Changes
/* Memory Access */
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.
=== BRK/IRQ/NMI/RESET ===
On a RESET, the CPU loads the vector from $FFFC/$FFFD into the program counter and continues fetching instructions from there.
On an NMI, the CPU pushes the low byte and the high byte of the program counter as well as the processor status onto the stack, disables interrupts and loads the vector from $FFFA/$FFFB into the program counter and continues fetching instructions from there.
On an IRQ, the CPU does the same as in the NMI case, but uses the vector at $FFFE/$FFFF.
On a BRK instruction, the CPU does the same as in the IRQ case, but sets bit #4 (B flag) in the copy of the status register that is saved on the stack.
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