4.7.3.6 Interrupts Quiz: AQA Computer Science, Unit 7

The 20 questions

1. What is an interrupt?

   • A command that clears the status register so that all of the flags are reset to their default values for the next task
   • A fault in the system clock that stops every instruction from running until the computer is restarted by the user
   • A message sent from main memory that the processor must store in its general-purpose registers for later use
   • A signal that tells the processor to pause the current task and deal with an event that needs attention

2. What is an interrupt service routine (ISR)?

   • A routine that the processor runs to handle a particular interrupt
   • A routine that disables all interrupts until the computer is next restarted
   • A part of the control unit that generates the clock pulses for the processor
   • A routine that writes the interrupt request into secondary storage for later review

3. Where does the processor typically check for interrupts in the fetch-execute cycle?

   • In the middle of an ALU operation, so that the calculation can be interrupted
   • Only during the decode stage, after the CIR has been loaded with the instruction
   • Only before the first instruction of a program, when the power is switched on
   • After an instruction has finished executing, before the next fetch

4. Why must the processor save the volatile environment when it services an interrupt?

   • So the interrupted program can resume exactly where it left off, with its registers and PC restored
   • So the MBR can hold the ISR's code permanently while the interrupted program waits
   • So the ISR can overwrite the program counter without ever needing to return
   • So the interrupt can be cleared from the status register automatically by the hardware

5. Which items form part of the volatile environment that is saved on an interrupt?

   • The ISR's machine code, which is stored in read-only memory
   • The PC, the status register and the contents of the general-purpose registers
   • The hard disk's directory of files, which is held on the drive
   • The clock circuit and its crystal oscillator, which keep the timing

6. Which event is a typical example of something that generates an interrupt?

   • The program counter is incremented by one after each instruction is fetched
   • A key is pressed on the keyboard, or a timer reaches its set value
   • The ALU completes an addition and stores the result in a register
   • The processor fetches the next instruction from main memory in sequence

7. What is the effect of an interrupt on the normal fetch-execute cycle?

   • The cycle runs twice for every instruction, once to execute and once more to check the interrupt line for any requests
   • The cycle skips the fetch stage for every remaining instruction of the program until the interrupt has been fully cleared
   • The processor diverts to the ISR and then returns to the interrupted point, so the cycle is temporarily changed
   • The cycle stops permanently and does not resume until the user restarts the computer manually from the power switch

8. Which statement about the order of events when an interrupt is handled is correct?

   • The state is restored before the ISR begins so that the ISR can use the previous values
   • The program is restarted from its first instruction after the ISR finishes its work
   • The ISR runs first and the state is saved afterwards in every case, to save time
   • The state is saved, the ISR runs, the state is restored, and the interrupted program continues

9. Why are interrupts useful to a computer system?

   • They remove the need for secondary storage, because data is always kept in memory
   • They let the processor respond to events such as input without continually checking each device
   • They allow every program to run at the same clock speed as the others
   • They make main memory non-volatile, so that data is kept after the power is switched off

10. A keyboard interrupt occurs while a program is running. Which register must be saved so the program can later resume at the right instruction?

    • The hard disk controller's register
    • The clock's crystal frequency register
    • The printer's status register
    • The program counter

11. What is the reason the processor must save the status register during an ISR?

    • Because the status register stores the address of the ISR in main memory so that the processor can jump there directly
    • Because the status register stores the machine code of the ISR so that it can be fetched again the next time it is needed
    • Because flags set by the interrupted program's last operation could otherwise be lost or overwritten
    • Because the status register holds the clock speed that the ISR needs to read before it can safely change any value

12. A computer receives 50 interrupts per second, and each ISR takes 2 ms. What fraction of processor time is spent in ISRs?

    • 2%
    • 100%
    • 50%
    • 10%

13. Why is it important for an ISR to be short?

    • A short ISR lets the program counter skip instructions in the main program safely
    • A long ISR delays the interrupted program and any other interrupts that are waiting to be serviced
    • A short ISR runs from the hard disk instead of main memory, which is faster for the processor
    • Only short ISRs are able to save the status register, so long ones lose flag values

14. What does an ISR typically do once the interrupt has been serviced?

    • Deletes the interrupted program from secondary storage to free up space
    • Sets the clock to zero and waits for the next interrupt before continuing
    • Restores the saved registers and returns control to the interrupted program
    • Clears main memory so that the program can restart from a clean state

15. After a timer interrupt is serviced, which register tells the processor where to resume?

    • The current instruction register, which holds the code of the ISR itself
    • The memory buffer register, which holds the last data item that was read
    • The memory address register, which holds the current value of the timer
    • The program counter, which holds the address of the next instruction

16. Why is the processor's state described as a 'volatile environment' when an interrupt occurs?

    • The registers and flags can change at any time during execution, so they must be saved when the interrupt occurs
    • The environment is stored in ROM, so it cannot be changed by the interrupt at any point
    • The environment is stored in volatile RAM only, so it is lost whenever the power is switched off
    • The environment refers to the operating system's files, which are deleted during each interrupt

17. Which statement about interrupts compared with polling is correct?

    • Interrupts require the processor to check every device on every clock cycle without a break, which uses up processor time
    • Interrupts let a device signal the processor when it needs attention, rather than the processor repeatedly checking it
    • Polling is faster because the processor never checks any device at any point, so no time is spent on checks at all
    • Polling and interrupts are identical methods that differ only in the name given to them by the manufacturers of devices

18. What is the first step when the processor accepts an interrupt?

    • The current state of the processor, such as the PC and the registers, is saved
    • The ISR's code is decoded into the CIR as the first action of the response
    • The status register is cleared to zero before anything else is done by the processor
    • The clock is reset to its initial frequency before any other action is taken

19. An interrupt arrives during the execute stage of an instruction. When is it normally serviced?

    • Immediately, in the middle of the ALU's calculation, without finishing the instruction
    • At the next power cycle, because the execute stage cannot be interrupted at all
    • During the next fetch, after the instruction has been partly decoded by the control unit
    • After the current instruction completes, at the next point where interrupts are checked

20. An ISR uses registers R1 and R2. What must happen to their values in the interrupted program?

    • They are saved before use and restored before the processor returns to the interrupted program
    • They are copied into the status register, where they are held during the ISR
    • They are overwritten and not restored, because the ISR runs in its own separate space
    • They are cleared to zero and then passed to the MBR as data for the ISR

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