* The 16-bit instructions ADD HL,ss, ADC HL,ss and SBC HL,ss exist but not the instruction SUB HL,ss.
* While the syntax of the 8-bit ADD, ADC and SBC instructions all explicitly mention the A register, the SUB instruction does not mention it. On the Zilog eZ80, the SUB instruction explicitly mention the A register.
* The Z80 has a lot of 8-bit registers but it can only really do arithmetic on one of them, : A.
* The IN r,(C) instruction updates flags but IN A,(n) does not.
* IN r,(C) and OUT (C),r instructions syntax is misleading as these instructions actually use the full 16-bit port address contained in BC. On the Zilog eZ80, these instructions are correctly named IN, r,(BC) and OUT (BC),r.