IBM XT diagnostic ROM

Tony Duell ard at
Thu May 29 14:03:42 CDT 2008

> > What you have found is called a Power On Self Test card, or "POST card".


> It was my understanding that only AT-class machines (by age, not 
> necessarily by architecture type) wrote to that port.  I've never seen a 
> PC or XT (or clone thereof) motherboard/BIOS write to that port.

Well, it's a different port address, but it's clear from the source 
listings in the IBM PC/XT Techref that it writes out POSTcodes as the 
tests progress.

Some (AT and later) clones used other port addresses, BTW. 

Soem time ago there was a design fro a POSTcard in elektor magazine. It 
was just a GAL for address decoding (equations not given, but very easy 
to work out), an 8 bit latch to hold the POSTcodes as they are output, 
and another GAL to decode that and drive a couple of 7 segment displays. 
There was a DIP switch to select the IBM XT port, IBM AT port, and IIRC 2 
or 3 of the odd clone ports. 


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