1

Context SwitchingGroup 1 – Presented by David Calhoun

Question 3.7: What are the actions taken by the kernel to context-switch between processes?

2

Multitasking (real life)

(Look carefully...)

3

Multitasking (OS)

(process scheduler)

4

What is a context switch?

-process of suspending and resuming processes

-triggered by interrupt (for instance, an IO interrupt), preemptive multitasking, or as part of

user/kernel mode switching

-process context represented by the Process Control Block (PCB)

5

Process Control Block (PCB)

The PCB is "the manifestation of a process in an operating system"

Harvey M. Deitel (1984) [1982]. An introduction to operating systems

6

What's in the PCB?

7

PCB components

-PCB implementations differ, but generally contain:

Process ID (PID) Process state (fig 3.2) (ready, running, waiting,

etc) Registers (including Program Counter) Memory management info (address space of

the process)

More complete list: p. 104

8

Overhead

Switching contexts

involves a lot of

overhead (typically a

few milliseconds).

Think of the overhead

involved in multitasking

in real life.