I recently discovered my first real program(*): from 1980, it was my attempt at Space Invaders for the RML 380Z. This was the government-subsidized computer for schools, my school (Chatham House Grammar School) was lucky enough to have one. One, yes, for the whole school. The Research Machines 380Z was a Z80 CP/M machine, I forget how much RAM but it couldn't have been more than 64k of course. The graphics abilities were primitive (and monochrome) but I did manage a fair duplication of the classic game (of course at the time was it was yet to become a classic). I never managed to code up the shields though, and it ran at a constant "full speed" which meant I could not speed up the Invaders as the game progressed.
The entire program was 1447 bytes long. Yup, less than 1.5k and that included the instructions! It is remotely possible that somewhere in my parent's attic are some 5.25" floppies with the actual source on, but my recent discovery was of a roll of brown paper with the listing, along with a few hand-written "improvements" that I planned on making. There were even a few comments, never more than two words at a time though.
Scans of the first couple of pages follow. I was impressed with the OCR software's attempts at understanding Z80 machine code, but not impressed enough to tidy it up and post it as real ASCII, you'll have to make do with the bitmaps.
(*) Technically this was my second real program: in order to qualify for access to the 380Z you had to prove yourself to the teacher on the other "computer" first: this was a teletype terminal connected via a phone line to God-Knows-What: some kind of mainframe I assume. For that fine device I wrote a version of Monopoly in BASIC, but no records of that exist because the punch tape hardware almost never worked, and even if I had a roll of tape with holes in it I wouldn't be too keen on hand-decoding it from EBCIDIC to ASCII.