Question

Why RETI instruction be the last instruction of ISR?

(a) because it returns with carry

(b) because it returns to the main program with all the flags of the SREG raised

(c) because it returns to the main program where interrupt is generated and set the global interrupt enable bit in SREG

(d) none of the mentioned

The question was asked in final exam.

The above asked question is from Interrupt Programming topic in section AVR Microcontroller of Microcontroller

Answer 1

The correct choice is (c) because it returns to the main program where interrupt is generated and set the global interrupt enable bit in SREG

To explain: When an interrupt occurs, the global interrupt enable bit is cleared. If global interrupt enable is cleared it means interrupts are disabled. RETI instruction must last instruction of ISR because it returns to the main program where interrupt is generated and sets the global interrupt enable bit in SREG.