PC AT Rom Patch?

[Richard A. Cini]

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