How an interrupt is generated

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Quick Questions

Shade in the A or B circle for each question.

1. What starts the process of generating an interrupt?

A An event happens that needs the CPU's attention

B The CPU deletes the running program

2. Which is the interrupt signal sent to?

A The monitor

B The CPU

3. A key is pressed on the keyboard. Which source generates the interrupt?

A Software (a running program)

B Hardware (the keyboard)

4. A running program tries to divide a number by zero. Which source generates the interrupt?

A Software (the running program)

B Hardware (the keyboard)

5. Worked example: a key is pressed. What is the correct order of the first two steps?

A Key is pressed, then the keyboard sends an interrupt signal to the CPU

B The keyboard sends an interrupt signal to the CPU, then the key is pressed

Match the Pairs

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

Left column		Right column
1interrupt signal		Adivide by zero error -> interrupt signal
2keyboard sends signal		Bkey pressed -> signal -> CPU
3A running program is a software source that can generate an interrupt when an error happens		CAn interrupt is only generated after an event happens, so the signal cannot be sent before the key press.
4The order of events when a key is pressed: event, signal, then the CPU is told		DA signal sent to the CPU by a device or a running program to report an event that needs attention
5CPU -> interrupt signal -> keyboard		EA hardware source generates an interrupt when a key is pressed
6interrupt signal sent before the key is pressed		FThe direction is wrong: the device sends the interrupt signal to the CPU, not the CPU to the device.

Definitions

interrupt signal

Fill in the Blanks

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

event ← "key press" SEND ________________ TO CPU

IF event = "key press" THEN SEND interrupt signal TO ___ ENDIF

device ← keyboard source ← ________

IF divisor = 0 THEN source ← ________ ENDIF

__ event = "key press" THEN SEND interrupt signal TO CPU ENDIF

Spot the Error

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

Question 1

A IF key is pressed THEN

B keyboard creates signal

C SEND interrupt TO printer

D ENDIF

Question 2

A result <- 10 / 0

B error <- TRUE

C source <- 'software'

D SEND interrupt TO the mouse

Question 3

A type <- 'software'

B source <- 'mouse'

C event <- 'button clicked'

D SEND interrupt TO CPU

Question 4

A EVENT: user presses a key

B keyboard detects the key

C SEND interrupt signal TO CPU

D CPU never learns of the event

Question 5

A user presses a key

B keyboard creates a signal

C signal is sent to the screen

D CPU may now handle the key

Answer Key

How an interrupt is generated

Section A: Quick Questions

A — An event happens that needs the CPU's attention

B — The CPU

B — Hardware (the keyboard)

A — Software (the running program)

A — Key is pressed, then the keyboard sends an interrupt signal to the CPU

Section B: Match the Pairs

interrupt signal → A signal sent to the CPU by a device or a running program to report an event that needs attention

keyboard sends signal → A hardware source generates an interrupt when a key is pressed

A running program is a software source that can generate an interrupt when an error happens → divide by zero error -> interrupt signal

The order of events when a key is pressed: event, signal, then the CPU is told → key pressed -> signal -> CPU

CPU -> interrupt signal -> keyboard → The direction is wrong: the device sends the interrupt signal to the CPU, not the CPU to the device.

interrupt signal sent before the key is pressed → An interrupt is only generated after an event happens, so the signal cannot be sent before the key press.

Section C: Definitions

interrupt signal — A signal sent to the CPU by a device (hardware) or a running program (software) when an event, such as a key press, needs the CPU's attention.

Section D: Fill in the Blanks

event ← "key press"
SEND interrupt signal TO CPU

IF event = "key press" THEN
    SEND interrupt signal TO CPU
ENDIF

device ← keyboard
source ← HARDWARE

IF divisor = 0 THEN
    source ← SOFTWARE
ENDIF

IF event = "key press" THEN
    SEND interrupt signal TO CPU
ENDIF

Section E: Spot the Error

C — An interrupt signal is sent to the CPU, not to the printer. The line should be SEND interrupt TO CPU.

D — The running program reports its error by sending the interrupt signal to the CPU. The mouse does not receive it. It should be SEND interrupt TO CPU.

A — A mouse is a hardware device, so the interrupt it generates is a hardware interrupt. It should be type <- 'hardware'.

D — The interrupt signal exists to tell the CPU about the event so it can give it attention. The CPU is alerted, not left unaware.

C — The interrupt signal from the keyboard goes to the CPU, not to the screen. It should be signal travels to the CPU.