If you, like myself, were wondering why the opcodes are written in octal, apparently modern hex notation was new at the time: PDP-11 was introduced in 1970, 0-F hex in 1966: https://en.wikipedia.org/wiki/Hexadecimal

Cool. I suppose the reason they're diodes and not just wires is the same reason n-rollover keyboards need diodes, so that current doesn't jump across rows you're not trying to read?

Luxury! (Monty Python voice) In my day we had to enter the bootstrap from the front panel switches.

Seriously though, we did, if you had money you bought this board - it wasn't as bad as you might think, the old PDPs had core which didn't lose it's contents when you powered them off, usually the bootstrap was still there

For obfuscation, the unused diode footprints could be populated with decoys that look like diodes but are open. You wouldn't want the competition easily understanding your 30 byte diode program.

Plug in the debugger and give it 5 minutes to warm up!

Until now I've never heard of anyone else doing this. I purchased a 6502 in Nov '75 without any way to program it. Out of desperation, I wired up a diode matrix using the 6502 machine codes and was delighted that it actually ran! Even though I had a bucket full of signal diodes from a nearby surplus store, this programming effort was more than sufficient motivation for purchasing some ram chips.

I was curios what does the code do and asked ChatGPT.

  Summary of Functionality:

  • This code seems to perform a sequence of loading data from memory into registers, performing bit tests, comparisons, and conditional branching based on the results of these tests.
  • It appears to be part of a loop or some repeated operation, as there are multiple backward branches that seem to control a flow of tests and memory manipulations.