Interrupts

Question 1

What is polling?

Answer 1

• The CPU checks with each device whether it requires attention
• This is in-efficient because if the device does not require attention there was no point checking it

Question 2

What is an interrupt?

Answer 2

• Device sends a single on the control bus to the CPU to indicate that it requires attention

Question 3

What happens with the ISR (Interrupt Service Routine)?

Answer 3

• A stack is used
• At the end of the execution step if there is an interrupt, the contents of the PC are transferred to the bottom of the stack
• The address of the first instruction of the ISR can be moved into the PC
• When the ISR is finished, the original contents of the PC are popped off the stack back into the PC

Question 4

Give two examples of hardware interrupts:

Answer 4

• Power/reset button is pressed

* Memory parity error

Question 5

Give three examples of software interrupts:

Answer 5

• Illegal instruction encountered
• Arithmetic overflow
• New log-on request

Question 6

Give two examples of Input/Ouput interrupts:

Answer 6

• Buffer nearly empty

* Signal the completion of a data transfer to/from a device