Checkin [9aa43b8c3e4060e8] is a brute force fix to a problem that reportedly crept in between 20170123 and 20170204, where DEP doesn't do what it ought to.
Simple test case:
1. sudo bin/pidp8i-sim
2. c
3. STOP
4. SR=0020, L_ADD
5. DEP
6. Ctrl-E, ex 20
In the older release (and in current tip of trunk) you get 0020 from SIMH, but in 20170204, you get 0000.
If you debug further, you find that this is because PC has some of its high bits (i.e. above the 12-bit contents of the actual register) set. How did this happen, and why did it change? Is it due to the SIMH register handling change patched in from upstream, perhaps?
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