Information about BIOS and the boot process

[Ivan Gustavo]

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.

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.

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

Tanks in advance.

[SONE Takeshi]

On Sat, Mar 29, 2003 at 08:45:30AM +0000, Ivan Gustavo wrote:
>    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 will really appreciate that work.
>   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.

CPU starts with last 16 byte of address space.
On a 386 or later it's FFFFFFF:0000??
Anyway that does not really matter since the code there is a simple 
long jump (jmp seg:off).

[ron minnich]

On Fri, 28 Mar 2003, SONE Takeshi wrote:
> CPU starts with last 16 byte of address space.
> On a 386 or later it's FFFFFFF:0000??

CPU starts out in "big real mode" at 0xffff0000, and most BIOSes 
(linuxbios too) do a jump cs:ip to 0xf000:0x0, which pops the CPU into the 
old 8086 real mode.

kron

[Stefan Reinauer]

* SONE Takeshi  [030328 14:49]:
> On Sat, Mar 29, 2003 at 08:45:30AM +0000, Ivan Gustavo wrote:
> >   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.
>
> CPU starts with last 16 byte of address space.
> On a 386 or later it's FFFFFFF:0000??
> Anyway that does not really matter since the code there is a simple 
> long jump (jmp seg:off).

Even i386+ CPUs come up in 16bit mode, so it is
F000:FFF0 (setting the 64k segment to F0000-FFFFF and having the entry
point 16 bytes before the end of this segment

Stefan

Best regards,
  Stefan Reinauer

[Ivan Gustavo]

ron minnich wrote:
> On Fri, 28 Mar 2003, SONE Takeshi wrote:
> > CPU starts with last 16 byte of address space.
> > On a 386 or later it's FFFFFFF:0000??
>
> CPU starts out in "big real mode" at 0xffff0000,

Tanks :)

But, other ask... Ok , the CS point to the last accessible segment, and
IP to the  last addess less  16 bytes...
What I do not understand is, how the bios data go to there  (may be
funny to you, so I dont find nothing documentation about)...
I ask it because a common bios have 256k and the segment which the
processor point is  752Kbytes far...
... so I guess I am not imagining right.
> and most BIOSes (linuxbios too) do a jump cs:ip to 0xf000:0x0, which

pops the CPU into the
> old 8086 real mode.
>
> kron

Ivan Gustavo  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.

Eric