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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<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>