What happens as a result of an interrupt

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Quick Questions

Shade in the A or B circle for each question.

1. A user presses a key while a word processor is open. What does the ISR do?

A It reads the character and passes it to the program

B It shuts down the computer and restarts it

2. A user moves the mouse. What is the result of the interrupt?

A The current program is deleted from memory

B The pointer position on the screen is updated

3. A program tries to divide a number by zero. What can the ISR do?

A Return the answer 0 and hide the problem

B Show an error message or stop the faulty program

4. Two processes try to use the same area of memory at once (a memory conflict). What can happen?

A One process is stopped or made to wait

B Both processes overwrite the memory at the same time

5. Once the ISR has finished handling an interrupt, what does the CPU do next?

A Returns to the task it was interrupted from and carries on

B Starts the interrupted program again from the beginning

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1key press interrupt -> ISR reads the character		AISR finished -> restore state -> resume task
2mouse move interrupt -> ISR updates pointer		BThe ISR action is wrong for the event. A divide by zero is dealt with by showing an error message or stopping the faulty program, not by reading a key press.
3Dividing by zero causes an interrupt, and the ISR shows an error message or stops the faulty program		CThe ISR works out the new position and the pointer on the screen is moved to it
4When the ISR has finished, the CPU goes back to the task it was running before the interrupt		DThe ISR reads the key that was pressed and passes the character to the program that is waiting for it
5divide by zero interrupt -> ISR reads a key press		EIF divisor = 0 THEN ISR shows error / stops program
6ISR finished -> CPU stops and never runs the task again		FAfter the ISR the CPU should return to the interrupted task and carry on. It does not stop for good.

Definitions

interrupt outcome

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF event = "key press" THEN character ← _______() passToProgram(character)

IF event = "mouse move" THEN _____________(newX, newY) ENDIF

IF event = "divide by zero" THEN _________("Cannot divide by zero") stopProgram()

IF event = "memory conflict" THEN ___________(faultyProcess) ENDIF

restoreState() ______(interruptedTask)

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A INTERRUPT key_press

B ISR: READ key FROM monitor

C PASS key TO program

D RESUME previous task

Question 2

A INTERRUPT mouse_move

B ISR: SHUT DOWN the computer

C REDRAW pointer on screen

D RESUME previous task

Question 3

A INTERRUPT mouse_move

B ISR: SHOW error message

C STOP faulty program

D CPU RESUMES other tasks

Question 4

A INTERRUPT memory_conflict

B ISR: FIND clashing process

C SPEED UP that process

D RESUME the interrupted task

Question 5

A INTERRUPT key_press

B SAVE state of CPU

C ISR: READ key

D PASS key TO printer

E RESUME interrupted task

Answer Key

What happens as a result of an interrupt

Section A: Quick Questions

A — It reads the character and passes it to the program

B — The pointer position on the screen is updated

B — Show an error message or stop the faulty program

A — One process is stopped or made to wait

A — Returns to the task it was interrupted from and carries on

Section B: Match the Pairs

key press interrupt -> ISR reads the character → The ISR reads the key that was pressed and passes the character to the program that is waiting for it

mouse move interrupt -> ISR updates pointer → The ISR works out the new position and the pointer on the screen is moved to it

Dividing by zero causes an interrupt, and the ISR shows an error message or stops the faulty program → IF divisor = 0 THEN ISR shows error / stops program

When the ISR has finished, the CPU goes back to the task it was running before the interrupt → ISR finished -> restore state -> resume task

divide by zero interrupt -> ISR reads a key press → The ISR action is wrong for the event. A divide by zero is dealt with by showing an error message or stopping the faulty program, not by reading a key press.

ISR finished -> CPU stops and never runs the task again → After the ISR the CPU should return to the interrupted task and carry on. It does not stop for good.

Section C: Definitions

interrupt outcome — What the ISR does depends on the event: it reads a key press, updates the pointer, shows an error or stops a faulty program. Then the CPU resumes the task it was running.

Section D: Fill in the Blanks

IF event = "key press" THEN
  character ← readKey()
  passToProgram(character)

IF event = "mouse move" THEN
  updatePointer(newX, newY)
ENDIF

IF event = "divide by zero" THEN
  showError("Cannot divide by zero")
  stopProgram()

IF event = "memory conflict" THEN
  stopProcess(faultyProcess)
ENDIF

restoreState()
resume(interruptedTask)

Section E: Spot the Error

B — A key press is read from the keyboard, not the monitor. The monitor is an output device, so it cannot supply the character. The line should be ISR: READ key FROM keyboard.

B — Moving the mouse does not need the computer to shut down. The ISR just updates the pointer position, so the line should be ISR: UPDATE pointer position.

A — The ISR here shows an error message and stops the faulty program. That outcome belongs to a divide by zero interrupt, not a mouse move. The line should be INTERRUPT divide_by_zero.

C — With a memory conflict, the clashing process is stopped or made to wait so it stops interfering. Speeding it up would make the clash worse. The line should be STOP or WAIT that process.

D — The character that was read is passed to the program the user is typing into, not to a printer. The line should be PASS key TO program.