Shutdown mode has the entire device powered down, and you must externally save all state, including cache and TCM state. The processor is returned to Run mode by the assertion of Reset. This state saving is performed with interrupts disabled, and finishes with a Drain Write Buffer operation. When all the state of the ARM1136JF-S processor is saved the ARM1136JF-S processor executes a Wait For Interrupt instruction. The signal STANDBYWFI is asserted to indicate that the processor can enter Shutdown mode.