The [[GBZ80]] (Sharp SM83) that powers the original [[Nintendo GameBoy]] is 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 Nintendo documentation does not mention M-cycles or T-states at all. They only mention CPU cycles, which are always equal to 4 T-states (like NOPs in the CPC world). Also, the GBZ80 has different timings than the Z80. For example, CALL nn takes 6 cycles on the GBZ80, but only 5 NOPs on the Z80. [https://archive.org/details/GameBoyProgManVer1.1/ Gameboy programming manual]
* The GBZ80 lacks the alternate register set, the dedicated I/O bus, the R register, 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]
* 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. [https://www.chibiakumas.com/z80/Gameboy.php Learn GBZ80 Assembly Programming with ChibiAkumas]
The Sega Mega Drive, Master System and Game Gear use a standard Z80 CPU. [https://drive.google.com/file/d/1v7H1rwjmYNSpvxWAp74wVIU15RyR0Dry/ Genesis sound software manual] [https://segaretro.org/images/d/d6/SoftwareReferenceManualForSegaMarkIIIEU.pdf Master System reference manual] [https://segaretro.org/images/1/16/Sega_Game_Gear_Hardware_Reference_Manual.pdf Game Gear reference manual]