The CPU checks for interrupts after each cycle

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Quick Questions

Shade in the A or B circle for each question.

1. When does the CPU check whether an interrupt signal has arrived?

A Only when the computer is switched off

B At the end of each fetch-decode-execute cycle

2. The CPU is halfway through executing an instruction when an interrupt signal arrives. What happens?

A The CPU finishes the current instruction, then checks and handles the interrupt

B The CPU stops mid-instruction and handles the interrupt straight away

3. At the end of a cycle the CPU finds NO interrupt signal. What does it do next?

A It fetches the next instruction and carries on as normal

B It stops and waits until an interrupt arrives

4. At the end of a cycle the CPU finds an interrupt signal. What does it do before fetching the next program instruction?

A It switches to handling the interrupt

B It ignores the interrupt and fetches the next instruction

5. Which statement about the interrupt check is correct?

A It is only done when the user presses a key

B It is done once every cycle, whether or not an interrupt has occurred

Match the Pairs

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

Left column		Right column
1IF interrupt_signal = TRUE THEN		AIF interrupt found THEN handle interrupt BEFORE next fetch
2ELSE fetch next instruction		BNo interrupt is present, so the CPU carries on as normal with the next fetch-decode-execute cycle
3The CPU only checks for an interrupt after the current instruction has finished, at the end of the cycle		Cfetch, decode, execute, THEN check interrupt
4If an interrupt is found during the check, the CPU switches to handling it before fetching the next instruction		DAt the end of a cycle, the CPU checks whether an interrupt has arrived and, if it has, switches to handling it
5check for interrupt, then fetch, decode, execute		EThe interrupt check is placed at the start of the cycle. The CPU checks at the END of each cycle, after the current instruction has been executed.
6No interrupt found, so the CPU stops and waits		FIf there is no interrupt, the CPU does not stop. It carries on as normal and fetches the next instruction.

Definitions

interrupt check

Fill in the Blanks

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

execute() __ interrupt = TRUE THEN handle_interrupt()

execute() IF interrupt = ____ THEN handle_interrupt()

IF interrupt = _____ THEN fetch_next_instruction() ENDIF

decode() execute() _______________()

IF interrupt THEN handle() ____ fetch()

Spot the Error

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

Question 1

A REPEAT

B FETCH instruction

C DECODE and EXECUTE

D IF no interrupt THEN run ISR

E UNTIL power off

Question 2

A FINISH current instruction

B SKIP the interrupt check

C IF none THEN FETCH next

Question 3

A Interrupt signal arrives

B CPU finishes the instruction

C CPU ignores the signal

Question 4

A IF no interrupt THEN

B FETCH next instruction

C ELSE ignore the interrupt

D ENDIF

Question 5

A AT END OF the whole program

B CHECK interrupt signal

C IF none THEN carry on

Answer Key

The CPU checks for interrupts after each cycle

Section A: Quick Questions

B — At the end of each fetch-decode-execute cycle

A — The CPU finishes the current instruction, then checks and handles the interrupt

A — It fetches the next instruction and carries on as normal

A — It switches to handling the interrupt

B — It is done once every cycle, whether or not an interrupt has occurred

Section B: Match the Pairs

IF interrupt_signal = TRUE THEN → At the end of a cycle, the CPU checks whether an interrupt has arrived and, if it has, switches to handling it

ELSE fetch next instruction → No interrupt is present, so the CPU carries on as normal with the next fetch-decode-execute cycle

The CPU only checks for an interrupt after the current instruction has finished, at the end of the cycle → fetch, decode, execute, THEN check interrupt

If an interrupt is found during the check, the CPU switches to handling it before fetching the next instruction → IF interrupt found THEN handle interrupt BEFORE next fetch

check for interrupt, then fetch, decode, execute → The interrupt check is placed at the start of the cycle. The CPU checks at the END of each cycle, after the current instruction has been executed.

No interrupt found, so the CPU stops and waits → If there is no interrupt, the CPU does not stop. It carries on as normal and fetches the next instruction.

Section C: Definitions

interrupt check — At the end of each fetch-decode-execute cycle the CPU checks for an interrupt signal. If none, it fetches the next instruction; if one, it switches to handling it first.

Section D: Fill in the Blanks

execute()
IF interrupt = TRUE THEN
  handle_interrupt()

execute()
IF interrupt = TRUE THEN
  handle_interrupt()

IF interrupt = FALSE THEN
  fetch_next_instruction()
ENDIF

decode()
execute()
check_interrupt()

IF interrupt THEN handle()
ELSE
  fetch()

Section E: Spot the Error

D — The CPU only switches to the interrupt service routine when an interrupt IS present. The line should be IF interrupt THEN run ISR.

B — The CPU checks for an interrupt signal at the end of every cycle, so it must not skip the check. The line should be CHECK for interrupt signal.

C — After the current instruction has finished, the CPU checks the interrupt signal and does not ignore it. The line should be CPU then checks the signal.

C — If an interrupt is present, the CPU switches to handling it before fetching the next instruction. It does not ignore it. The line should be ELSE handle the interrupt.

A — The interrupt check happens at the end of each fetch-decode-execute cycle, not only when the whole program ends. The line should be AT END OF each cycle.