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<concept id="GUID-BDB847A2-557A-5902-AA6D-C1AE10D8E493" xml:lang="en"><title>Code

Paging Guide</title><shortdesc>Code paging is the application of demand paging to executable code. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-CD6AC3BC-AF81-4342-9B66-34ACDD573093"><title>Purpose</title> <p>This document explains the principles of

code paging in Symbian platform. </p> <p><b>Intended

Audience:</b> </p> <p>This document is intended to be read by those interested

in the Symbian platform kernel. </p> </section>

<section id="GUID-FC1B1DC9-133A-43BA-BAEC-711718817B82"><title>Basics of code paging</title> <p>Code paging means the use

of demand paging to executable code. Demand paging increases the apparent

size of available RAM on a device by loading data into RAM when needed. Since

the memory locations used by the code cannot be determined before it is loaded,

the code needs to be modified when it is paged into RAM. </p> <p>Classes explained

here. </p> <p>Executable code is paged in and out of memory in accordance

with the demand paging algorithm which is discussed in the document. The algorithm

involves four basic operations: </p> <ul>

<li id="GUID-AE260EC1-A9F4-53B9-A0B7-1681FAFA42A5"><p>paging in, </p> </li>

<li id="GUID-F870B9A6-6B57-5988-93D7-C87A58B8F94D"><p>aging, </p> </li>

<li id="GUID-4C9897F4-8A64-556E-939A-0687E775E4EC"><p>rejuvenating, and </p> </li>

<li id="GUID-E9CF60A1-162C-540A-839A-475D99802B85"><p>freeing. </p> </li>

</ul> <p>The remainder of this document discusses the kernel side implementation

of each of these operations in turn. Most of the work is done by the <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref> class. </p> </section>

<section id="GUID-FC5D5EE4-EEE2-4630-8321-0BB3855CB300"><title>Paging in paged code</title> <p>When a program accesses an

item of paged code for the first time the code needs to be paged into RAM.

The initial call generates a data abort: this is caught by the exception handler

which calls the <xref href="GUID-1AA89F20-7D14-35DB-92FE-06670B2BA484.dita"><apiname>HandleFault()</apiname></xref> function of the <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref> class.

The function call performs the paging in as follows. </p> <ul>

<li id="GUID-B944B6DF-3DC9-5FF4-92B1-2F6A5298767A"><p>Checks the MMU page

table entry for the address which caused the data abort. If the entry is not <xref href="GUID-4C8E5466-82E2-3B45-81C5-980FC521DD3A.dita"><apiname>KPteNotPresentEntry</apiname></xref> then

there is no memory mapped at that address and it may need paging in. </p> </li>

<li id="GUID-DB0C8B0C-A38D-555C-98F7-08040771AD65"><p>Verifies that the exception

was caused by an access to the code chunk memory region. </p> </li>

<li id="GUID-F00DF257-8807-5F31-9FED-4626746ED421"><p>Finds the code segment

which is at the current address. </p> </li>

<li id="GUID-7A5442AF-A66D-56CA-8921-B7BA3907B624"><p>Verifies that the code

segment is the one being demand paged. </p> </li>

</ul> <p>The <xref href="GUID-1AA89F20-7D14-35DB-92FE-06670B2BA484.dita"><apiname>HandleFault()</apiname></xref> function of <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref> then

calls the <xref href="GUID-A7C5F674-CF18-308B-A07F-6C057AB29180.dita"><apiname>PageIn()</apiname></xref> function of <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref>,

which performs the following steps. </p> <ul>

<li id="GUID-EAE80357-A36C-5DF6-BCA5-C1F704388DCF"><p>Obtains a <xref href="GUID-535DF46C-C681-3167-9AE1-E927D3B3B928.dita#GUID-535DF46C-C681-3167-9AE1-E927D3B3B928/GUID-E9132C24-94CE-35FE-8E32-943C2134BD1C"><apiname>DemandPaging::DPagingRequest</apiname></xref> object

by calling <xref href="GUID-535DF46C-C681-3167-9AE1-E927D3B3B928.dita#GUID-535DF46C-C681-3167-9AE1-E927D3B3B928/GUID-D70D74D4-EEDD-3F58-AF6D-160E9DF707DC"><apiname>DemandPaging::AcquireRequestObject()</apiname></xref>. </p> </li>

<li id="GUID-08CE4C36-B2BD-5DB8-BBB5-D3259D63C496"><p>Obtains a physical page

of RAM by calling <xref href="GUID-535DF46C-C681-3167-9AE1-E927D3B3B928.dita#GUID-535DF46C-C681-3167-9AE1-E927D3B3B928/GUID-BF59E426-A1C8-3491-9AC8-929A1DF8EE42"><apiname>DemandPaging::AllocateNewPage()</apiname></xref>. </p> </li>

<li id="GUID-33BA3BF8-999B-5691-B8A6-8905ED3E7B61"><p>Maps the RAM at the

temporary location <xref href="GUID-E9132C24-94CE-35FE-8E32-943C2134BD1C.dita#GUID-E9132C24-94CE-35FE-8E32-943C2134BD1C/GUID-53AF13F7-94B7-3EDE-B0FF-6F33FF16D208"><apiname>DemandPaging::DPagingRequest::iLoadAddr</apiname></xref>. </p> </li>

<li id="GUID-5AA0C439-42E3-5E70-BC65-B2E7D7C38909"><p>Reads the correct contents

into the RAM page by calling <xref href="GUID-535DF46C-C681-3167-9AE1-E927D3B3B928.dita#GUID-535DF46C-C681-3167-9AE1-E927D3B3B928/GUID-4CBDC66C-B779-3894-9B32-726911E3D9E2"><apiname>DemandPaging::ReadCodePage()</apiname></xref>. </p> </li>

<li id="GUID-4217C1A9-5930-505E-9E8C-864691C68B35"><p>Initialises the <xref href="GUID-C5F2C8BC-9C8E-3BC1-AB24-99E23A38152B.dita"><apiname>SPageInfo</apiname></xref> structure

for the physical page of RAM and marks it as type <xref href="GUID-BB093184-8193-3157-883B-9264CD6389C7.dita"><apiname>EPagedCode</apiname></xref>. </p> </li>

<li id="GUID-13948D5A-1C02-5A0D-A1F7-0C13E8B700F2"><p>Maps the page at the

correct address in the current process. </p> </li>

<li id="GUID-903E5F84-C4C2-509A-BB31-073780C895A7"><p>Adds the <xref href="GUID-C5F2C8BC-9C8E-3BC1-AB24-99E23A38152B.dita"><apiname>SPageInfo</apiname></xref> to

the beginning of the live page list. </p> </li>

</ul> <p>When these calls have completed they return control to the program

which caused the data abort. </p> </section>

<section id="GUID-C6956FFF-0138-45B9-87AA-66B7609B2B3E"><title>Aging paged code</title> <p>The demand paging algorithm defines

pages in the live list to be either young or old. When a page changes status

from young to old, the kernel changes the MMU mappings for the page to make

it inaccessible. It does so by calling the <xref href="GUID-2625AEBC-E316-3CC4-B840-DF2148BCAD65.dita"><apiname>SetOld()</apiname></xref> function

of the <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref> class. The implementation of

this procedure is different in the Moving Memory Model and the Multiple Memory

Model. </p> <p>In the Moving Memory Model, the call to <xref href="GUID-2625AEBC-E316-3CC4-B840-DF2148BCAD65.dita"><apiname>SetOld()</apiname></xref> acts

as follows: </p> <ul>

<li id="GUID-B01591FD-2D3F-5752-B38B-5BC69B9FAE49"><p>Finds the MMU page table

entry for the page and clears the bits <xref href="GUID-37E290ED-FFF8-3A6B-9CA9-07863E340F0D.dita"><apiname>KPtePresentMask</apiname></xref>. </p> </li>

</ul> <p>In the Multiple Memory Model, <xref href="GUID-2625AEBC-E316-3CC4-B840-DF2148BCAD65.dita"><apiname>SetOld()</apiname></xref> calls the

kernel function <xref href="GUID-EAF2FEE1-BA70-32E5-8D6A-E55F0A814E4B.dita"><apiname>DoSetCodeOld()</apiname></xref> which acts as follows: </p> <ul>

<li id="GUID-7573F92E-441B-55AD-AB0D-D4341AF22E3A"><p>Examines the bit array <xref href="GUID-DD3F8638-685C-3246-A493-34C8BBA51566.dita#GUID-DD3F8638-685C-3246-A493-34C8BBA51566/GUID-241CBE5A-0A84-3F26-BAFD-7FCC33F788E7"><apiname>DMemModelCodeSegMemory::iOsAsid</apiname></xref> s

to determine the processes into which the code segment is loaded. </p> </li>

<li id="GUID-3AFE85EA-6967-5E89-B277-CDBD17315B06"><p>Updates each mapping

in turn. </p> </li>

</ul> <p>The status of a page may change during a call to <xref href="GUID-EAF2FEE1-BA70-32E5-8D6A-E55F0A814E4B.dita"><apiname>DoSetCodeOld()</apiname></xref>,

either because it has been rejuvenated or paged out. In these cases <xref href="GUID-EAF2FEE1-BA70-32E5-8D6A-E55F0A814E4B.dita"><apiname>DoSetCodeOld()</apiname></xref> simply

ends, as the aging operation is no longer appropriate. </p> </section>

<section id="GUID-1B0F49F2-4B9C-4B60-AFA6-521C33951576"><title>Rejuvenating paged code</title> <p>When a program accesses

a program held in an old page, it generates a data abort because the kernel

made the page inaccessible when it was set to old. The data abort is caught

by the exception handler which calls the <xref href="GUID-1AA89F20-7D14-35DB-92FE-06670B2BA484.dita"><apiname>HandleFault()</apiname></xref> function

of the <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref> class. It is this call which

performs the rejuvenation as follows. </p> <ul>

<li id="GUID-4494FF5B-8F24-5973-9178-F1134A3B7313"><p>Gets the MMU page table

entry for the address which caused the abort. If the bits <xref href="GUID-37E290ED-FFF8-3A6B-9CA9-07863E340F0D.dita"><apiname>KPtePresentMask</apiname></xref> are

clear then the page needs rejuvenating. If all the bits are clear then the

page needs to be paged in, not rejuvenated. </p> </li>

<li id="GUID-4708382F-4E31-5448-B958-A919C1E2C31A"><p>Finds the <xref href="GUID-C5F2C8BC-9C8E-3BC1-AB24-99E23A38152B.dita"><apiname>SPageInfo</apiname></xref> for

the page, using the physical address stored in the page table entry. </p> </li>

<li id="GUID-E235D0FB-7A9B-5A1C-BC84-782A9F8A29FD"><p>If it finds that the

state of the page is <xref href="GUID-9C190095-2B44-31A5-A898-E362FCC19EDC.dita"><apiname>EStatePagedDead</apiname></xref> then the page is dead

rather than old and needs to be paged in, not rejuvenated. </p> </li>

<li id="GUID-B1AE6505-05B7-5ACA-B284-C95AF360AADE"><p>Updates the page table

entry to make the page accessible. </p> </li>

<li id="GUID-D39CCEFF-087C-5F07-B935-B52877D35817"><p>Moves the <xref href="GUID-C5F2C8BC-9C8E-3BC1-AB24-99E23A38152B.dita"><apiname>SPageInfo</apiname></xref> for

the page to the beginning of the live list, making it the youngest page in

the list. </p> </li>

</ul> <p>These steps are performed with the system lock held. </p> </section>

<section id="GUID-C53B418C-46BC-43C5-940D-94667DD00872"><title>Freeing paged code</title> <p>When a physical page of RAM

holding demand-paged code is needed for other purposes, it must be freed up.

The kernel does this by calling the <xref href="GUID-9C1DEEB8-2C7E-3AD1-97B1-58E64AAAF1B1.dita"><apiname>SetFree()</apiname></xref> function

of the <xref href="GUID-B718F920-45E2-3DD6-927F-0465B156993D.dita"><apiname>MemModelDemandPaging</apiname></xref> class. The implementation of

this procedure is different in the Moving Memory Model and the Multiple Memory

Model. </p> <p>In the Moving Memory Model, the call to <xref href="GUID-9C1DEEB8-2C7E-3AD1-97B1-58E64AAAF1B1.dita"><apiname>SetFree()</apiname></xref> acts

as follows: </p> <ul>

<li id="GUID-57E4C0C5-0D5D-5C0F-B324-46F4E7AD6739"><p>Finds the MMU page table

entry for the page and sets it to <xref href="GUID-4C8E5466-82E2-3B45-81C5-980FC521DD3A.dita"><apiname>KPteNotPresentEntry</apiname></xref>. </p> </li>

</ul> <p>In the Multiple Memory Model, the call to <xref href="GUID-9C1DEEB8-2C7E-3AD1-97B1-58E64AAAF1B1.dita"><apiname>SetFree()</apiname></xref> calls

the kernel function <xref href="GUID-D16A900C-A40C-3D6A-A77E-4008376DBAAE.dita"><apiname>DoSetCodeFree()</apiname></xref> which acts as follows: </p> <ul>

<li id="GUID-F1CDC949-F1FC-5E87-A4B2-A5F12F099B9A"><p>Examines the bit array <xref href="GUID-DD3F8638-685C-3246-A493-34C8BBA51566.dita#GUID-DD3F8638-685C-3246-A493-34C8BBA51566/GUID-30F9613A-C06F-3185-94B5-4CAE20B7A052"><apiname>DMemModelCodeSegMemory::iOsAsids</apiname></xref> to

determine the processes into which the code segment is loaded. </p> </li>

<li id="GUID-4D104936-767C-514C-9253-1F8CC80B92A9"><p>Makes each page inaccessible

in turn. </p> </li>

</ul> </section>

</conbody><related-links>

<link href="GUID-E21E7992-607A-5A49-B022-189ECA9E76D1.dita"><linktext>Code Paging

Overview</linktext></link>

<link href="GUID-B35A70D2-1BC8-51DE-95BF-F315DB394582.dita"><linktext>Demand Paging

Overview</linktext></link>