• 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.
• mrs(Rd, special_reg) Rd = special_reg copy a special register to a general register. The special register may be IPSR (Interrupt Status Register) or BASEPRI (Base Priority Register). The IPSR provides a means of determining the exception number of an interrupt being processed. It contains zero if no interrupt is being processed.

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.