PDP-8/S Incompatibilities......

I've had my PDP-8/S up and running for some time - but have recently experienced failures using some PDP-8 software. My 8/S passes all the DEC diagnostics - and it runs FOCAL-69 and 71 perfectly. So I decided to track down what was causing some programs to fail - and I discovered that the problem is PDP-8/S incompatibilities. PDP-8/S "Reality" ----------------- The PDP-8/S User Manual lies! It makes the following claim regarding Group 1 Operate Instructions: "The only restriction on combining OPR 1 (Group 1) operations within one instruction, other than logical conflicts, is that a rotate operation (bits 8, 9 or 10) may not be combined with the increment AC operation (bit 11) since they are executed during the same bit times." I decided to test that claim on my PDP-8/S and got the following results (via single stepping through the test program below.) The "8/S" column is Link AC contents (L AAAA): *200 Page 1 1 0200 *200 2 /AC 8/S NOTES 3 00200 7200 CLA /0 0 0000 4 00201 7201 CLA IAC /1 0 0001 5 00202 7326 CLA CLL CML RTL /2 0 0002 6 00203 7325 CLA CLL CML IAC RAL /3 1 0003 8/I AND UP 7 00204 7307 CLA CLL IAC RTL /4 1 0001 8/I AND UP 8 00205 7327 CLA CLL CML IAC RTL /6 1 0003 8/I AND UP 9 00206 7215 CLA IAC RAL RAR /10 1 0003 6120 10 00207 7203 CLA IAC BSW /100 1 0001 8/E AND UP 11 00210 7332 CLA CLL CML RTR /2000 0 2000 12 00211 7354 CLA CLL CMA RAR RAL /3776 1 7777 8/I OR 8/L 13 00212 7350 CLA CLL CMA RAR /3777 1 7777 14 00213 7330 CLA CLL CML RAR /4000 0 4000 15 00214 7352 CLA CLL CMA RTR /5777 1 7777 16 00215 7333 CLA CLL CML IAC RTR /6000 1 0003 8/I AND UP 17 00216 7346 CLA CLL CMA RTL /7775 1 7777 18 00217 7344 CLA CLL CMA RAL /7776 1 7777 19 00220 7240 CLA CMA /7777 ? 7777 20 00221 7402 HLT 21 $ No detected errors No links generated Note that all of the CMA shift instructions failed - even though the test (from Programming Languages) thought they should work in pre-8/I machines. At first I thought my PDP-8/S was failing - until I found the following from the PDP-8/S FAQ (What is a PDP-8/S?): "Compatibility: The core of the PDP-8 instruction set is present, but there are a sufficient number of incompatibilities that, as with the PDP-5, many otherwise portable "Family of 8" programs will not run on the PDP-8/S. Perhaps the worst incompatibility is that the Group 1 OPR instruction CMA cannot be combined with any of the rotate instructions; as with the PDP-8, IAC also cannot be combined with rotate." I decided to understand WHY this happens - and the answer is in the PDP-8/S Maintenance Manual and the PDP-8/S schematics. I've abreviated the sequence of execution below. (The "Ax" references are bit timimg): Group 1 Instruction Sequencing ------------------------------ Fetch ----- A12 Set or Clear Link, Clear AC per instruction Execute ------- A00 For right rotation (A00-01), shift AC+L together once or twice per instruction A(00-11) IAC: Adds one to AC CMA: Complements the AC IAC*CMA: Complement and add one to AC Left Rotate: AC+L shifted right 11 or 12 times per instruction A12 IAC: Complements L on overflow (CMA appears in gating, but has no effect) As can be seen, while the system clears the AC and clears or sets the Link during Fetch - it rotates right BEFORE complementing the AC! It also attempts to execute a complement of the AC at the SAME TIME as a left rotate - which simply does not work! Finally, just to verify the correctness of the test program, I single stepped it on my PDP-8/E and got the same results as the SIMH trace below: SIMH PDP8 Trace of the above: ----------------------------- sim> sh cpu history PC L AC MQ ea IR 00200 1 7777 0000 CLA 00201 1 0000 0000 CLA IAC 00202 1 0001 0000 CLA CLL CML RTL 00203 0 0002 0000 CLA CLL CML IAC RAL 00204 0 0003 0000 CLA CLL IAC RTL 00205 0 0004 0000 CLA CLL CML IAC RTL 00206 0 0006 0000 CLA IAC RAL RAR 00207 0 0001 0000 CLA IAC BSW 00210 0 0100 0000 CLA CLL CML RTR 00211 0 2000 0000 CLA CLL CMA RAL RAR 00212 0 7354 0000 CLA CLL CMA RAR 00213 1 3777 0000 CLA CLL CML RAR 00214 0 4000 0000 CLA CLL CMA RTR 00215 1 5777 0000 CLA CLL CML IAC RTR 00216 0 6000 0000 CLA CLL CMA RTL 00217 1 7775 0000 CLA CLL CMA RAL 00220 1 7776 0000 STA 00221 1 7777 0000 HLT sim> Cheers, Lyle -- Bickley Consulting West Inc. http://bickleywest.com "Black holes are where God is dividing by zero"