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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    31 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>

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

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

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

    35 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.

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

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

    38 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

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

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

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

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

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

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

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

    46 </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

    47 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>,

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

    49 <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

    50 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>

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

    52 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>

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

    54 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>

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

    56 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>

    57 <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

    58 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>

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

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

    61 <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

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

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

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

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

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

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

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

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

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

    71 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

    72 as follows: </p> <ul>

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

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

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

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

    77 <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

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

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

    80 in turn. </p> </li>

    81 </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>,

    82 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

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

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

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

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

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

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

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

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

    91 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

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

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

    94 <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

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

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

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

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

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

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

   101 <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

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

   103 the list. </p> </li>

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

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

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

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

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

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

   110 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

   111 as follows: </p> <ul>

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

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

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

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

   116 <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

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

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

   119 in turn. </p> </li>

   120 </ul> </section>

   121 </conbody><related-links>

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

   123 Overview</linktext></link>

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

   125 Overview</linktext></link>

   126 </related-links></concept>