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<concept id="GUID-9781C95D-4606-57FB-9F62-B7CC45DDD968" xml:lang="en"><title>Flexible

Memory Model Guide</title><shortdesc>Overview of the Symbian platform flexible memory model</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-8F4D5E77-D4CC-48DD-AA11-77191A19C6FE"><title>Design</title> <p>The flexible memory model implements the

address space using these objects: </p> <ul>

<li id="GUID-416CE3F1-C16C-5686-BAAD-79CAD188DC28"><p>memory objects, </p> </li>

<li id="GUID-512A3E93-FAF3-593B-A490-5BB8C7A0925D"><p>memory managers, </p> </li>

<li id="GUID-31E356AA-A82F-5BDD-BEB5-1F97596BC642"><p>memory mappings, and </p> </li>

<li id="GUID-E67E19A3-E970-575E-9D80-FD50F756EB8B"><p>address space objects. </p> </li>

</ul> </section>

<section id="GUID-50C7D629-8818-4863-AE74-EF9835D1BC97"><title>Memory Objects</title><p>A memory object abstracts physical

memory. It represents storage for chunks, code and thread stacks. A memory

object is implemented as an array of physical page addresses. Attributes such

as 'cached', 'uncached', 'device' and 'strongly ordered' are common to all

the memory in the object. </p></section>

<section id="GUID-F1B26B2A-87F8-4363-902A-525B6AB92F8E"><title>Memory Managers</title><p>A memory manager performs operations

on memory objects such as allocating and freeing memory. Different types of

memory have different kinds of memory manager, for instance paged and unpaged

memory. </p></section>

<section id="GUID-A2CEF774-C4E9-4401-81A7-EB038C52BDC6"><title>Memory Mappings</title><p>Memory mappings exist to map virtual

memory on to physical memory. One or more memory mappings are associated with

one or more memory objects and their managers. Permissions, sharing and pinning

are implemented at the level of the memory mapping. Permissions include read/write

permissions, user/supervisor permissions and execute/no-execute permissions. </p></section>

<section id="GUID-FA9AD4F3-863F-4471-9DB4-AFAF3525D657"><title>Address Space Objects</title><p>Address space objects model

the address space of a process, that is the virtual address region used by

the process. An address space contains a list of memory mappings through which

the corresponding physical memory is accessed. </p></section>

<section id="GUID-B810FD8F-9C64-40D7-ABCD-87708A7EDBB0"><title>Changes</title> <p>Kernel developers need to know about certain

differences between the flexible memory model and the multiple memory model

which was used in previous versions of Symbian platform. </p> <p>In the multiple

memory model, memory was managed at the level of the chunk. Chunks were wrapped

round page tables of 1Mb and permissions and sharing were properties of the

chunk. Address space was partitioned into local, shareable and code memory

and shared memory had to be reserved at this level (that is, in units of 1Mb). </p> <p>The

transition to the flexible memory model involved two important changes. </p> <ul>

<li id="GUID-F1BA82A1-FC0B-5D27-8106-61313B0FC7C8"><p>Two processes sharing

the same chunk may use different virtual addresses to access it. Old code

which assumes that a chunk has a single virtual address may need to be rewritten. </p> </li>

<li id="GUID-FB107346-DB07-5BC6-AA04-A94E0E62F958"><p>The kernel no longer

creates chunks to store thread stacks and static data for each process. </p> </li>

</ul> <p>In two other respects the multiple memory model and the flexible

memory model are exactly the same: </p> <ul>

<li id="GUID-FB17CC7E-7187-5857-A124-4BA7E08F68ED"><p>Hardware chunks are

addressed globally. </p> </li>

<li id="GUID-34F3B419-D83A-5EFB-BDAA-2D518E78EC20"><p>Shared chunks have fixed

kernel addresses which are accessible in supervisor mode. </p> </li>

</ul> </section>

<section id="GUID-23B4066A-C3B1-47D9-834E-45F4F17C7222"><title>See Also</title> <p> <xref href="GUID-DB3A6401-D25E-57E7-A8BF-3B9E0DE3B1D6.dita">Flexible

Memory Model Tutorial</xref>  </p> </section>

</conbody></concept>