Kernel +Extension

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.

The DECLARE_STANDARD_EXTENSION macro +is used to provide an entry point for a kernel extension (see Entry +Points).

Extensions are built into the ROM image, by specifying +the extension keyword in the .iby file. +This enables the ROM build tool to build the ROM header. The extensions are +loaded in the order specified in the kernel.iby file.

extension[VARID]=\Epoc32\Release\<assp>\urel\KDEBUG.DLL \System\Bin\kdebug.dll

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 DECLARE_STANDARD_EXTENSION entry +point. They then use this object to call the exported API.

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 elocd.ldd.

To do +this, drivers have to declare DECLARE_EXTENSION_LDD in +addition to the DECLARE_STANDARD_EXTENSION macro. In this +model, extensions generally call Kernel::InstallLogicalDevice() /Kernel::InstallPhysicalDevice() 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.

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; + }