Lecture 4 - Processes

Question 1

Why are processes useful?

Answer 1

Makes full utilisation of the CPU

If processes with multiprogramming didn’t exist then each job would have sole control of the computer until it terminated

Question 2

What is a process?

Answer 2

The OS’s abstraction for execution, i.e. a process is a program being executed

Question 3

What is a sequential process?

Answer 3

An address space (abstraction of memory) being executed by a single thread of execution (abstraction of cpu)

The unit of execution, the unit of scheduling, the dynamic execution context vs the static program

Question 4

What 3 basic things does a process consist of?

Answer 4

Address space - instructions, data

CPU State - program counter, stack pointer, registers

OS Resources - open files, network connections..

Question 5

What are the 4 areas of a process’s address space

Answer 5

Stack (dynamic mem)

Heap (dynamic mem)

Static data (data segment)

Code (text segment)

Question 6

What data structure does the OS maintain to keep track of a process’s state?

Answer 6

Process Control Block (PCB) or process descriptor

Question 7

What sort of information does a Process Control Block include?

Answer 7

Process ID, Parent ID, execution state, Program counter, Stack pointer, registers etc.

Question 8

What is the first process created when a UNIX system starts up?

Answer 8

init

Question 9

How are new processes created in UNIX?

Answer 9

fork() system call

Question 10

How does the fork() syscall work?

Answer 10

creates and initialises new PCB (and kernel resources the same as the parent)

Initialises program counter, stack pointer to be the same

Copies the address space of parent to child.

i.e. duplicates the process

returns the child’s PID to the parent

returns 0 to the child

Question 11

How can a different process be started?

Answer 11

First fork then exec()

Question 12

What does exec() syscall do?

Answer 12

stops current process

loads program in arg0 into address space (overwrites existing image)

Initialises hardware context and args for new program

Place pcb onto ready queue

Question 13

How does a process exit?

Answer 13

once finished executing, calls exit()

Question 14

What does the exit() syscall do?

Answer 14

releases all resources

gets rid of most OS structures supporting the process

checks if parent is alive

if os, enters “zombie” state

otherwise dies

Question 15

How does a parent process arrange to get the return value of a child

Answer 15

wait() syscall

Question 16

How does the wait() syscall work?

Answer 16

Puts the parent to sleep until the child exits

When child calls exit(), OS wakes parent and returns value, and gets rid of the zombies

Question 17

What are the 3 process execution states?

Answer 17

ready: waiting to be assigned to a cpu (could run, but another process has the cpu)
running: executing on a cpu
waiting: aka blocked, waiting for event such as I/O completion or message from another process

Question 18

What are the state transitions in and out of the running state?

Answer 18

In: dispatch/schedule from ready state

Out: interrupt to ready state

terminate to terminated state

trap or exception to blocked state

Question 19

What are the state transitions in and out of ready state?

Answer 19

in: create process

interrupt from blocked state (e.g. I/O complete)

interrupt from running state (cpu scheduling)

out: dispatch/schedule to running state

Question 20

What are the state transitions in and out of blocked state?

Answer 20

in: trap or exception from running state
out: interrupt (event completed e.g. I/O)

Question 21

Draw the process state transitions

Answer 21

Question 22

What is a context switch?

Answer 22

When the cpu switches from executing one process to another

Question 23

What is involved in a context switch?

Answer 23

CPU saves old PCB

and loads new PCB

Question 24

What is the problem with context swtiches?

Answer 24

While CPU is performing them no useful stuff can happen

100% overhead