Lecture 2: Virtualization and system calls

Question 1

Layers

Answer 1

Applications
|
Libraries
|
Kernel
|
hardware

Question 2

What is a process and how they work

Answer 2

It’s a running instance of a program

Gives the machine an illusion that it has the whole machine to itself through the use of Virtual memory addresses

Question 3

System calls

Answer 3

A system call API is an interface that makes OS services available to user programs

Question 4

What is an exception?
How does the OS handle it?

Answer 4

an exception is the transfer of control to the os kernel in response to some event

The kernel gets a corresponding exception handler to process the exception

Question 5

System call numbers

Answer 5

To associate each system call with a number, the kernel uses the system call dispatch table, which is stored in the sys_call_table array

Question 6

Control flow & altering the control flow

Program state?
System state?

Answer 6

From start to shutdown, the cpu executes a sequence of instructions one at a time.

We can alter the control flow using
- Jumps and Branches
- Call and Return
These react to changes in a program’s state

Not sufficient for changes in system state

So the system needs a mechanism for exception control

Hi-level
- exceptions
low-level
- process context switch

Question 7

Synchronous and Asynchronous exceptions

Answer 7

Asynchronous: Caused by events external to the processor
- Timer interrupt
- I/O interrupt

Synchronous: Caused by executing an instruction
- Traps: intentional
- Faults: unintentional but recoverable
- Aborts: Unintentional and unrecoverable

Question 8

Fault examples

Answer 8

Page Fault: User writes to memory location
Invalid Memory reference fault: sends SIGSEGV to the user process, and the user process exits with a segmentation fault (accessing in invalid index in an array)