LOCK NET hangs ITS

[larsbrinkhoff]

On a KA10 with NCP, running the NET command from LOCK hangs the system. I have not tested the KSIMP or without NCP.

The sysjob calls IMPINI as expected, but the PUSHJ P,UFLS ;WAIT FOR HALF SECOND never returns. It looks like TIME isn't updated, so the half second never ends.

[larsbrinkhoff]

There seems to be a problem with the clock interrupt. A while after typing the NET command, the machine enters a state where the clock interrupt at level 7 is held (as per the PIH register), and never released. Maybe the JRST 12, instruction isn't called to dismiss a clock interrupt.

[larsbrinkhoff]

Execution of the clock interrupt handler reaches VSSLCK, which probably calls SYSIMU to bring up the IMP.

And then the handler calls SSLCK which calls IMPOST to trigger an IMP output interrupt. However after this, the IMP interrupt handler is called over and over again, so IMPOST never returns.

[larsbrinkhoff]

The IMP interrupts are due to the IMPR CONI bit, i.e. "IMP Ready". This is actually a long standing problem that I have dodged in the past. I re-read ITS hardware memo 10 and noticed that the "IIR" (called IMPIC in the PDP-10 emulator) should be copied from CONO bit 4000. Previously, there was only an action when 4000 was set. ITS seems to have the bit set initially to get an interrupt when the IMP becomes ready, and then clears the bit to only get an interrupt on IMP error. The latter part wasn't working.

[eswenson1]

Where can I find ITS Hardware Memo 10?

[larsbrinkhoff]

Here: https://github.com/larsbrinkhoff/its-archives/blob/master/ailab/ITS_Hardware_Memo_10.pdf

[larsbrinkhoff]

I posted a pull request that fixes the hang in ITS:
open-simh/simh#522

Since there is no bug in ITS itself, I'll close this now.