Hurray! CP/M on an Amstrad 464 in the 80s was my first contact with an operating system that back then felt to me professional - like those in the movies. And it brought compilers.
It was the only environment I could find compilers and an assembler for, along with a book in a local library. It ended up teaching me Z80 assembler, but with Motorola style syntax!
I remember writing an invaders style game and a disk sector editor with it. Fun times and the beginning of a long journey that still continues to this day.