The Sinclair ZX81 manual had the full list of Z80 opcodes in it - Appendix A in http://zxnext.narod.ru/manuals/ZX81_Manual.pdf http://zxnext.narod.ru/manuals/ZX

The Sinclair ZX81 manual had the full list of Z80 opcodes in it - Appendix A in http://zxnext.narod.ru/manuals/ZX81_Manual.pdf http://zxnext.narod.ru/manuals/ZX81_Manual.pdf . So with that you could start PEEKing and POKEing and experimenting. I never really did any Z80 coding on the ZX81 itself, but that section of the manual came in really useful when I got an Amstrad CPC, which is what I learned Z80 on. To that I added The Ins & Outs of the Amstrad (https://cpcrulez.fr/book_english-the_ins_and_outs_of_the_amstrad.htm https://cpcrulez.fr/book_english-the_ins_and_outs_of_the_ams...), which documented the firmware and other CPC-specific stuff.