Protection and Syscalls

Question 1

describe Dual Mode Operation

Answer 1

• can distinguish between tasks executed by OS and user
• user mode (ring 3) or kernel mode (ring 0)
• allows privileged instructions
• ISR code executes in kernel mode
• user code must appeal to OS to run privileged instructions using system calls

Question 2

what is a System Call and how is it called

Answer 2

• user programs request system calls
• a syscall traps to OS
• dedicated ISR for syscall interupts
• write the syscall id into a particular register
• execute the instruction

Question 3

Explain user mode to kernel mode transition

Answer 3

• user process calls system call
• mode bit = 0
• parameters pass to registers
• system call is executed
• mode bit = 1
• return from system call

Question 4

What is a system library?

Answer 4

A library of wrapper functions for the system calls to simplify their use. e.g. printf uses the write

Question 5

What happens when a user mode application tries to exe a privileged instruction?

Answer 5

The CPU generates a protection fault (a type of interupt) and traps to the OS to be handled

Question 6

what are the 3 protection mechanism?

Answer 6

dual mode
hardware timers - os regularly interupts user programs to ensure it maintains control of the cpu
memory protection