不可思議之水晶礦脈

1. 1
Context Switching
Group 1 – Presented by David Calhoun
Question 3.7: What are the actions taken by the
kernel to context-switch between processes?

2. 2
Multitasking (real life)
(Look carefully...)

3. 3
Multitasking (OS)
(process scheduler)

4. 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. 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. 6
What's in the PCB?

7. 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. 8
Overhead
Switching contexts
involves a lot of
overhead (typically a
few milliseconds).
Think of the overhead
involved in multitasking
in real life.