[coreboot] Understanding BIOS I/O Adresses

panic lists at xandea.de
Wed Nov 18 00:38:15 CET 2015

Peter Stuge:
> Please don't post disassembly of code from outside the project. Thanks.

ok. noted.

>> In particular I'm interested in 0x80, 0x84/0x85, 0x8c/0x8d.
>>From ports.lst:
> ----------P0080008F--------------------------
> PORT 0080-008F - DMA PAGE REGISTERS (74612)
> ..
> 0084  RW  extra page register
> 0085  RW  extra page register
> 008C  RW  extra page register
> 008D  RW  extra page register
> So either something is being done with the DMA controller (seems
> unlikely in early boot) or these registers have a completely
> different meaning at that time in the platform lifecycle.

An idea is that, probably, these registers serve as scratchpad copy and
recover ax and dx because they are modified during the subroutines.

-- panic

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