Suppose a program is running on a non-pipelined single processor computer system. The computer is connected to an external device that can interrupt the processor asynchronously. The processor needs to execute the interrupt service routine (ISR) to serve this interrupt. The following steps (not necessarily in order) are taken by the processor when the interrupt arrives:
(i) The processor saves the content of the program counter.
(ii) The program counter is loaded with the start address of the ISR.
(iii) The processor finishes the present instruction.
Which ONE of the following is the CORRECT sequence of steps?
Answer
(A)
Explanation
A non-pipelined processor checks for interrupts only between instructions, so it first completes the instruction it is executing (iii). Next it saves the program counter (i), which now holds the address of the next instruction, so the interrupted program can continue from there once the ISR returns. Only then is the program counter loaded with the ISR's start address (ii); doing (ii) before (i) would destroy the return address.
Answer: A