers {in linux/drivers/atm/*}

  �  block drivers {in linux/drivers/block/*}

  �  CD-ROM drivers {in linux/drivers/cdrom/*}

  �  character drivers {in linux/drivers/char/*}

  �  I2O drivers {in linux/drivers/i2o/*}

  �  IDE drivers {in linux/drivers/ide/*}

  �  input drivers (keyboard/mouse/joystick) {in linux/drivers/input/*}

  �  ISDN drivers {in linux/drivers/isdn/*}

  �  md, LVM, and RAID drivers {in linux/drivers/md/*}

  �  radio drivers {in linux/drivers/media/radio/*}

  �  video drivers {in linux/drivers/media/video/*}

  �  MTD drivers {in linux/drivers/mtd/*}

  �  network drivers, including PLIP, PPP, dummy, Ethernet, bonding,
     Arcnet, hamradio, PCMCIA, Token Ring, and WAN

  �  SCSI logical and physical drivers {in linux/drivers/scsi/*}

  �  sound drivers {in linux/drivers/sound/*}

  �  telephony drivers {in linux/drivers/telephony/*}

  �  USB host controllers and device drivers {in linux/drivers/usb/*}

  �  video frame buffer drivers {in linux/drivers/video/*}


  5.4.12.  Filesystems


  Call filesystem_setup():

  �  init_devfs_fs();  /*  Header file may make this empty  */

  �  For CONFIG_NFS_FS configurations, call init_nfs_fs().

  �  For CONFIG_DEVPTS_FS configurations, call init_devpts_fs().

     [in linux/fs/filesystems.c]


  5.4.13.  IRDA


  For CONFIG_IRDA configurations, call irda_device_init().
  /* Must be done after protocol initialization */
  [in linux/net/irda/irda_device.c]


  5.4.14.  PCMCIA


  /* Do this last */
  For CONFIG_PCMCIA configurations, call init_pcmcia_ds().
  [in linux/drivers/pcmcia/ds.c]


  5.4.15.  Mount the root filesystem



               mount_root();



  [in linux/fs/super.c]


  5.4.16.  Mount the dev (device) filesystem



               mount_devfs_fs ();



  [in linux/fs/devfs/base.c]


  5.4.17.  Switch to the Initial RamDisk



  #ifdef CONFIG_BLK_DEV_INITRD

          if (mount_initrd && MAJOR(ROOT_DEV) == RAMDISK_MAJOR && MINOR(ROOT_DEV) == 0) {
                  // Start the linuxrc thread.
                  pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);
                  if (pid > 0)
                          while (pid != wait(&i));
                  if (MAJOR(real_root_dev) != RAMDISK_MAJOR
                       || MINOR(real_root_dev) != 0) {
                          error = change_root(real_root_dev,"/initrd");
                          if (error)
                                  printk(KERN_ERR "Change root to /initrd: "
                                      "error %d\n",error);
                  }
          }

  #endif /* CONFIG_BLK_DEV_INITRD */



  See "linux/Documentation/initrd.txt" for more information on initial
  RAM disks.

  {end of do_basic_setup()}



  6.  Glossary


  AP:  Application Processor, any x86 processor other than the Bootstrap
  Processor on IA-32 SMP systems

  ACPI:  Advanced Configuration and Power Interface

  APIC:  Advanced Programmable Interrupt Controller

  APM:  Advanced Power Management, a BIOS-managed power management
  specification for personal computers

  BSP:  Bootstrap Processor, the primary booting processor on IA-32 SMP
  systems

  BSS:  Block Started by Symbol: the uninitialized data segment

  BKL:  Big Kernel Lock, the Linux global kernel lock

  CRn:  Control Register n, i386-specific control registers

  FPU:  Floating Point Unit, a separate math coprocessor device

  GB:  gigabyte (1024 * 1024 * 1024 bytes)

  GDT:  Global Descriptor Table, an i386 memory management table

  IA:  Intel Architecture (also i386, x86)

  IDT:  Interrupt Descriptor Table, an i386-specific table that contains
  information used in handling interrupts

  initrd:  initial RAM disk (see "linux/Documentation/initrd.txt")

  IPC:  Inter-Process Communication

  IPI:  Inter-processor Interrupt, a method of signaling interrupts
  between multiple processors on an SMP system

  IRDA:  InfraRed Data Association

  IRQ:  Interrupt ReQuest

  ISA:  Industry Standard Architecture

  KB:  kilobyte (1024 bytes)

  LDT:  Local Descriptor Table, an i386-specific memory management table
  that is used to describe memory for each non-kernel process

  MB:  megabyte (1024 * 1024 bytes)

  MCA:  Micro Channel Architecture, used in IBM PS/2 computers

  MP:  Multi-processor

  MSW:  Machine Status Word

  MTRR:  Memory Type Range Registers

  PAE:  Physical Address Extension: extends the address space to 64 GB
  instead of 4 GB

  PCI:  Peripheral Component Interconnect, an industry standard for
  connecting devices on a local bus in a computer system
  PCMCIA:  Personal Computer Memory Card International Association;
  defines standards for PCMCIA cards and CardBus PC Cards

  PIC:  Programmable Interrupt Controller

  PNP:  Plug aNd Play

  PSE:  Page Size Extension: allows 4 MB pages

  SMP:  Symmetric Multi Processor/Processing

  TLB:  Translation Lookaside Buffer, i386-specific processor cache of
  recent page directory and page table entries

  TSS:  Task State Segment, an i386-specific task data structure

  UP:  Uniprocessor (single CPU) system.



  7.  References



  1. Tigran Aivazian, "Linux Kernel Internals" (URL:
     http://www.moses.uklinux.net/patches/lki.html)

  2. Werner Almesberger, x86 Booting.  (URL:
     ftp://icaftp.epfl.ch/pub/people/almesber/booting/)

  3. Werner Almesberger, "LILO Generic boot loader for Linux: Technical
     overview."  December 4, 1998.  Included in LILO distribution.

  4. Werner Almesberger and Hans Lermen, Using the initial RAM disk
     (initrd).  (file: linux/Documentation/initrd.txt)

  5. H. Peter Anvin, "The Linux/I386 Boot Protocol (file:
     linux/Documentation/i386/boot.txt)

  6. Michael Beck et al, "Linux Kernel Internals," second edition.
     Addison-Wesley, 1998.

  7. Ralf Brown's Interrupt List, URL: http://www.ctyme.com/intr/int.htm
     {browsable}

  8. Ralf Brown's Interrupt List, URL:
     http://www.delorie.com/djgpp/doc/rbinter/ix/ {browsable}

  9. Ralf Brown's Interrupt List, URL:
     http://www.cs.cmu.edu/ ralf/files.html {zipped, not browsable}

  10.
     E820 memory sizing method: URL:
     http://www.teleport.com/ acpi/acpihtml/topic245.htm

  11.
     IBM Personal Computer AT Technical Reference.  1985.

  12.
     IBM Personal System/2(r) and Personal Computer BIOS Interface
     Technical Reference, second edition.  1988.

  13.
     Hans Lermen and Martin Mares, "Summary of empty_zero_page layout."
     (file: linux/Documentation/i386/zero-page.txt)

  14.
     linux/Documentation directory files

  15.
     Martin Mares, "Video Mode Selection Support." (file:
     linux/Documentation/svga.txt)

  16.
     Scott Maxwell, "Linux Core Kernel Commentary."  Coriolis Press,
     1999.

  17.
     Mindshare, Inc., Tom Shanley, "Pentium(r) Pro and Pentium(r) II
     System Architecture," second edition.  Addison-Wesley, 1998.

  18.
     Allesandro Rubini, "Linux Device Drivers."  O'Reilly and
     Associates, 1998.


  19.
     URL: ftp://linux01.gwdg.de/pub/cLIeNUX/interim/Janet_Reno.tgz

  20.
     URL: http://www.eecs.wsu.edu/~cs640/ (was dead at last check)

  21.
     URL: http://www.linuxbios.org + "Papers"



                            Table of Contents


  1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .   5
  1.1. Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
  1.2. This document . . . . . . . . . . . . . . . . . . . . . . . .   5
  1.3. Contributions . . . . . . . . . . . . . . . . . . . . . . . .   6
  1.4. Trademarks  . . . . . . . . . . . . . . . . . . . . . . . . .   6
  1.5. License . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
  2. Linux init ("ASCII art")  . . . . . . . . . . . . . . . . . . .   7
  3. Linux early setup . . . . . . . . . . . . . . . . . . . . . . .  10
  3.1. IA-32 Kernel Setup  . . . . . . . . . . . . . . . . . . . . .  10
  3.1.1. start_of_setup: . . . . . . . . . . . . . . . . . . . . . .  11
  3.1.1.1. Read second hard drive DASD type  . . . . . . . . . . . .  11
  3.1.1.2. Check that LILO loaded us right . . . . . . . . . . . . .  11
  3.1.1.3. Check old loader trying to load a big kernel  . . . . . .  12
  3.1.1.4. Determine system memory size  . . . . . . . . . . . . . .  12
  3.1.1.5. Video adapter modes . . . . . . . . . . . . . . . . . . .  12
  3.1.1.6. Get Hard Disk parameters  . . . . . . . . . . . . . . . .  13
  3.1.1.7. Get Micro Channel bus information . . . . . . . . . . . .  13
  3.1.1.8. Check for mouse . . . . . . . . . . . . . . . . . . . . .  13
  3.1.1.9. Check for APM BIOS support  . . . . . . . . . . . . . . .  13
  3.1.1.10. Prepare to move to protected mode  . . . . . . . . . . .  14
  3.1.1.11. Enable address line A20  . . . . . . . . . . . . . . . .  14
  3.1.1.12. Make sure any possible coprocessor is properly reset
   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
  3.1.1.13. Mask all interrupts  . . . . . . . . . . . . . . . . . .  15
  3.1.1.14. Move to Protected Mode . . . . . . . . . . . . . . . . .  15
  3.1.1.15. Jump to startup_32 code  . . . . . . . . . . . . . . . .  15
  3.2. Video Setup . . . . . . . . . . . . . . . . . . . . . . . . .  15
  3.2.1. video:  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
  3.2.1.1. basic_detect: . . . . . . . . . . . . . . . . . . . . . .  16
  3.2.1.2. mode_params:  . . . . . . . . . . . . . . . . . . . . . .  17
  3.2.1.3. mopar_gr: . . . . . . . . . . . . . . . . . . . . . . . .  17
  3.2.1.4. mode_menu:  . . . . . . . . . . . . . . . . . . . . . . .  17
  3.2.1.5. mode_set: . . . . . . . . . . . . . . . . . . . . . . . .  17
  3.2.1.6. store_screen: . . . . . . . . . . . . . . . . . . . . . .  18
  3.2.1.7. restore_screen: . . . . . . . . . . . . . . . . . . . . .  18
  3.2.1.8. mode_table: . . . . . . . . . . . . . . . . . . . . . . .  19
  3.2.1.9. mode_scan:  . . . . . . . . . . . . . . . . . . . . . . .  19
  3.2.1.10. svga_modes:  . . . . . . . . . . . . . . . . . . . . . .  19
  4. Linux architecture-specific initialization  . . . . . . . . . .  21
  4.1. startup_32: . . . . . . . . . . . . . . . . . . . . . . . . .  21
  4.2. Set segment registers to known values . . . . . . . . . . . .  21
  4.3. SMP BSP (Bootstrap Processor) check . . . . . . . . . . . . .  21
  4.4. Initialize page tables  . . . . . . . . . . . . . . . . . . .  21
  4.5. Enable paging . . . . . . . . . . . . . . . . . . . . . . . .  21
  4.6. Clear BSS . . . . . . . . . . . . . . . . . . . . . . . . . .  22
  4.7. 32-bit setup  . . . . . . . . . . . . . . . . . . . . . . . .  22
  4.8. Copy boot parameters and command line out of the way  . . . .  22
  4.9. checkCPUtype  . . . . . . . . . . . . . . . . . . . . . . . .  22
  4.10. Count this processor . . . . . . . . . . . . . . . . . . . .  22
  4.11. Load descriptor table pointer registers  . . . . . . . . . .  22
  4.12. Start other processors . . . . . . . . . . . . . . . . . . .  23
  5. Linux architecture-independent initialization . . . . . . . . .  24
  5.1. start_kernel: . . . . . . . . . . . . . . . . . . . . . . . .  24
  5.1.1. More architecture-specific init . . . . . . . . . . . . . .  24
  5.1.2. Continue architecture-independent init  . . . . . . . . . .  24
  5.1.3. Parsing command line options  . . . . . . . . . . . . . . .  24
  5.1.4. trap_init . . . . . . . . . . . . . . . . . . . . . . . . .  25
  5.1.5. init_IRQ  . . . . . . . . . . . . . . . . . . . . . . . . .  25
  5.1.6. sched_init  . . . . . . . . . . . . . . . . . . . . . . . .  26
  5.1.7. time_init . . . . . . . . . . . . . . . . . . . . . . . . .  26
  5.1.8. softirq_init  . . . . . . . . . . . . . . . . . . . . . . .  26
  5.1.9. console_init  . . . . . . . . . . . . . . . . . . . . . . .  26
  5.1.10. init_modules . . . . . . . . . . . . . . . . . . . . . . .  26
  5.1.11. Profiling setup  . . . . . . . . . . . . . . . . . . . . .  27
  5.1.12. kmem_cache_init  . . . . . . . . . . . . . . . . . . . . .  27
  5.1.13. sti  . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
  5.1.14. calibrate_delay  . . . . . . . . . . . . . . . . . . . . .  27
  5.1.15. INITRD setup . . . . . . . . . . . . . . . . . . . . . . .  27
  5.1.16. mem_init . . . . . . . . . . . . . . . . . . . . . . . . .  27
  5.1.17. kmem_cache_sizes_init  . . . . . . . . . . . . . . . . . .  28
  5.1.18. proc_root_init . . . . . . . . . . . . . . . . . . . . . .  28
  5.1.19. mempages = num_physpages;  . . . . . . . . . . . . . . . .  28
  5.1.20. fork_init(mempages)  . . . . . . . . . . . . . . . . . . .  28
  5.1.21. proc_caches_init() . . . . . . . . . . . . . . . . . . . .  28
  5.1.22. vfs_caches_init(mempages)  . . . . . . . . . . . . . . . .  29
  5.1.23. buffer_init(mempages)  . . . . . . . . . . . . . . . . . .  29
  5.1.24. page_cache_init(mempages)  . . . . . . . . . . . . . . . .  29
  5.1.25. kiobuf_setup() . . . . . . . . . . . . . . . . . . . . . .  29
  5.1.26. signals_init() . . . . . . . . . . . . . . . . . . . . . .  29
  5.1.27. bdev_init()  . . . . . . . . . . . . . . . . . . . . . . .  29
  5.1.28. inode_init(mempages) . . . . . . . . . . . . . . . . . . .  29
  5.1.29. ipc_init() . . . . . . . . . . . . . . . . . . . . . . . .  30
  5.1.30. dquot_init_hash()  . . . . . . . . . . . . . . . . . . . .  30
  5.1.31. check_bugs() . . . . . . . . . . . . . . . . . . . . . . .  30
  5.1.32. Start other SMP processors (as applicable) . . . . . . . .  30
  5.1.33. Start init thread  . . . . . . . . . . . . . . . . . . . .  31
  5.1.34. unlock_kernel()  . . . . . . . . . . . . . . . . . . . . .  31
  5.1.35. current->need_resched = 1; . . . . . . . . . . . . . . . .  31
  5.1.36. cpu_idle() . . . . . . . . . . . . . . . . . . . . . . . .  31
  5.2. setup_arch  . . . . . . . . . . . . . . . . . . . . . . . . .  32
  5.2.1. Copy and convert system parameter data  . . . . . . . . . .  32
  5.2.2. For RAMdisk-enabled configs (CONFIG_BLK_DEV_RAM)  . . . . .  32
  5.2.3. setup_memory_region . . . . . . . . . . . . . . . . . . . .  32
  5.2.4. Set memory limits . . . . . . . . . . . . . . . . . . . . .  32
  5.2.5. parse_mem_cmdline . . . . . . . . . . . . . . . . . . . . .  32
  5.2.6. Setup Page Frames . . . . . . . . . . . . . . . . . . . . .  32
  5.2.7. Handle SMP and IO APIC Configurations . . . . . . . . . . .  32
  5.2.8. paging_init() . . . . . . . . . . . . . . . . . . . . . . .  33
  5.2.9. Save the boot-time SMP configuration  . . . . . . . . . . .  33
  5.2.10. Reserve INITRD memory  . . . . . . . . . . . . . . . . . .  33
  5.2.11. Scan for option ROMs . . . . . . . . . . . . . . . . . . .  33
  5.2.12. Reserve system resources . . . . . . . . . . . . . . . . .  33
  5.3. init thread . . . . . . . . . . . . . . . . . . . . . . . . .  33
  5.4. do_basic_setup {part of the init thread}  . . . . . . . . . .  34
  5.4.1. Be the reaper of orphaned children  . . . . . . . . . . . .  34
  5.4.2. MTRRs . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
  5.4.3. SYSCTLs . . . . . . . . . . . . . . . . . . . . . . . . . .  34
  5.4.4. Init Many Devices . . . . . . . . . . . . . . . . . . . . .  34
  5.4.5. PCI . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
  5.4.6. Micro Channel . . . . . . . . . . . . . . . . . . . . . . .  34
  5.4.7. ISA PnP . . . . . . . . . . . . . . . . . . . . . . . . . .  34
  5.4.8. Networking Init . . . . . . . . . . . . . . . . . . . . . .  35
  5.4.9. Initial RamDisk . . . . . . . . . . . . . . . . . . . . . .  35
  5.4.10. Start the kernel "context" thread (keventd)  . . . . . . .  35
  5.4.11. Initcalls  . . . . . . . . . . . . . . . . . . . . . . . .  35
  5.4.12. Filesystems  . . . . . . . . . . . . . . . . . . . . . . .  37
  5.4.13. IRDA . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
  5.4.14. PCMCIA . . . . . . . . . . . . . . . . . . . . . . . . . .  37
  5.4.15. Mount the root filesystem  . . . . . . . . . . . . . . . .  37
  5.4.16. Mount the dev (device) filesystem  . . . . . . . . . . . .  37
  5.4.17. Switch to the Initial RamDisk  . . . . . . . . . . . . . .  37
  6. Glossary  . . . . . . . . . . . . . . . . . . . . . . . . . . .  39
  7. References  . . . . . . . . . . . . . . . . . . . . . . . . . .  41







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