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<concept id="GUID-C244D421-8BD0-4212-A5C5-47A8B1E0C1E2" xml:lang="en"><title>Kernel

Extension</title><shortdesc>This document describes how to implement a device driver as a kernel

extension.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-D3EEB917-368E-45A3-866E-C63490994C59">             <p>Device

drivers can also be kernel extensions, which means that they are loaded by

the Kernel when it boots. They are used for extending the Kernel, as the name

suggests. Generally, kernel extensions provide early initialisation of devices

that must be permanently available, such as LCD, DMA, and I2C and other peripheral

bus controllers. Because kernel extensions are loaded by the Kernel, they

are never unloaded and so their destructors are never called. </p> <p>The <codeph>DECLARE_STANDARD_EXTENSION</codeph> macro

is used to provide an entry point for a kernel extension (see <xref href="GUID-B94FFCA4-1EB3-46A7-9FF9-54C55D67FFE8.dita">Entry

Points</xref>). </p> <p>Extensions are built into the ROM image, by specifying

the <codeph>extension</codeph> keyword in the <filepath>.iby</filepath> file.

This enables the ROM build tool to build the ROM header. The extensions are

loaded in the order specified in the <filepath>kernel.iby</filepath> file. </p> <codeblock id="GUID-1AABD1B3-BF5A-50FC-9B1F-86E657F9890D" xml:space="preserve">extension[VARID]=\Epoc32\Release\&lt;assp&gt;\urel\KDEBUG.DLL \System\Bin\kdebug.dll</codeblock> <p>A

kernel extension's interface to other Kernel side components is usually exported

using a static interface. Clients can access this interface by using the global

instance of the object created and initialised in the <codeph>DECLARE_STANDARD_EXTENSION</codeph> entry

point. They then use this object to call the exported API. </p> <p>Kernel

extensions can also be implemented that let user code open channels on them

to use the interface. This model is used for devices where initialisation

has to be done at system boot up, but which can then be used by the clients,

for example, the media driver <filepath>elocd.ldd</filepath>. </p> <p>To do

this, drivers have to declare <xref href="GUID-38771B51-195D-3148-A462-277DA3696117.dita"><apiname>DECLARE_EXTENSION_LDD</apiname></xref> in

addition to the <xref href="GUID-8B6DF6D7-4995-3564-9303-272500D7E747.dita"><apiname>DECLARE_STANDARD_EXTENSION</apiname></xref> macro. In this

model, extensions generally call <xref href="GUID-671F731F-428F-379D-8260-D9F18CAC25CF.dita#GUID-671F731F-428F-379D-8260-D9F18CAC25CF/GUID-ADB60188-13D4-3245-96D0-4D44CB983551"><apiname>Kernel::InstallLogicalDevice()</apiname></xref> /<xref href="GUID-671F731F-428F-379D-8260-D9F18CAC25CF.dita#GUID-671F731F-428F-379D-8260-D9F18CAC25CF/GUID-703E2FAD-A71C-377B-86F0-D19643D4CDA9"><apiname>Kernel::InstallPhysicalDevice()</apiname></xref> to

install the logical device. Later clients can open channels on this driver

and use the interface in the same way as a standard driver. </p> <codeblock id="GUID-29B10898-2CCA-5D03-B14A-CAD3C56EB73F" xml:space="preserve">DECLARE_STANDARD_EXTENSION()

    {

    ...

    // Create factory object

    DExDriverLogicalDevice* device = new DExDriverLogicalDevice;

    if (device==NULL)

        r=KErrNoMemory;

    else

        {

        // Installs the logical device by calling the second

        // phase constructor

        r=Kern::InstallLogicalDevice(device);

        }

    return r;

    }

DECLARE_EXTENSION_LDD()

    {

    return new DExDriverLogicalDevice;

    }</codeblock>     </section>