 Preserving the MPU state means that the ISR must assure that whatever values were in the MPU registers at the time of the interrupt are there when the ISR terminates.A part of the preservation process is automatically handled by the MPU when it acknowledges the interrupt, as it will push the program counter (and program bank in the 65C816/65C802) and status register to the stack prior to executing the ISR.At the completion of the ISR, when the instruction is executed, the MPU will reverse the process.