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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-D557CC2C-C045-5818-B2F7-9BD8BC39257D" xml:lang="en"><title>Address
Space and Process</title><shortdesc>Programs in Symbian platform consist of a number of processes,
each of which contains one or more conceptually concurrent threads of execution.
Each thread has at least one <i>chunk</i> of memory.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Each user process has its own private address space, that is, a collection
of memory regions which that process can access. A user process cannot directly
access 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 (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>Shared Memory
Between Threads</linktext></link>
</related-links></concept>