A printer runs out of paper, a key is pressed, a program tries to divide by zero. The processor cannot keep checking every device, so devices and programs send it a signal instead. Learn what causes an interrupt, when the processor notices it and exactly how it is handled.
An interrupt is a signal from a device or a program to the processor, showing that something needs attention. Interrupts are checked at the end of each fetch-execute cycle. If there is one with a higher priority than the current task, the processor saves the contents of its registers (including the PC) on the stack, loads the start address of the interrupt service routine (ISR) into the PC and runs it. When the ISR finishes, the saved values are restored and the interrupted task carries on from where it stopped.
Fig. 1: the processor leaves the main program, runs the ISR, then returns
Section 01
Why interrupt?
Without interrupts, the processor would have to keep checking every device to see whether it needed anything, wasting time. With interrupts, devices tell the processor when they need it, so it can get on with other work in the meantime.
Source
Examples of causes
Hardware
A key is pressed or the mouse moves; a printer is out of paper or has finished printing; data has arrived at the network card; power is failing.
Software
A run-time error such as division by zero or an attempt to access memory the program does not own; a program requests an operating system service.
Timer
The clock signals that a program's time slice is over, so the operating system can switch to another process.
User
The user presses a key combination to stop or break into a program.
Your turn
Give one application of interrupts in a multitasking operating system.
A timer interrupt ends each process's time slice, so the operating system can switch the processor to another process and several programs appear to run at once.
Examiner trap
An interrupt is a signal to the processor. Do not describe it as "an error": many interrupts, such as a key press or a finished print job, are normal events.
Section 02
When and how it is handled.
In the lab, start an interrupt, then fire a higher priority one while the first ISR is running, and watch what is saved on the stack. The handling steps below are the ones the mark scheme looks for.
The processor finishes the current fetch-execute cycle, then checks for an interrupt (by checking the interrupt flags in the status register).
If there is an interrupt, its priority is compared with the current task. A lower priority interrupt waits; a higher priority one is serviced.
The contents of the registers, including the PC, are saved on the stack.
The start address of the ISR is loaded into the PC, and the ISR is executed to deal with the cause of the interrupt.
While the ISR runs, an interrupt of even higher priority can interrupt it in the same way.
When the ISR has finished, the saved values are restored from the stack, and the interrupted program continues from where it left off. Further pending interrupts are then checked.
Examiner trap
"The computer stops what it is doing" earns nothing. Say that the registers are saved (on the stack) so the task can be resumed later, and that the ISR is executed.
Stage 1 · Learn
Check what the sim just showed you
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What is an interrupt?
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When does the processor check for interrupts?
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In the lab, what happens to the PC when an interrupt is accepted?
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Which is a software interrupt?
Section 03
Priorities in action.
Questions often describe two or three interrupts arriving close together and ask for the order in which they are serviced. Use the priorities, and remember that an interrupt is only noticed at the end of a cycle.
Worked example Order of servicing
A program is running. The printer sends a "finished" interrupt (priority 1, lowest). While its ISR runs, a key press interrupt (priority 2) arrives, and then a power failure interrupt (priority 5, highest) arrives during the key press ISR. Describe what happens.
At the end of a cycle the printer interrupt is detected. It has higher priority than the program, so the program's registers are saved on the stack and the printer ISR starts.
The key press (priority 2) is higher than the printer ISR (1), so at the end of a cycle the printer ISR's registers are saved and the key press ISR runs.
The power failure (priority 5) interrupts the key press ISR in the same way and is serviced at once, for example by saving data before the power goes.
If the power holds, each finished ISR restores the values from the stack in reverse order: key press ISR, then printer ISR, then the program.
Answer: Power failure first, then key press, then printer, then the program resumes
Your turn
Why is a stack a suitable structure for saving register contents when interrupts can be nested?
A stack is last in, first out: the most recently interrupted task is the first one to be restored, which is exactly the order needed to return through nested ISRs.
Stage 2 · Exam
Exam-style questions
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An interrupt arrives with a lower priority than the ISR currently running. What happens?
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Why are the register contents saved before the ISR runs?
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What is loaded into the PC so that the ISR runs?
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Which is an application of a timer interrupt?
Skill unlocked
Interrupts, mastered.
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