<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
<!-- This component and the accompanying materials are made available under the terms of the License 
"Eclipse Public License v1.0" which accompanies this distribution, 
and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
<!-- Initial Contributors:
    Nokia Corporation - initial contribution.
Contributors: 
-->
<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<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>
</conbody></concept>