ANSWER: Weekly Classic Computer Trivia Question (20150121)

And this question must have been too obscure to elicit any response. The answer is C) 50. After posting the trivia question I came up with a way on the PDP8 to measure time events to a resolution of 1.5 microseconds and went back and retested the High Speed Paper tape reader hardware. The earlier version could only measure to 3.0 microseconds resolution. Here is the 1.5 microsecond version of the program: / ADVANCED HIGH SPEED PAPER TAPE TIMING PROGRAM / / USES FIELD 1 AND LINK/AC TO MEASURE TIME DELAY / WITH INTERRUPTS. *0000 .-. / INTERRUPT ENTRY POINT RSF / MAKE CERTAIN IT WAS A HSR INTERRUPT HLT / HOUSTON WE HAVE A PROBLEM DCA I AUTO0 / SAVE COUNT AND LINK AS A 24 BIT RAL DCA I AUTO0 / SAVE LINK JMP MORE *0010 AUTO0, .-. / MORE, ISZ Z FRM1 / CHECK FOR DONE JMP NEXT / EXIT INTERRUPT AND CONTINUE TIMING TAD Z COUNTS / POINT AT START OF ARRAY DCA Z AUTO0 TAD Z CNTS / GET BACK ORIGINAL COUNT OF SAMPLES DCA Z FRM1 TLS / PRIME THE PUMP PRLP, TAD I Z AUTO0 / GET 12 BIT PART DCA Z BASE TAD I Z AUTO0 / GET LINK PART CLL RAR / RESTORE THE LINK TAD Z BASE / COUNT IS IN LINK AND AC TAD Z C0002 / ADD 2 WHICH CORRECTS TIMING FOR START DELAY DCA Z BASE / SAVE FOR PRINTING RAL / PRINT THE LEADING OCTAL DIGIT TAD Z CHZERO / ADD IN '0' TO GET LEADING DIGIT JMS PRINT / AND WE DONT HAVE TO WORRY ABOUT THE LINK ANYMORE TAD Z M0004 / INITIALIZE DIGIT COUNTER DCA Z DIGCNT DIGLP, TAD Z BASE / OCTAL DIGIT IS IN UPPER 3 BITS RTL RAL DCA Z BASE / NEXT DIGIT IS IN UPPER 3 BITS TAD Z BASE / NEED TO SHIFT ONCE MORE TO GET IN LOWER 3 BITS RAL AND Z C0007 / KEEP JUST THE LOWER 3 BITS TAD Z CHZERO / ADD IN THE OFFSET FOR ASCII 0 CHARACTER JMS PRINT / AND GO PRINT IT ISZ Z DIGCNT JMP DIGLP TAD Z CHSPC / PRINT A SPACE JMS PRINT TAD Z CHCR / PRINT CR JMS PRINT TAD Z CHLF / PRINT LF JMS PRINT ISZ Z FRM1 / ARE WE DONE? JMP PRLP / NO HLT / ALL DONE JMP START / GO AGAIN NEXT, CLA CLL / LINK AND AC MAKE A 13 BIT COUNTER ION / INTERRUPTS ON. CIF THAT FOLLOWS DELAYS THIS CIF 10 / SWITCH TO FIELD 1 AT THE JMP I CZERO RFC / START THE NEXT READ JMP I C0000 / START COUNTING (ADDS 2 TO THE ACTUAL COUNT) PRINT, .-. / PRINT THE CHARACTER IN THE AC TSF / WAIT FOR LAST CHARACTER TO FINISH PRINTING JMP .-1 / WAIT TLS / SHIP IT CLA CLL / RETURN WITH AC CLEAR JMP I PRINT / RETURN / CONSTANTS C0000, 0 C0002, 2 C0007, 0007 / MASK CIAC, IAC / IAC CONSTANT CHLF, 0012 / CONSTANT FOR LINE FEED CHCR, 0015 / CONSTANT FOR CARRIAGE RETURN CHSPC, 0040 / CONSTANT FOR A SPACE CHARACTER CHZERO, 0260 / CONSTANT FOR '0' CHARACTER COUNTS, ARRAY-1 / POINTER TO COUNTS ARRAY-1 FOR AUTO INCREMENT INITIALIZE M0004, -4 / VARIABLES FRM1, 0000 / FRAME COUNT KEPT HERE CNTS, 0000 / COPY OF FRAME COUNT READ FROM SWITCHES BASE, 0000 / TEMPORARY STORAGE OF WORD BEING PRINTED DIGCNT, 0000 / COUNT OF DIGITS WHEN PRINTING *0200 START, CLA DCA FRM1 / FILL ALL OF FIELD 1 WITH IAC INSTRUCTIONS CDF 10 FILL, TAD CIAC / GET THE CIAC INSTRUCTION DCA I FRM1 / PUT IT ON FIELD 1 ISZ FRM1 / BUMP AND CHECK FOR DONE JMP FILL / NOT DONE CDF 00 / RESTORE TO FIELD ZERO AGAIN, LAS / READ SR CMA / INITIALIZE FRAME COUNT FROM SR FOR ISZ DCA Z FRM1 TAD Z FRM1 DCA Z CNTS / KEEP A COPY FOR USE WHEN PRINTING TAD Z COUNTS / INITIALIZE THE AUTO INCREMENT REGISTER DCA Z AUTO0 JMP NEXT / AND GO MAKE A PASS *0400 ARRAY, .-. /STORE THE COUNTS HERE $ The program fills field 1 with IAC instructions and then reads the switch register to find out how many frames to process. Each frame is timed and the result stored in words pairs starting at 0400. Counts are 0-8191 which on a straight 8 would represent 0 - 12286.5 microseconds. The actual time is 2 counts more than this because of the JMP I C0000 to switch fields and start execution of the timer. Once the switch register count of frames is reached the contents of the array plus 2 is dumped out as 5 octal digits to the teletype. In the case of the Type 750 and PC01 the first character read sets the flag immediately so the first entry in the table shows as 2 which for the purpose of my emulator I fixed the first entry. My emulator of course gives the same answers as the 8 but then it has to because it uses the table generated by the real 8. The only other place I ran it was on Sim8 which always gives 0145 as the answer which is not even close. I will come up with something of more general interest for the next trivia question. I have something in mind already. Doug Ingraham PDP-8 S/N 1175 On Wed, Jan 21, 2015 at 11:27 PM, Doug Ingraham <dpi at dustyoldcomputers.com> wrote: -- Doug Ingraham PDP-8 SN 1175