PC AT Rom Patch?
Richard A. Cini
rcini at optonline.net
Wed Nov 7 19:18:23 CST 2007
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.
Collector of Classic Computers
Build Master and lead engineer, Altair32 Emulator
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