For what it's worth, if you don't have the BIOS code, I have a ZIP file on
my Web site with the code in it.
On 11/7/07 8:11 PM, "Chuck Guzis" <cclist at sydex.com> wrote:
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
Rich
--
Rich Cini
Collector of Classic Computers
Build Master and lead engineer, Altair32 Emulator
http://www.altair32.com
http://highgate.comm.sfu.ca/~rcini/classiccmp