fritzm.github.io

PDP-11/45: CPU debug V -- chasing lights!

Sun 19 June 2016 by Fritz Mueller

Tracked down the source of the inverted result after register-to-register move problem on GRA: outputs of the subsidiary ALU control ROM E74 on pins 6 and 7 are floating. Will need some closer inspection to determine if this is a board fault or a chip fault. In the meantime, I have a spare GRA which I had been reluctant to try because it has a "bad" sticker on it... Decided to give it a try anyway, and it seems to be working much better than the one I have been debugging.

Now have enough of the CPU debugged to toggle in and run a simple light chaser program:

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000000  013700  177570  L0:     MOV     @#177570, R0    ;LOAD COUNT FROM SWITCH REGISTER
000004  005300          L1:     DEC     R0              ;COUNT DOWN
000006  001376                  BNE     L1              ;LOOP UNTIL ZERO
000010  006301                  ASL     R1              ;SHIFT DISPLAY VALUE
000012  001002                  BNE     L2              ;SKIP AHEAD IF NOT SHIFTED OUT
000014  012701  000001          MOV     #1,R1           ;ELSE RELOAD
000020  010137  177570  L2:     MOV     R1,@#177570     ;STORE TO DISPLAY REGISTER
000024  000765                  BR      L0              ;REPEAT FROM THE TOP

Some notes on the program and video above since I've received some questions:

  • The listing here is shown assembled at location 000000, but the program is relocatable and can be toggled in at any convenient address (000000, on top of the trap vectors, probably isn't the best choice!)

  • Data display should be on "DISPLAY REGISTER" to see the chase.

  • The front panel toggles are loaded into a counter to control the speed of the chase. Without some of the most significant bits set the chase may go too fast to see, especially on older 11's with incandescent indicators. All toggles off is a special case: this will be the slowest chase, since as written the counter wraps around when decremented before being checked for zero. The video has toggles 15 and 14 up.

  • If you look at the address lights in the video, you can see that I ran this program from address 100000. This was because at the time I had a fault in the first 16KW of memory on my MS11-L so I couldn't execute any code at lower addresses.