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<concept id="GUID-3A625B23-354E-5CB4-98CF-FF53AD724FA0" xml:lang="en"><title>Migration
Tutorial: Demand Paging</title><shortdesc/><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Demand paging is a change to how the Kernel can use RAM from Symbian platform
v9.3. This topic describes the possible results for base port. </p>
<p>When demand paging is used, the contents of memory are available to a program
when they are required - i.e. 'on demand'. When the contents are no longer
required, the RAM can be used for other content. In this way, the total RAM
required to store content is less than if it were all permanently available. </p>
<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
techniques for mitigating the effects of demand paging</xref> for writers
of device drivers. These recommendations can result in a more ‘multithreaded’
base-port. This may have the following impact that needs to be considered: </p>
<ul>
<li id="GUID-DB6CC462-FB46-5D3B-8BAA-5B21FA06AF9D"><p>A base-port component
may provide services to device drivers, exposing to them a shared resource;
either hardware or software: </p> <ul>
<li id="GUID-6B9E9433-EA19-54AE-B7A5-6E2C3341723B"><p>hardware - may be a
hardware controller whose non-instantaneous operation, once initiated, cannot
be disturbed until it completes </p> </li>
<li id="GUID-92AD6E2B-34CC-59B4-8D52-FD739C3DD841"><p>software - may be a
list of requests for services. </p> </li>
</ul> </li>
<li id="GUID-45F7D284-C216-5684-8C7B-2958C809B1BC"><p>A hardware component
has a control interface that can be used by a number of drivers. Operations
on the control interface although near instantaneous, are not atomic and cannot
be interrupted. </p> </li>
</ul>
<p>In the case of the base-port component, when the state of a resource needs
to be protected from the effects of pre-emption for a non-negligible period
of time, the recommended approach is to use mutual exclusion, protecting the
resource with a mutex: unless there is any chance that the same driver may
trigger the same operation before the previous one completed. For example,
when operations are non-blocking and happen in a context different from the
initiator’s, a <xref href="GUID-D5B555DA-3D17-3ED2-A931-CB35BD93A953.dita"><apiname>NFastMutex</apiname></xref> should suffice. </p>
<p>An example of the hardware component situation is a set-clear control interface,
where a pair of registers (one containing the bits to be set, the other the
bits to be cleared) have to be written to produce the desired change. If the
operation is pre-empted after bits are set but before they are cleared for
a desired final output, and a new set-clear operation is initiated, the final
state of the interface may be undetermined. Pre-emption protection in this
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
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
it completes. If the interface is to be used from an interrupt context disabling
all interrupts is sufficient to protect against thread concurrency. </p>
</conbody><related-links>
<link href="GUID-90B5FDD9-7D59-5035-BF53-2B177655DCD6.dita"><linktext>Migration
                Tutorial: Demand Paging and Internal MMC Cards</linktext>
</link>
<link href="GUID-EB2566BD-8F65-5A81-B215-E8B05CFE21C3.dita"><linktext>Migration
Tutorial: Demand                 Paging and Media Drivers</linktext></link>
</related-links></concept>