* The Nintendo documentation does not mention M-cycles or T-states at all. They only mention CPU cyclesGBZ80 lacks the alternate register set, which are always equal to 4 T-states (like NOPs in the CPC world). Alsodedicated I/O bus, the GBZ80 has different timings than R register, the Z80. For exampleindex registers (thus no DD and FD prefixed opcodes), CALL nn takes 6 cycles on the GBZ80ED prefixed opcodes (including block transfer), but only 5 NOPs on the Z80sign and parity/overflow flags (and all conditional instructions that used them), the undocumented flags (thus no leaking of WZ and Q internal registers). [https://archivewww.orgpastraiser.com/detailscpu/GameBoyProgManVer1.1gameboy/ Gameboy programming manualgameboy_opcodes.html GBZ80 opcodes]
* The GBZ80 also lacks the alternate register set, the dedicated I/O bus, the R register, the index registers NMI pin (thus no DD IFF2 and FD prefixed opcodesno RETN), the ED prefixed opcodes (including block transfer), the sign IM instructions and parity/overflow flags (and all conditional instructions that used them), the undocumented flags (thus no leaking of WZ and Q internal registers)I register. It has a different interrupt system than the Z80. [https://wwwgbdev.pastraiser.comio/cpupandocs/gameboy/gameboy_opcodesInterrupts.html GBZ80 opcodesSource]
* The GBZ80 also lacks the NMI pin 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 (thus no IFF2 and no RETNlike NOPs in the CPC world). Also, the IM instructions and the I register. It GBZ80 has a different interrupt system timings than the Z80. For example, CALL nn takes 6 cycles on the GBZ80, but only 5 NOPs on the Z80. [https://gbdevarchive.ioorg/pandocsdetails/InterruptsGameBoyProgManVer1.html Source1/ Gameboy programming manual]
* 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]
