IBM PC hacking

[Michael B. Brutman]

 > The problem with those diagnostics (and most built-in diagnostics) is
 > that a fairly large part of the machine has to be operational (at
 > least the CPU, some memory, ROM, video) for them to be at all useful.

 > -tony


One of the lesser known features of the PCjr was the manufacturing test 
mode.  I think output was via the ports on the back.  (Not the famous 
'L' port though, which appears to have no use.)  The manufacturing test 
mode required a lot less of the machine to be functional than the user 
level diagnostics or the advanced diagnostics.

Even for machines that run diagnostics from disk, assuming that the 
memory or CPU is ok is a big leap.  To do diagnostics correctly you 
really need an external processor or an onboard service processor like 
midrange machines do, and the appropriate debug ports on the parts you 
are going to test.


Mike