fritzm.github.io

PDP-11/45: Diagnostics IX - FP11 FPU, cont.

Sat 10 September 2016 by Fritz Mueller

Did a lot of reading on the FP11 design. A few interesting notes that are buried in the maintenance manual:

  • When debugging FP11 microcode with a KM11 in single-microstep mode, the 11/45 front panel microcode display shows the address of the next microinstruction, NOT the current microinstruction. This is because the stop-point for single microinstruction is at a point between T2 and T3, just after the next microinstruction addr has been calculated. This is different behavior than the 11/45 CPU front panel microaddress display.

  • There's a note in the maintenance manual that explicitly cautions that when using extender boards for debug, the RC maintenance clock should be used, and set with period >50ns. I had not been doing similar while debugging the KB11-A CPU, and maybe this explains the occasional different behavior I'd see when throwing boards out on extenders... In particular, I had seen this when debugging a spare CPU GRA; next time I return to that board I will try the CPU RC clock.

Okay, so here's my first simple test program for STST:

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000000                          AC0=%0
000000                          .ASECT
001000                          .=1000
001000  170127  044000  START:  LDFPS   #044000         ;FID+FIUV
001004  172467  000004          LDF     NEGZ,AC0        ;LOAD A MINUS-ZERO
001010  170300                  STST    R0              ;STORE FEC TO R0
001012  000000                  HALT
001014  100000  000000  NEGZ:   .WORD   100000,000000   ;MINUS-ZERO
001000                          .END    START

This would be expected to produce the 000014 "Floating Undefined Variable" (minus-zero) exception code in R0, but I see an incorrect value of 177417. Using the KM11 on the FPU shows the -0 trap and STST microstate flow is per expectation.

Put the FRL out on the extender and started stepping the microcode, examining the state of the pins at the AC register file along the way. In the -0 trap flow, the FEC code 000014 presented (inverted) at TRP.50 via the EALU, and subsequently retrieved at TRP.60 looks correct. However, the value presented at TRP.70 via QR, BR, and the FALU does not. Out of time this weekend; Will have to chase signals back through those paths next time!