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Thread: PC boot process

  1. #1
    AO Curmudgeon rcgreen's Avatar
    Join Date
    Nov 2001

    Thumbs up PC boot process

    A few basic facts on how the (IBM compatible) PC boots itself.

    1. The first thing your processor does when powered on, is to seek
    a standard address in memory, and execute whatever code it finds.

    This address is F000:FFF0h.
    All x86 Intel or clone chips are hard-wired to seek this address.

    2. At memory address F000:FFF0h, there will be a jmp (jump)
    instruction to another address where a program is located.
    This program (in the BIOS), hard wired in ROM somewhere
    between 640k and 1 meg in memory will perform rudimentary
    hardware testing called Power On Self Test (POST).

    3.BIOS will seek boot drive. (the one given priority in your setup (CMOS).
    BIOS will get the first sector of this drive (cylinder 0, head 0, sector 1),
    load this sector into RAM and execute the code found therein.

    It is important to remember that this code will be executed,
    no matter what it says to do. If it is virus code, it will execute,
    because there is no antivirus software running yet. The OS hasn't
    loaded yet.

    4. The first sector contains two items, first; the boot loader.
    The boot loader is a short program that seeks the bootable
    partition, and loads the first sector of the partition into
    memory, and jumps there to execute its code. Second; the first sector
    also has four data fields for partition info. This is the Partition Table.
    So, this first sector is sometimes called the Master Boot Record,
    and sometimes it is called the Partition Table, because it contains
    both of these items.

    5.In the standard MSDOS MBR/boot loader, the code will consult the
    partition table, looking for the "bootable", or "active" flag on one of
    the partition entries. The first sector of the bootable partition will
    be loaded into memory and executed. This sector is called the
    Boot Sector, and is often confused with the Master Boot Record,
    discussed above. It is also, especially in the past, a common
    place for virus code.

    6. The code in the boot sector looks for the OS kernel and loads
    it into memory and executes it. Originally, the dos kernel had to
    ocuppy the first contiguous data sectors of the partition,
    because the boot sector code wasn't very sophisticated.

    If you install a different OS than the dos/win type, or if you
    dual boot, the standard boot loader in the master boot record
    can be replaced by a boot loader that can give you a choice.
    The standard Dos boot loader, can only boot the partition
    marked bootable, but LILO, the linux loader, ignores the
    bootable flag, and loads whatever OS you select.
    I came in to the world with nothing. I still have most of it.

  2. #2
    Fastest Thing Alive s0nIc's Avatar
    Join Date
    Sep 2001
    kewl info there..

  3. #3
    Senior Member
    Join Date
    Jan 2002

    Thumbs up Bravo

    After the FreeAgent fiasco. People have reverted back to posting intelligent threads such as this one. It's a relief to find good solid info in a post. Interesting at that. This is what "greenie" material is made of. Keep up the good posts rcgreen, as well as all the other recent informative posts.
    The COOKIE TUX lives!!!!
    Windows NT crashed,I am the Blue Screen of Death.
    No one hears your screams.

  4. #4
    Senior Member
    Join Date
    Oct 2001
    cmos setup
    Wine maketh merry: but money answereth all things.
    --Ecclesiastes 10:19

  5. #5
    Hi mom!
    Join Date
    Aug 2001
    Good, clear post. Thanks rcgreen - keep them coming.
    I wish to express my gratitude to the people of Italy. Thank you for inventing pizza.

  6. #6
    Leftie Linux Lover the_JinX's Avatar
    Join Date
    Nov 2001
    Beverwijk Netherlands
    thanks for the info
    ASCII stupid question, get a stupid ANSI.
    When in Russia, pet a PETSCII.

    Get your ass over to SLAYRadio the best station for C64 Remixes !

  7. #7
    I am a cracker
    Excellent post we need MORE like this one here! keep them rollin

  8. #8
    Senior Member
    Join Date
    Jan 2002
    Without wanting to nice post rcgreen

  9. #9
    Senior Member
    Join Date
    Dec 2001

    Thumbs up

    good post. A pity some were taken in by that individual claiming a system was his. Oh well, a good lesson to them to be careful to check out things first.

    rcgreen - good idea and here is some more:

    post/boot process on -

    cisco routers

    sparc servers

    I'd post them but I'm not where I can get the details today....


  10. #10
    Senior Member
    Join Date
    Nov 2001
    good post! Thanks for the info rcgreen!

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