← IGCSE Computer Science Paper 1

Interrupt service routine (ISR)

A small program the CPU runs to deal with one particular interrupt. Each type of interrupt has its own ISR, kept in memory, and it is short so the CPU can quickly return to its task.

Part of: IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 57

Understanding Interrupt service routine (ISR) in IGCSE Computer Science Paper 1

What it does: The OS matches the type of interrupt to the correct ISR and then runs it. The routine does only the few steps needed and then hands control straight back to the interrupted task. An ISR is software. It is the code that handles an interrupt. It is not the device or the signal that causes it. ISRs are stored in memory so the CPU can find and run the right one straight away when an interrupt happens. The operating system works out which type of interrupt has happened and picks the matching ISR to run. A short ISR finishes quickly, so the CPU soon goes back to its original task and the computer stays responsive.

Common mistakes: Each type of interrupt has its own ISR. A printer interrupt must run printer_ISR, not the keyboard ISR. An ISR must be short so the CPU can return quickly. It should end with RETURN TO previous_task, not wait for more keys.

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Learning

Duo

Which describes an interrupt service routine (ISR)?

A small program the CPU runs to deal with one particular interruptA device that sends signals to the CPU to ask for attention

Matches

One ISR is used to deal with every type of interrupt

↓

Each type of interrupt has its own ISR, because each one needs different instructions to deal with it.

Applying

Meanings

Term

interrupt service routine (ISR)

Meaning

A small program the CPU runs to deal with one particular interrupt. Each type of interrupt has its own ISR, kept in memory, and it is short so the CPU can quickly return to its task.

Practice

PROCEDURE KeyboardISR()

Testing

Gaps

IF interrupt = KEYBOARD THEN keyISR

Check Answer

Spots

2 KEYBOARD: CALL key_ISR

3 PRINTER: CALL key_ISR

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Quick Questions

1. Which describes an interrupt service routine (ISR)?

A. A device that sends signals to the CPU to ask for attention

B. A small program the CPU runs to deal with one particular interrupt ✓

An ISR is software. It is the code that handles an interrupt. It is not the device or the signal that causes it.

2. How many ISRs are there for the different types of interrupt?

A. One single ISR handles every type of interrupt

B. A different ISR for each type of interrupt ✓

Each type of interrupt, such as a key press or a divide by zero error, needs its own ISR with code suited to that event.

3. Where are ISRs kept so they are ready to run?

A. In memory ✓

B. Only on the keyboard or mouse that causes the interrupt

ISRs are stored in memory so the CPU can find and run the right one straight away when an interrupt happens.

4. Which part of the computer chooses which ISR to run?

A. The operating system ✓

B. The keyboard

The operating system works out which type of interrupt has happened and picks the matching ISR to run.

5. Why is an ISR kept short?

A. So the CPU can return quickly to what it was doing ✓

B. So the ISR can run forever without stopping

A short ISR finishes quickly, so the CPU soon goes back to its original task and the computer stays responsive.

Match the Pairs

ISR → A small program the CPU runs to deal with one particular interrupt

ISR_keyboard() and ISR_printer() → A different ISR for each type of interrupt

The operating system chooses which ISR to run for the interrupt that has occurred → interrupt = printer_error run ISR_printer_error

An ISR is short so the CPU can return quickly to the task it was doing → ISR_keypress read key store in buffer return to task

One ISR is used to deal with every type of interrupt → Each type of interrupt has its own ISR, because each one needs different instructions to deal with it.

The ISR is a long program that takes a long time to finish before the CPU returns to its task → An ISR should be short so the CPU can return quickly to what it was doing.

Key Term

interrupt service routine (ISR)

A small program the CPU runs to deal with one particular interrupt. Each type of interrupt has its own ISR, kept in memory, and it is short so the CPU can quickly return to its task.

Fill in the Blanks

1. IF interrupt = KEYBOARD THEN CALL keyISR

Hint: Type the keyword that runs the routine written for this type of interrupt

2. IF interrupt = MOUSE THEN CALL mouseISR

Hint: Type the interrupt type that matches the ISR being called

3. DECLARE ISRtable : ARRAY[1:5] OF STRING

Hint: Type the data structure keyword used to keep a list of ISRs in memory, ready to run

4. interruptType ← GetInterrupt() CALL ISRtable[interruptType]

Hint: Type the variable that holds which kind of interrupt happened, so the OS can choose the right ISR

5. PROCEDURE mouseISR() OUTPUT "Mouse clicked" ENDPROCEDURE

Hint: Type the keyword that closes this short routine so the CPU can return quickly

Spot the Error

Question 1

A CASE OF interrupt

B KEYBOARD: CALL key_ISR

C PRINTER: CALL key_ISR

D ENDCASE

Line C — Each type of interrupt has its own ISR. A printer interrupt must run printer_ISR, not the keyboard ISR.

Question 2

A PROCEDURE keyboard_ISR()

B READ key FROM keyboard

C ADD key TO input_buffer

D WAIT FOR next_key

E ENDPROCEDURE

Line D — An ISR must be short so the CPU can return quickly. It should end with RETURN TO previous_task, not wait for more keys.

Question 3

A STORE ISR_table IN HARD_DISK

B table[KEYBOARD] ← key_ISR

C table[MOUSE] ← mouse_ISR

Line A — ISRs are kept in memory (RAM) so they are ready to run straight away. A hard disk is too slow to fetch them from.

Question 4

A PROCEDURE timer_ISR()

B WAIT 10 SECONDS

C RESET timer

D ENDPROCEDURE

Line B — An ISR should be short so the CPU can quickly return to its task. Waiting 10 seconds would hold the CPU up. It should be clock ← clock + 1.

Question 5

A IF interrupt = DIVIDE_BY_ZERO THEN

B CALL error_ISR

C ELSE IF interrupt = KEYPRESS

D CALL key_ISR

E ENDIF

Line C — The condition is missing THEN. It should be ELSE IF interrupt = KEYPRESS THEN, so the correct ISR is chosen for each interrupt type.

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