Protection

Question 1

Why is protection needed?

Answer 1

Must prevent programs from causing disruption from each other when sharing HW & resources

Question 2

What is dual mode operation?

Answer 2

Allows us to differentiate between OS code and User code using rings.

Allows the concept of privileged instructions

Question 3

What ring is user mode?

Answer 3

Ring 3

Question 4

What ring is kernel mode?

Answer 4

Ring 0

Question 5

How is dual mode implemented/differentiated?

Answer 5

The mode-bit in hardware shows what mode the instruction is to be implemented as.

Question 6

What mode is the hardware in at boot time?

Answer 6

Starts in kernel mode so that the operating system can be loaded. This then starts user processes in user mode

Question 7

How can a switch be made from user to kernel mode?

Answer 7

On interrupt, Interrupt Service Routine code executes in kernel mode. Mode-bit must be reset before returning control

Question 8

What does the Hardware Timer do?

Answer 8

Generate regular interrupts in order to ensure OS maintains control of the CPU.

At each interrupt, ISR decides whether or not to return control to the current user program.

Question 9

How are Privileged operations carried out?

Answer 9

In order to carry out kernel-mode operations, need to use system calls in order to appeal to the Operating system to allow privileged operation