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<concept id="GUID-D557CC2C-C045-5818-B2F7-9BD8BC39257D" xml:lang="en"><title>Address

Space and Process</title><shortdesc>Programs in Symbian OS 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>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. Threads belonging

to a user process run at user privilege level. </p>

<p>There is a special process, the Kernel process, whose threads run at supervisor

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

<ul>

<li id="GUID-5D2E1FB5-0DA1-5880-933C-A2335465D4A1"><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 deallocation

on the Kernel heap </p> </li>

<li id="GUID-1F10F528-B4B0-5DCD-8D39-15A8C3CDBF03"><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>The address space of a process consists of a number of <i>chunks</i>, where

a chunk is a region of RAM mapped into contiguous virtual addresses. On creation,

a user process contains one thread (the main thread) and one to three chunks;

these are: </p>

<ul>

<li id="GUID-48BE05FA-5AB5-5178-9C43-E7B8B4909856"><p>The <b>stack/heap</b> chunk

containing the stack and the heap used by the main thread of the process;

this chunk always exists. </p> </li>

<li id="GUID-09FDE7D1-37C9-5988-B06A-9A3588081AC1"><p>The <b>code</b> chunk;

this exists only if the process is loaded into RAM (i.e. it is not ROM resident). </p> </li>

<li id="GUID-F7F06C52-468D-5C29-B985-B8615530786D"><p>The <b>data</b> chunk;

this exists only if the process has static data. </p> </li>

</ul>

<p>If a process creates additional threads, then a new chunk is created for

each new thread. Each chunk contains the thread's stack; if a new thread is

not sharing an existing heap, then the chunk also contains a new heap. </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>