digital equipment corporation · maynard, massachusetts

FOCAL Bug Story from Rick Merrill

Only one bug in FOCAL was ever reported from the field through the chain of command. Individuals reported other bugs directly to me. A student at Sudbury high school had written a program in Focal to calculate the digits of PI. When he checked his first printout against the documentation he found two digits transposed as if the computer were dyslectic. He was understandably miffed. However, no one could repeat that bug!

We were running four users on a PDP-8 with a full 16K with teletypes at 110 baud when we got a Tektronix display that ran 2400 baud. Then I saw the problem happening at random! Suspecting what the issue was I asked Dave Plumber to set me up with his PDP8 simulator on the PDP6. Then I set the interrupt frequency to match the word fetch frequency of the PDP8 - in other words interrupt at each instruction: bingo, A dyslectic computer. We needed to move the IOF instruction (turn interrupts off) to the start of the output routine. Next week we would be running 4-users on the brand new PDP-8I never tested with 4 users in Maynard to a big show in Anaheim, CA.

FOCAL Saved The Day

Once in Anaheim I loaded the program and started running the PDP-8I with 1,2,3, and 4 users when suddenly the output from each user terminals all started showing on TTY #1 which was of course running in the lowest memory bank. Within a few minutes Louis Klotz and I figured out that the interrupt was simply clearing the high memory bits, EVEN when the instruction was IOF! With prints all over the floor Louis and his team made a green wire fix that night and the next day we were up and running with a waiting line of people wanting to try four users on a PDP-8I.

Suddenly the lights went out, the computer fans sighhhhed and all users the cavernous show room went AWWWW with a few other choice words thrown in. The loudspeakers announced that they apologized for the outage and power would be restored in fifteen minute exactly. The wait was to let the big lights cool enough to restart!

Sure enough the lights came on, and the fans went wwwoooOOO, BUT the ONLY CHEERING came from the Digital booth because each and every user was back to exactly where they were at power out! No reboot required! It was a Big score for Core that day!

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Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with.  I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation. 

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