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more insight into BIOS MTRRs (Users)

posted by Ninho(R) E-mail, 20.03.2011, 19:30

>> I had not had an incentive to look into this before, now digging a little
>> further, (plain DOS) : VESA BIOS reports the LFB at C0000000h (3 Giga), and
>> that is correct. But at the same time for some unfathomable reason
>> AMI/SiS BIOS erroneously has pointed the appropriate MTRR (MSR 20E/20F) to
>> phys address D0000000h instead of C0000000. "Plug and play" they say, let
>> me laugh :=)

> That's the plug and pray - the right meaning ;)

I always thought the right interpretation was pay and pray ;-)

...However, I gave the whole story a second look & changed my opinion, it's
no BIOS bug; rather the region, starting at D0000000, set to write-combining is the "AGP
aperture", not the LFB - the latter is left for an OS to set presumably.

Looking from Windows now, neither Win 98 nor 2k set an MTRR over the frame
buffer. I guess for such purpose they are using the PAT instead.

---
Ninho

 

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