* Conditional jumps are only 8-bit relative. And unconditional jumps are only 16-bit absolute.
* LDX absolute,Y and LDY absolute,X instructions exist but not the corresponding instructions STX absolute,Y and STY absolute,X do not.
* The CLV (Clear Overflow Flag) instruction exist but not the SEV (Set Overflow Flag) instruction does not.
* The ROR instruction didn't exist in the very earliest (pre-1977) chips.