>>I once wrote my own printer driver for ZX... ok, copied what was there, and reworked it from ZX's thermal printer to Seikosha's 7-dot matrix printer. It even worked with COPY command - which was the best misnomer ever :). And it still fit into the 256 byte printer buffer.

The TRS-80 initially came with just 4K RAM; the Level II had a 16K option or you could purchase the mighty expansion interface to take memory to 48K. So every byte counted. I wrote a Forth system that used 1-byte Z80 RST Vectors that on the TRS-80 would translate to a 3-byte memory jump. Ended up with code smaller than the equivalent ASM, but also slower unfortunately- you could see screen repaints caused by the slow RST overhead. And then all such concerns were rendered obsolete by the PC with its essentially infinite 640K memory, clearly more than you'll ever need.

FWIW, DJNZ decremented the 8-bit B register towards 0 for relatively quick loops. This left 16-bit HL and DE free for the good stuff. Amazing to think that the quite good ZBasic that came on ROM for the TRS-80 eventually all boiled down to a handful of registers and a stack.