7 Nov
2007
7 Nov
'07
5:11 p.m.
To follow up, it seems as if the code at offset 058Dh in the Version
3 PC AT BIOS is the speed-tester. Curiously, it seems not simply to
test for an overfast CPU clock, but a slow one also. The timebase
for the check is the "refresh" bit (bit 4 of port 61h). Values lying
outside of the acceptable range result in doing a 0101h beep.
A workable patch would seem to be to insert an unconditional jump at
5BC to 5CB, which would bypass both tests. When I get a chance, I'll
burn some ROMs and try it, after recomputing the ROM checksum.
Cheers,
Chuck