Altair can't Jump

Jeff G jeffg at
Wed Jun 17 23:18:28 EDT 2020

Thanks Herb, its actually my intention that since I have the real deal, to
get the experience of messing with it as someone may have back in the day.
For the most part, I haven't used almost any test equipment other than a
logic probe and multimeter so far.

I'm no stranger to test gear though, I cut my teeth learning digital
electronics on a Space Invaders arcade board, which happened to be powered
by an 8080. I've fixed a bunch of other boards and have some decent test
gear, like a Fluke 9010A with 8080 pod, O-scopes and an HP 16500C logic
analyzer. That said, I'm working on this in the house, away from the bench,
under a desk lamp, and so far, no cheater toys yet. YET. (evil grin)

I'm at my wits end with it though, the total echo program which is about as
simple as it gets really, runs about 60 bytes that I'm entering at the
front panel every time. This thing has certainly given me an honorary
computer science degree, as every little error and attempt to cut a corner
was thrown back at me. I can speak binary and octal fluently even in my
sleep, and I've also gone over this program enough times against an opcode
list to make my head hurt, and all while knowing that I'm not 100% sure
that either the serial board OR the CPU is 100% operational, although I'm
leaning towards the CPU being good enough as it did run the kill-the-bit
game and played music through an AM radio, with no problems with ram. Even
if this was on the bench, I guess I'd have to run NOPs or Port IOs and look
at the bus with the LA or o-scope. If the LA didn't weight more than my car
I'd have lugged it into the house by now.

Hardware wise, on the serial board, I found a bad voltage regulator,
replaced the 1488 and 1489 ics that were missing, and even found the -16v
was even missing on the motherboard (cold solder point) . I inspected it
for jumpers/mods and while it does have one, it doesn't seem to be part of
the serial ports nor hurting anything.

As of today, I eliminated the ram board being culprit with an DeRamp FDC+
board doing ram duties. I also learned about interrupts and initially
thought that perhaps they were somehow getting enabled and processed, as my
code was starting at 0 and interrupt vectors are at very low addresses,
like 0008h. I messed with the EI/DI opcodes to make sure the INTE light
works. I've now bumped the code up to 1000h, and added a DI opcode to it,
but nothing changed. I also found one final error that was likely causing a
stack issue (jumping to a push instead of the following opcode), and fixed

NOW, what did change, is that I've sat here and single stepped the entire
thing, and found it works perfectly in doing so. A character I enter is
being replied back no problem, which tells me serially everything's fine
Yet if I run it, I'm just getting a solid stream of garbage, although
interestingly, repetitive garbage.

So my next thoughts are that the bus is ringing/noisy, or perhaps there's
bad buffers on either the serial board or the CPU board, as it seems that
reading ports 0/1 is flaky or having a timing issue, or the serial board
itself has other issues. Again, works absolutely great if I single step it
all the way through.

If anyone wants to look at the code, you can find it here:

It was mostly taken straight from the TU-ART manual, except I changed JR's
to Jump if Zeros since JR was a Z80 opcode and not supported by 8080.


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