* On NMOS, an indirect JMP will behave unexpectedly when the indirect address crosses a page boundary, because the 6502 does not add the carry to calculate the address of the high byte. For example, JMP ($19FF) will use the contents of $19FF and $1900 for the JMP address. On CMOS, this issue was fixed, at the cost of 1 additional cycle. In our example, JMP ($19FF) will use the contents of $19FF and $2000 for the JMP address.
* Some instructions, particularly those involving branches or indexed addressing modes, incur an extra cycle if the processor has to cross a memory page boundary. This is problematic for time-sensitive code.
* Conditional jumps are only 8-bit relative. And unconditional jumps are only 16-bit absolute.
* 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.
* The CLV (Clear Overflow Flag) instruction exist but not the SEV (Set Overflow Flag) instruction.
* The NOP instruction takes 2 full-cycles. This is the minimum amount of cycles an instruction can take. It is necessary because, while the instruction itself does nothing, it still has to increment the 16-bit PC register.
* The alternate NOPs are not created equal. Some have one- or two-byte operands (which they don't do anything with), and they take different amounts of time to execute.
* The 6502 post-decrements on PHA 6502’s Decimal (BCD) mode automatically adjusts ADC and pre-increments on PLASBC results, while the Z80 pre-decrements on PUSH requires a DAA instruction after each BCD addition and post-increments on POP, making them behave in opposite wayssubtraction.
* The 6502 uses only one addressing mode per instruction, while the Z80 can combine two different addressing modes within a single instruction.
* The 6502 post-decrements on PHA and pre-increments on PLA, while the Z80 pre-decrements on PUSH and post-increments on POP.
* 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.
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