PDP-11/45: M9301 Troubles

After a lot of recent progress with the RK11, suffered a setback: after trying once again to image a fresh RT-11 pack, the machine began to behave erratically at boot. Sometimes the boot monitor would run fine, sometimes it would run for a while and then take a machine exception or vector off to bad address, and sometimes the machine would fail to boot at all, immediately taking exceptions of various sorts. Getting so close, but then lost a lot of ground; I guess that's sort of the way it is going to be with a machine that is this old...

So, back to the top without a working boot monitor. Microcode sequencing and seemed to be working fine -- exam/deposit/step working reliably from the front panel. Checked execution of the first few instructions of the boot ROM in detail, a branch and some single operand instructions, and they seemed to execute correctly.

Assuming some sort of new failure in the CPU, I prepared to instrument the Unibus with the HP logic analyzer; it seemed that way I could trigger on machine exceptions then look back over the captured address traces for a pattern. To make sense of those, I'd need a listing of the boot ROM in hand. Noel Chiappa has a dump for this and a partial disassembly on his site here, but is it the same as mine? Better check...

Started to run through Noel's listing and compare to the ROM contents on my machine via front panel exam. Sure enough, there seemed to be some words different in a few places. Maybe the CPU is fine, and the ROM card is failing? Swapped out the M9301 for a simpler M792 diode matrix boot ROM, and sure enough -- was able to boot straight away off my RKDP pack, and from there reliably beat up the CPU with diagnostics. So, great news: just a failing M9301!

Alright, so now I want to capture a dump of the M9301 so I can systematically compare with Noel's listing to see if there's a pattern in failed/flaky bits to help guide my repair. For this I need a memory dump utility that I can toggle in from the front panel. Came up with this:

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001000  012705  START:  MOV     #177564,R5      ;CONSOLE XCSR
        177564
001004  012700  L0:     MOV     #177570,R0      ;SWITCH REGISTER
        177570
001010  000000          HALT
001012  011004          MOV     @R0,R4          ;READ ADDR FROM SWITCHES
001014  000000          HALT
001016  011003          MOV     @R0,R3          ;READ COUNT FROM SWITCHES
001020  012401  L1:     MOV     (R4)+,R1        ;GET NEXT WORD TO DUMP
001022  012702          MOV     #6,R2           ;SIX DIGITS TO PRINT
        000006
001026  005000          CLR     R0              ;R0 GETS MSB R1
001030  073027          ASHC    #1,R0
        000001
001034  062700  L2:     ADD     #60,R0          ;MAKE INTO ASCII DIGIT
        000060
001040  010065          MOV     R0,2(R5)        ;OUTPUT
        000002
001044  105715          TSTB    (R5)
001046  100376          BPL     .-2
001050  005000          CLR     R0              ;R0 GETS 3 MSB R1
001052  073027          ASHC    #3,R0
        000003
001056  077212          SOB     R2,L2           ;LOOP DIGITS
001060  012765          MOV     #15,2(R5)       ;OUTPUT '\R'
        000015
        000002
001066  105715          TSTB    (R5)
001070  100376          BPL     .-2
001072  012765          MOV     #12,2(R5)       ;OUTPUT '\N'
        000012
        000002
001100  105715          TSTB    (R5)
001102  100376          BPL     .-2
001104  077333          SOB     R3,L1           ;LOOP WORDS
001106  000736          BR      L0              ;START OVER

Executed this a few times and got slightly different results, then things settled into a pattern where the lowest nybble of every word was consistently zeroed but everything else was fine. Smoking gun pointing to a single PROM on the M9301. Pulled and reseated that chip, and did the same for the other three while I was at it, and ... everything 100% after that. Wow, really should have just tried that first...

Well, at least I'm up and running again! The memory dump program might come in useful again at some other time, and as a byproduct after I fixed my M9301 I got 100% agreement with Noel's listing. So I think that listing can be considered authoritative now; good enough to generate replacement PROMs should anybody ever need to do so.