* All 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.
* The 6502 post-decrements on PHA and pre-increments on PLA, while the Z80 pre-decrements on PUSH and post-increments on POP, making them behave in opposite ways.
* The 6502 uses only one addressing mode per instruction, while the Z80 can combine two different addressing modes within a single instruction.
* 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.