|| Flags are affected by most operations.
DF is unchanged following an interrupt of any kind. This can cause unexpected bugs in the interrupt handler if Decimal Mode is enabled when an interrupt occurs.
Also, DF is not defined after RESET.
|-
| S (Stack Pointer) || 8-bit || Points to the current location in the stack || Stack is located in page 1 ($0100-$01FF), 8-bit S is offset to this base