This! Still have fond memories of reverse-engineering the screen-buffer encoding on my Amstrad CPC 464 by poking values directly into the buffer and watching what happened on the display (i.e. my TV). Somehow got hold of the Z80 datasheet for opcodes - but working out the relative jumps with pen an paper was a slow process. Upgraded to Amiga around the time when I realized that assemblers had to be a thing...