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

Sun 30 October 2016 by Fritz Mueller

Some spare 74194 arrived in the mail; popped one in to the socket I had prepared at E15 on the FRL board, and the FP add/subtract problem is fixed. The following FP11 diagnostics now pass:

Diagnostic	Description	Status
CFPAB0.BIC	LDFPS,STFPS,SETI,SETL,SETF,SETD,CFCC	pass
CFPBB0.BIC	STST	pass
CFPCD0.BIC	LDF,LDD,STF,STD	pass
CFPDC0.BIC	ADDF,ADDD,SUBF,SUBD	pass
CFPEB0.BIC	CMPF,CMPD	pass
CFPFB0.BIC	MULF,MULD	pass
CFPGC0.BIC	DIVF,DIVD	pass
CFPHB0.BIC	CLR,TST,ABS,NEG	pass
CFPIB0.BIC	LDCDF,LDCFD,STCFD,STCDF	pass
CFPJB0.BIC	LDCJX,STCXJ	pass
CFPKB0.BIC	LDEXP	pass
CFPMB0.BIC	MAINT	pass

...which is almost everything. The last failing diagnostic is CFPLB0, which tests MODF and MODD. Set up a similar test program for this instruction:

        000000                          AC0=%0
        000001                          AC1=%1
000000                                  .ASECT
        001000                          .=1000
001000  170011                  START:  SETD                ;SET DOUBLE PRECISION MODE
001002  172467  000020                  LDD     D1,AC0      ;FETCH FIRST OPERAND FROM D1
001006  172567  000024                  LDD     D2,AC1      ;FETCH SECOND OPERAND FROM D2
001012  171401                          MODD    AC1,AC0     ;MOD (FRAC IN AC0, INT IN AC1)
001014  174067  000026                  STD     AC0,D3      ;STORE FRAC TO D3
001020  174167  000032                  STD     AC1,D4      ;STORE INT TO D4
001024  000000                          HALT
001026  040200  000000  000000  D1:     .WORD   040200,000000,000000,000000 ;1.0
001034  000000
001036  040300  000000  000000  D2:     .WORD   040300,000000,000000,000000 ;1.5
001044  000000
001046  000000  000000  000000  D3:     .WORD   000000,000000,000000,000000
001054  000000
001056  000000  000000  000000  D4:     .WORD   000000,000000,000000,000000
001064  000000
        001000                          .END    START

This does show a problem: after exection, the integer result at D4 seems correct, but the fractional result in D3 is incorrect (037777 177777 177777 177777). Verified the correct microflow with the KM11.

Stopped in microstate MOD.22, and examined ALUs on FRL where the fractional result is masked. ALU function selects (for A & ~B) and B inputs (all zeros for mask) look correct throughout. A inputs, however, are all ones except the least significant bit, which seems incorrect. All for now -- will dig a little deeper on the microcode flows and follow up on this lead next time...