9.3 Interrupts

Question 1

What is an interrupt

Answer 1

Async signal. Ie not occuring at specified time, hence special mechanism to deal with

Question 2

How does the CPU detect interrupts

Answer 2

• There is a special interrupt input wire
• Interrupt checking step in instruction cycle (usually just before execute)
• External interrupt controller used to prioritise interrupts

Question 3

How does the CPU handle interrupts (ie what steps does it need to do)

Answer 3

• Store PC and data on stack
• ‘Jump’ to interrupt handler
• Return back to current program.

Question 4

What architectural support can be put in place for interrupts

Answer 4

• SPC register (save program counter)
• Pre-defined interrupt addresses
• Status bit for interrupts

Question 5

What is the relation between the kernel and the wider operating system.

Answer 5

Provides core functionality for OS, scheduling IO, system calls. Is itself a program.

Question 6

What are software interrupts

Answer 6

Like hardware interrupts but controlled by kernel. System calls etc.

Question 7

What are timing interrupts

Answer 7

Time-slicing to share processor between multiple programs

Question 8

How does the kernel scheduler work

Answer 8

• Has queue of programs to schedule
• When programs need to await something can start another
• Otherwise time-slicing, interrupts etc.
• When program is rescheduled is placed at back of queue

Question 9

What are some example interrupts for the kernel scheduler

Answer 9