6. Limited Direct Execution

Question 1

What are the challenges in virtualisation?

Answer 1

Performance and control

Question 2

What is direct execution?

Answer 2

Running the program directly on the CPU

Question 3

How to prevent programs from running malicious actions?

Answer 3

Introduce privilege levels which are user mode and kernel mode.

Question 4

What happens when a system call is performed?

Answer 4

1. Program execute a special trap instruction
2. The trap handler will raise privilege to kernel mode and execute the privileged operations
3. The OS will call a special return-from-trap instruction that returns to the user program while reverting back to user mode

Question 5

How and when are the trap handlers defined?

Answer 5

The kernel will set up a trap table during boot time that configures all trap handlers.

Question 6

How are system calls performed in the low level?

Answer 6

The user must specify the specific system-call number in the register or the stack. The mandatory information must also be supplied.

Question 7

What are the approaches when switching between processes?

Answer 7

Cooperative: The OS waits for processes to give back control (yield) or to perform an illegal operation.
Non-cooperative approach: Timer interrupt

Question 8

What is the solution when the program is running an infinite loop in the cooperative approach?

Answer 8

Reboot the machine

Question 9

Which component saves the state of the running process when an interrupt occurs?

Answer 9

Hardware

Question 10

What is a context switch?

Answer 10

A mechanism to switch between processes for execution

Question 11

What is the kernel stack for?

Answer 11

It is a per-process stack to store key information that will be used by the kernel. For example, the OS will restore information into registers after reverting to user mode.

Question 12

What happens during a context switch?

Answer 12

The OS will save the registers, PC and kernel stack pointer of the current process. To perform the switch, it will restore the registers, PC and kernel stack of the next process.

Question 13

What are the two types of register saves that happen in the OS?

Answer 13

1. Timer interrupt will trigger the hardware to implicitly save all the user registers into the kernel stack.
2. A context switch will trigger the OS to explicitly save all the kernel registers into the process structure block

Question 14

What are the two types of handlers that must be established during boot time?

Answer 14

Trap handlers and interrupt handlers.

Question 15

What does limited direct execution do?

Answer 15

1. Create privilege mode (limited access)
2. Introduce timer interrupts (limited time)