CPU Modes

Question 1

The 2 CPU Modes

Answer 1

Kernel and User Mode

Question 2

How are CPU modes implemented?

Answer 2

Using protection rings (privilege levels).
Lower numbered rings have higher privilege.

Question 3

Why are protection rings needed?

Answer 3

1. Fault isolation (handled in more privileged ring)
2. Privileged instructions
3. Privileged memory space

Question 4

When is Kernel mode used?

Answer 4

1. Faults are captured and handled by the kernel
2. Issuing privileged instructions (i.e. disk I/O)
3. Accessing kernel memory space

Question 5

Privileged Instruction Examples

Answer 5

• I/O operations!!
• Enable/disable interrupts
• Changing from kernel to user mode

Question 6

System Call

Answer 6

Call to the kernel to execute a privileged instruction

Question 7

PC

Answer 7

Program Counter.
Register that stores the address of the instruction to be executed.
Incremented after fetching an instruction.
Can only be incremented by Kernel code.

Question 8

Process for Linux System Call

Answer 8

1. Privileged instruction is encountered (printf)
2. System call type is stored in register (%eax = sys_write;)
3. Interrupt is sent (0x80)
4. Enters Kernel mode
5. Interrupt code is used to look up handlers in IDT, Interrupt descriptor table
6. Previously saved sys call is used to look up code in previously found handler table (syscalls[%eax])
7. Needed code is run

Question 9

What is interrupt code for a system call?

Answer 9

0x80

Question 10

strace

Answer 10

Intercepts and records process’s system calls and the process’s received signals

Question 11

ltrace

Answer 11

Intercepts and records executed process’s dynamic library calls and the process’s received signals

Question 12

Hypervisor

Answer 12

VMM (Virtual Machine Monitor) that runs and manages virtual machines.
Has its own separate permission ring that has the highest privileges.