IBM PC hacking

[Tony Duell]

> > 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.
> 
> While it is rare that a minor error will surface in, say, the CPU (I have NEVER 
> seen a CPU go gradually, like "whoops, now I can't ADD or SHL" -- it's all or 

I've never had a CPU do that, actually, but I have had other LSI devices 
fail in really odd ways, so I could well believe it could happen.

> nothing), the diagnostics run in something like 1K of ROM with no memory use 
> other than video so they are helpful in diagnosing RAM problems.  And the

IIRC, there is no separate video RAM on the PCjr, the video circuit just 
takes some of the main RAM. And while you can have random bit failures in 
RAM, I've found that either complete chips failing or address inputs 
failing (so that locations appear to repeat) is much more common. 
 
> sound, joystick, video, and floppy drive tests are helpful in determining if 
> the machine is functional on a basic level -- I always run them on any new PCjr 
> I come across to determine if it's worth hauling away or not :-)

Over here, PCjr machines are sufficiently rare that just about any is 
worth grabbing.

Apart from a defective gate array chip, I think just about any problem is 
repairable.

-tony