PDP-11/70 progress (and a cry for help)

Fritz Mueller fritzm at fritzm.org
Tue Feb 16 04:36:35 CST 2021

> 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.


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