I've been working on a software emulation of the Extended Arithmetic Element for the PDP-8/E. I've been struggling a little with the multiply (MUY) instruction in particular. I don't have an actual EAE to test this all with, but I'd appreciate anyone with one to test out a bit of code to confirm. In any event, both SimH and the OS X PDP-8/E Simulator exhibit the same symptoms, which seem to multiply (PC + 1) by MQ and add AC, placing that result into AC, MQ. Expected behavior, best I can tell from the Maintenance Manual Volume 2, would be (PC + 1) * MQ -> AC, MQ, without the addition of the previous AC prior to executing MUY. If this is the case, then clearing AC would be essentially required, unless you want to add that value after the multiply (which could be a useful mathematical operation in some circumstances). There is a little code found on bitsavers that seems to emulate these EAE instructions: http://bitsavers.trailing-edge.com/bits/DEC/pdp8/From_Vince_Slyngstad/digit… You'll note that there is no DCA at the top of the MUY subroutine to save AC in order to add it to MQ (and complete the carry) afterward. I've been writing a comparison tool after finding that one program changed to support the software EAE failed to function correctly, hence attempting to track down some differences. If you're interested in trying on real hardware (which would be appreciated), here's what I'm running: MUY=7405 DVI=7407 NMI=7411 SHL=7413 ASR=7415 LSR=7417 MQL=7421 SCL=7403 SCA=7441 MQA=7501 CAM=CLA MQL *200 CLA CLL TAD MQVAL MQL TAD ACVAL HLT MUY 7777 HLT MQVAL, 7777 ACVAL, 7777 $ After the first halt, AC=MQ=7777, as expected. After continuing, AC=7777, MQ=0000. This would be equivalent to 7777*7777+7777=77770000, rather than the expected value of 7777*7777=77760001. Any help would greatly ease my concerns! Kyle