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<concept id="GUID-21A43565-FC69-553E-9617-4832AB776004" xml:lang="en"><title>Introduction

to Threads and Processes</title><shortdesc>Programs consist of a number of processes, each of which contains

one or more conceptually concurrent threads of execution.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p/>

<p>A thread is the unit of execution within a process. Every time a process

is initialized, a primary thread is created. For many applications the primary

thread is the only one that the application requires; however, processes can

create additional threads.</p>

<p>Each user process has its own private address space, i.e. a collection

of memory regions which that process can access. A user process cannot directly

address memory areas in the address space of another process. There is also

a special process, the Kernel process, whose threads run at supervisor privilege

level. This process normally contains two threads:</p>

<ul>

<li id="GUID-00725755-A077-58CA-9AFE-1EDABCD56274"><p>the Kernel server thread,

which is the initial thread whose execution begins at the reset vector, and

which is used to implement all Kernel functions requiring allocation or de-allocation

on the Kernel heap. This is the highest priority thread in the system.</p> </li>

<li id="GUID-085FA786-6536-5778-A041-FD06BEF5D200"><p>the null thread, which

runs only when no other threads are ready to run. The null thread places the

processor into idle mode to save power.</p> </li>

</ul>

<p>Threads execute individually and are unaware of other threads in a process.

The scheduling of threads is pre-emptive, i.e. a currently executing thread

may be suspended at any time to allow another thread to run. </p>

<p>Each thread is assigned a priority; at any time, the thread running is

the highest priority thread which is ready to run. Threads with equal priority

are time-sliced on a round-robin basis. Context switching between threads

involves saving and restoring the state of threads. This state includes not

only the processor registers (the thread context) but also the address space

accessible to the thread (the process context). The process context only needs

switching if a reschedule is between threads in different processes.</p>

<p>Compare this with active objects which allow non pre-emptive multi-tasking

within a single thread.</p>

<p>A thread can suspend, resume, panic and kill another thread.</p>

<p>When a thread is created it is put into a suspended state, it does not

begin to run until that thread’s <codeph>Resume()</codeph> member function

is called.</p>

<p>When a thread is created, it is given the priority <codeph>EPriorityNormal</codeph> by

default. The fact that a thread is initially put into a suspended state means

that the thread priority can be changed (RThread::SetPriority()) before the

thread is started (RThread::Resume()).</p>

</conbody><related-links>

<link href="GUID-5FBA9BB1-94A2-470C-9932-C4255E3774C0.dita"><linktext>Use Of Shared

Memory Between Threads</linktext></link>

</related-links></concept>