16 Feb
2021
16 Feb
'21
11:08 a.m.
On Tue, Feb 16, 2021 at 2:36 AM Fritz Mueller via cctalk <
cctalk at classiccmp.org> wrote:
I can't, though -- all of this stuff works fine when running slowly :).
It's possible that while running in single-clock mode I might be able to
see a waveform that's slightly off on the 'scope, if I can pinpoint the
failure.
Right now I'm thinking it is most likely the AMX selecting the wrong input
(I'm guessing that BMX is correctly selecting KOMX, to get the constant "2"
for the add operation).
On Feb 16, 2021, at 1:13 AM, Josh Dersch
<derschjo at gmail.com> wrote:
My money's on t5 going wrong -- an ALU input mux or operation being
selected
incorrectly, possibly. This also somewhat explains why execution
doesn't trap due to executing a HALT from an odd address -- it isn't
actually executing from the wrong address, because the bus address is
loaded from a still-good PCB in t1.
Yes -- that seems to match the observations. So you could set up on t4 or
t5 of that microinstruction with the KM11 in single-clock-phase mode, and
then have at the ALU with a logic probe to check...
There is a small ALU control ROM on the GRA (E74 in the upper left of
sheet GRAA), which selects the ALU mode and operation based on lines coming
over from the IRC. I had a failure of this part on my 11/45, and ended up
with incorrect ALU setup in some circumstances. That part is a 256-bit
DM8598 tri-state bipolar mask ROM, and the truth table is on sheet GRAK.
Looking back at my notes, I think it was Glen who informed me (on this
list, in 2016!) that the Signetics 82S123 PROM could be substituted here.
Programmed one up, and it worked, in case you run into the same thing and
it is useful info.
I think I may have ended up doing a similar substitution with a microcode
ROM on my 11/05. Have the 11/70 microcode PROMs been dumped? I had to
recreate the 11/05 PROM from the listings in the engineering drawings...
- Josh
cheers,
--FritzM.