The [[GBZ80]] (Sharp SM83) that powers the original [[Nintendo GameBoy]] is an in-between the [[Intel 8080]] and Z80. [https://gbdev.io/pandocs/CPU_Comparison_with_Z80.html Source] [[Media:GBCPUman.pdf|Game Boy CPU Manual]] [https://gekkio.fi/files/gb-docs/gbctr.pdf Game Boy complete technical reference] [https://youtu.be/GBYwjch6oEE Reverse engineering the Game Boy hardware] [https://youtu.be/HyzD8pNlpwI The Ultimate Game Boy Talk] [https://emudev.de/ Emudev (gb-si)] [https://github.com/c-sp/game-boy-test-roms Game Boy test roms]
The GBZ80 lacks the alternate register set, the NMI pin, the dedicated I/O bus, the NMI pin, the I and R registers, the index registers (DD and FD prefixed opcodes are missing), ED prefixed opcodes (including block transfer), the sign and parity/overflow flags (and all conditional instructions that used them), the undocumented flags (thus no leaking WZ register). [https://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html GBZ80 opcodes]
Fun fact: Way more GBZ80 cores were produced for Gameboy hardware (118 million Gameboys and 81 million GBA) than all the Z80 chips produced for home computers and game consoles.