Operating System - Dual Mode

Question 1

User Mode

Answer 1

User program executes in user mode

Certain areas of memory are protected from user access

Certain instructions may not be executed

Question 2

Kernel Mode

Answer 2

Monitor executes in Kernel mode

Privileged instructions may be executed

Protected areas of memory may be accessed

Question 3

System Call - Kernel Mode

Answer 3

When we make use of a system call we go into the Kernel Mode

The actual execution of a system calls occurs on the kernel side.

Question 4

What is a System call

Answer 4

A mechanism for programs to request services or resources
i.e file operation, memory management, process control from the operating system’s kernel

Question 5

Kernel mode bit

Answer 5

0

Question 6

User mode bit

Answer 6

1

Question 7

Privileged/ Kernel Mode instructions

Answer 7

Instructions which can only be executed in the Kernel mode

Question 8

Why do we need dual mode?

Answer 8

• Stops user program attempting to execute an illegal instruction or to access the memory of other uses

Question 9

What happens if a user program attempts to execute an illegal instruction or attempts to access the memory of other users

Answer 9

• OS must terminate the program
• OS gives error message
• Produces memory dump file