Question

The use of which of the following AVR instructions within an interrupt service routine would necessitate saving the status register at the beginning of the routine? a. reti b. in temp, PIND c. rcall routine d.sts $123, r6 e. None of the above

Could you please explain the answer besides simply giving it?

Answer 1

Answer)

Option A) is correct.

Explanation:-

RETI: Returns from interrupt service program. The return address from the called location is loaded from the Stack and the global interrupt flag bit in the status register is set.

Note that the status register flag bits are not automatically stored, before entering an interrupt service routine, and it is not restored back when returning from an interrupt routine.

This step of storing and restoring into stack the status register, must be handled by the application program. The stack pointer uses a pre-increment scheme during RETI.

Example :-

extint1: push r0 ; Save status register r0 on the stack
...
pop r0 ; Restore status register r0
reti ; Return and enable interrupts if any