[LinuxBIOS] MB1030 / 3036 VGA resume

Stefan Reinauer stepan at coresystems.de
Thu May 11 15:51:54 CEST 2006

* Christian Sühs <chris at suehsi.de> [060511 14:51]:
> Ok, There are a few questions for me exspecially for the cr0 register.
> as I say default is 60000010h
> for that default the CPU is in real Mode. If I understand that right, it 
> is a 16bit machine ?!
yes, the x86 architecture is a drilled 16bit architecture with 32bit and
nowadays 64bit plugged on top. When the CPU is powered on, it still
comes up in 16bit mode.

One of the many delicacies of LinuxBIOS is that it enters protected mode
(32bit mode) after only 17(!) CPU instructions. 

> mainboardint cpu/x86/fpu/enable_fpu.inc
This enables the FPU so the romcc generated code can use the FPU
registers for spilling.

> that file does something on register cr0, but for the GX1 in cr0 is 
> nothing about fpu?!

I would assume it is there anyways, or the FPU is always enabled for the

> andl $~(1<<2), %eax /* found in enable_fpu.inc */
this clears bit 1.

 1<<2 is the value with only bit 2 set. The decimal value is 4

 $~(x) inverts the value of x, so the result is a value with all
 bits set except bit 2.

 the and operator sets all bits in eax that are
   - already set in eax
   - set in the value you provide
 so since only 1 bit is cleared in the value, the expression clears that
 one bit in eax.

> can anybody explain this in easy words ;)
> which bits are set in that case.

in easy words: none. bit 2 is cleared though. 

To set bits you would OR/ORL them into a value. Rather simple bitwise

  set bit 5:

   orl $(1<<5), %eax

NOTE: bits are counted from 0 here. So bit 2 is the third least
significant bit.

Hoe this helps..


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