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The 8Mhz IBM PC AT 5170

I just added my first 8Mhz IBM PC AT machine configuration to the list of IBM PC Machine Configurations, and not surprisingly, the new machine fails to boot.

This machine uses the 3rd ROM BIOS that IBM released for the PC AT, a revision that included support for 3.5-inch 1.44Mb diskettes – which will be nice, because I have a number of 1.44Mb diskette images I would like to be able to read in a PCjs machine.

Since machines with this BIOS also ran at 8Mhz, I’ve bumped the CPU speed up to 8,000,000 cycles/second. It’ll be interesting to see whether this BIOS also increased any of its hard-coded timing delay-loops as a result.

Anyway, when I enabled ChipSet I/O port messages in the Debugger:

m chipset on
m port on

I can see that the BIOS Power-On Self Test (POST) progresses nicely until it starts generating lots of port 0x61 activity:

chipset.inPort(0x0061,8042_RWREG): 0x30 at F000:05A8
chipset.inPort(0x0061,8042_RWREG): 0x20 at F000:05AE

I know what I’m going to be doing this afternoon now.

Jeff Parsons
Oct 13, 2014