* CALL nn takes 6 cycles on the GBZ80, but 5 NOPs on the Z80
* ADD HL,ss takes 2 cycles on the GBZ80, but 3 NOPs on the Z80
* JP cc,nn has different timings depending on whether the jump is taken. This is not the case on Z80.
Flags can differ too:
* Arithmetic can only really be done on the A register.
* PUSH and POP instructions utilize a 16-bit operand and the high-order byte is always pushed first and popped last. PUSH HL is PUSH H then L. POP HL is POP L then H.
* JP cc,nn has different timings depending on whether the jump is taken. This is not the case on Z80.
* ADD SP,e takes 4 cycles, while LDHL SP,e takes only 3 cycles.