> On Feb 16, 2021, at 1:13 AM, Josh Dersch <dersc...@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.
cheers,
--FritzM.
