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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-3A625B23-354E-5CB4-98CF-FF53AD724FA0" xml:lang="en"><title>Migration

    13 Tutorial: Demand Paging</title><shortdesc/><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>Demand paging is a change to how the Kernel can use RAM from Symbian platform

    15 v9.3. This topic describes the possible results for base port. </p>

    16 <p>When demand paging is used, the contents of memory are available to a program

    17 when they are required - i.e. 'on demand'. When the contents are no longer

    18 required, the RAM can be used for other content. In this way, the total RAM

    19 required to store content is less than if it were all permanently available. </p>

    20 <p>The Device Driver Guide provides <xref href="GUID-469EC7BB-8697-57FE-A487-016882A0BEA8.dita#GUID-469EC7BB-8697-57FE-A487-016882A0BEA8/GUID-E382AA71-8BB5-5567-8578-51598D3131FD">Suggested

    21 techniques for mitigating the effects of demand paging</xref> for writers

    22 of device drivers. These recommendations can result in a more ‘multithreaded’

    23 base-port. This may have the following impact that needs to be considered: </p>

    24 <ul>

    25 <li id="GUID-DB6CC462-FB46-5D3B-8BAA-5B21FA06AF9D"><p>A base-port component

    26 may provide services to device drivers, exposing to them a shared resource;

    27 either hardware or software: </p> <ul>

    28 <li id="GUID-6B9E9433-EA19-54AE-B7A5-6E2C3341723B"><p>hardware - may be a

    29 hardware controller whose non-instantaneous operation, once initiated, cannot

    30 be disturbed until it completes </p> </li>

    31 <li id="GUID-92AD6E2B-34CC-59B4-8D52-FD739C3DD841"><p>software - may be a

    32 list of requests for services. </p> </li>

    33 </ul> </li>

    34 <li id="GUID-45F7D284-C216-5684-8C7B-2958C809B1BC"><p>A hardware component

    35 has a control interface that can be used by a number of drivers. Operations

    36 on the control interface although near instantaneous, are not atomic and cannot

    37 be interrupted. </p> </li>

    38 </ul>

    39 <p>In the case of the base-port component, when the state of a resource needs

    40 to be protected from the effects of pre-emption for a non-negligible period

    41 of time, the recommended approach is to use mutual exclusion, protecting the

    42 resource with a mutex: unless there is any chance that the same driver may

    43 trigger the same operation before the previous one completed. For example,

    44 when operations are non-blocking and happen in a context different from the

    45 initiator’s, a <xref href="GUID-D5B555DA-3D17-3ED2-A931-CB35BD93A953.dita"><apiname>NFastMutex</apiname></xref> should suffice. </p>

    46 <p>An example of the hardware component situation is a set-clear control interface,

    47 where a pair of registers (one containing the bits to be set, the other the

    48 bits to be cleared) have to be written to produce the desired change. If the

    49 operation is pre-empted after bits are set but before they are cleared for

    50 a desired final output, and a new set-clear operation is initiated, the final

    51 state of the interface may be undetermined. Pre-emption protection in this

    52 case is achieved by simply locking the Kernel using <xref href="GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02.dita#GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02/GUID-7CBBF72B-4519-38DD-92CA-38AF636AFD8A"><apiname>NKern::Lock()</apiname></xref> before

    53 the operation starts and unlocking it with <xref href="GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02.dita#GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02/GUID-A1A42137-906C-30F1-AF61-4F786FC372DE"><apiname>NKern::Unlock()</apiname></xref> after

    54 it completes. If the interface is to be used from an interrupt context disabling

    55 all interrupts is sufficient to protect against thread concurrency. </p>

    56 </conbody><related-links>

    57 <link href="GUID-90B5FDD9-7D59-5035-BF53-2B177655DCD6.dita"><linktext>Migration

    58                 Tutorial: Demand Paging and Internal MMC Cards</linktext>

    59 </link>

    60 <link href="GUID-EB2566BD-8F65-5A81-B215-E8B05CFE21C3.dita"><linktext>Migration

    61 Tutorial: Demand                 Paging and Media Drivers</linktext></link>

    62 </related-links></concept>