Changes

Instructions PHA and PHP push the accumulator and processor status onto the stack, while PLA and PLP pull them back. Subroutine calls with JSR store the return address on the stack, and RTS retrieves it to continue execution. Similarly, interrupts (BRK) push the program counter and status, while RTI restores them. The stack has a 256-byte limit, and overflow occurs if not managed properly.

On a RESET, the CPU loads the vector from $FFFC/$FFFD into the program counter and continues fetching instructions from there.