• nop() pass no operation.
• wfi() Suspend execution in a low power state until an interrupt occurs.
• cpsid(flags) set the Priority Mask Register - disable interrupts.
• cpsie(flags) clear the Priority Mask Register - enable interrupts.

Currently the cpsie() and cpsid() functions are partially implemented. They require but ignore the flags argument and serve as a means of enabling and disabling interrupts.