|| Flags are affected by most operations.
BF is not a physical flag implemented in a register. It only appears on the stack when the P register is pushed to it.
PHP (Push Processor Status) and PLP (Pull Processor Status) can be used to set or retrieve P directly via the stack.
Interrupts (BRK/NMI/IRQ) implicitly push P to the stack. Interrupts returning with RTI will implicitly pull P from the stack.
|-
| S (Stack Pointer) || 8-bit || Points to the current location in the stack || Stack is located in page 1 ($0100-$01FF), 8-bit S register is offset to this base