* The Z80 pre-decrements on PUSH and post-increments on POP, while the 6502 post-decrements on PHA and pre-increments on PLA, making them behave in opposite ways.
* The 6502 saves flags automatically during interrupts; while the Z80 requires PUSH AF and POP AF.
* The 6502’s Decimal (BCD) mode automatically adjusts ADC and SBC results, while the Z80 requires a DAA instruction after each BCD addition and subtraction.
* ADC is the only command for addition. To perform an addition without carry, the carry flag must be cleared manually first. Same with SBC for subtract.