Page 65 - DCAP403_Operating System

Operating System

Notes 4.7 Concept of Thread

Threads are a way for a program to fork (or split) itself into two or more simultaneously (or
pseudo-simultaneously) running tasks. A thread is a single sequence stream within in a process.
Because threads have some of the properties of processes, they are sometimes called lightweight
processes. In a process, threads allow multiple executions of streams. In many respect, threads
are popular way to improve application through parallelism.
The CPU switches rapidly back and forth among the threads giving illusion that the threads are
running in parallel. Like a traditional process i.e., process with one thread, a thread can be in any
of several states (Running, Blocked, Ready or Terminated). Each thread has its own stack. Since
thread will generally call different procedures and thus a different execution history. This is why
thread needs its own stack.

Figure 4.6: Threading

Scheduler

Context Switcher
.....
Mega32 State

R1, R2, R3
R4, ...

R30, R31,
etc.

T1 T2 T3
Thread Thread Thread
P1 P2 P1
Msg Msg Msg
Sleep=50ms Box Running Box Blocked Box
Stack Stack Stack
Semaphore Semaphore

Held by Held by

Waiting Waiting

Waiting
Msg Msg Msg
Waiting
Msg Msg

Msg Msg Msg

An operating system that has thread facility, the basic unit of CPU utilization is a thread. A
thread has or consists of a program counter (PC), a register set, and a stack space. Threads are not
independent of one other like processes as a result threads shares with other threads their code

section, data section, OS resources also known as task, such as open files and signals.

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