The GBZ80 (Sharp SM83) is the CPU that powers the original Nintendo Gameboy and Gameboy Color handheld consoles. It is kind of an in-between the [[Intel 8080]] and [[Z80]]. [https://gbdev.io/resources.html Awesome Gameboy resources] [https://gbdev.gg8.se/wiki/ GBDev wiki] [https://emudev.de/ Emudev (q00.gb)]
The GBZ80 lacks the alternate register set, the dedicated I/O bus, the R register(thus no M1), the index registers (thus no DD and FD prefixed opcodes), the 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 of WZ and Q internal registers). [https://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html GBZ80 opcodes]
The GBZ80 also lacks the NMI pin (thus no IFF2 and no RETN), the IM instructions and the I register. It has a different interrupt system than the Z80. [https://gbdev.io/pandocs/Interrupts.html Source]