The “Return from Interrupt” instruction (RETFIE) is used to
mark the end of an ISR.
When the RETFIE 1 instruction is executed, the PC is loaded
with the saved PC value from the top of the PC stack. Saved context is also restored
with the execution of this instruction. Thus, execution returns to the state of
operation that existed before the interrupt occurred.
When the RETFIE 0 instruction is executed, the saved
context is not restored back to the registers.