Information about BIOS and the boot process

Eric W. Biederman ebiederman at
Mon Mar 31 15:11:00 CEST 2003

Ivan Gustavo <ivan at> writes:

>         I am writing some code to fixup the video chipset trident cyberblade and
> V1621 RGB->CS encoder ,
> and also trying to fix the IDE_BOOT for my mainboard (B860T or EPIA), so I am
> looking for information
> about the real bios process, and about the layout of the common bios files, and
> of course, any information
> about bios as possible.

You might try the IDE_BOOT option of etherboot...
> My main problem is to find good documentation about the post, the boot process.
>        I read on some places which the CS is set F000 and EIP FFF0, so in other
> websites I had seen CS is FFF0,
>  in other 000F , causing some confusion with me.

So far I have not seen a good reply so:

Since there continues to be confusion.  The cpu starts running at:
0xffff0000:0xfff0  An impossible to load CS selector but the cpu manages it.

Many times there is a far jmp to 0xf000:???? so there is a rom alias there.

While not universally used in LinuxBIOS there is code present to do a short
backwards jump.  And at the location of that jump transition into
32bit protected mode.

> I need some documentation about 20bits addressing lines, and the GATE A20
> function also.

As far as gateA20.  It is for legacy compatibility and it has been
since the introduction of the 286.  When a machine boots up all
address lines are enabled and active.  When going into dos
compatibility mode gate a20 is disabled.

Under LinuxBIOS gateA20 is always left enabled.


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