IBM XT diagnostic ROM

Fred Cisin cisin at
Wed May 28 15:32:15 CDT 2008

On Wed, 28 May 2008, Marcus Bennett wrote:

My favorite line of the description is:
"If the pivotal part is OK, you can have a test of the parts that
unimportant, this may not halt the work even if any error occurs"

> How about somebody get the ISA version of such a card and see if it's at all
> useful.
> I suppose it might be able to help you fault find a seemingle dead system
> though I am somewhat suspicious.

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

During the cold start process, the computer sends numbers at various
stages to a port.  By putting in a card that decodes the port and displays
those numbers, you can see which stages of the start up process succeeded.

It will only work if the computer is making it at least partway through
the cold start boot process, and will certainly not tell you anything if
the CPU is dead, no matter what some brain-dead moron might say.  (Well,
OK, some also have a few LEDs to tell you if the power supply is live,

They used to be readily available from JDR, etc.  In fact, one of the
magazines published a DIY version that was an exact match for the one that
JDR sold.

In addition to trouble-shooting boot problems, they can be incredibly
useful in software debugging, (not quite as useful as an NMI based
debugger).  Sometimes, it isn't feasable to insert enough code to display
a "milestone" point on the video, but OUTing a value to a port can be
squeezed in in a lot more places.  "The program made it to location L133,
therefore, the problem must be betwenn there and the next breakpoint."
"Why didn't it take that JG conditional jump??  Oh, yeah.  81H is "ABOVE"

Grumpy Ol' Fred     		cisin at

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