Yeah me too in 1982, using the Melbourne House Z80 reference, aged a young 10 years old. Working with POKE and no macro-assembler, I wrote mnemonics then translated them to machine-code by hand. A baptism of fire that to this day that I've not forgotten.

This book was the ignition that changed my life... https://archive.org/details/z-80-reference-guide-alan-tullya...