digital equipment corporation · maynard, massachusetts

Alan Weisman PDP-8 Memories

When I was in high school in the late 1970s, they had a PDP-8/E in the building and it was the pride of the town as most towns didn't have their own computers yet. It was used for 2 purposes - teaching the students BASIC and performing tasks relating to report cards, grades, attendance etc. To take the BASIC class, you needed certain prerequisite math courses and grades that I lacked so I wasn't allowed. However, there was "open" time where anyone could walk in and mess around. For some reason, my friends and I were drawn to learning the assembler language instead of BASIC and the manuals were stored there and available. We learned so much. Here's how we got into mischief and it's from memory so sorry if a few technical details are wrong.

There was a manual for their time sharing OS there that described the data structures used to store terminal sessions including the character buffers from the teletype activity. It the very top of the data structure was an "eye-catcher" numerical value but we didn't know the location where it was stored. We would halt the machine after hours in a way that would force a line printer dump. I spent endless evenings (because none of us had social lives or dates) looking for that eye-catcher. We finally found it. Memory wasn't protected from references in assembly language so in a program we addressed the top of that structure and walked it down to the terminal buffers. When running, another person would type something it would echo on our teletype. We would then tell that person what they were typing!

We weren't looking to cause damage. It was more of an exercise to see if we could do it. It was that experience that got me my first programming job later on, programming in assembly on a Prime Computer.

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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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