* 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 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.
* When an LDxR / CPxR / INxR / OTxR instruction is interrupted, the interrupt handler sees some flags in a different state. [https://github.com/hoglet67/Z80Decoder/wiki/Undocumented-Flags#interrupted-block-instructions Source]