DispatchDeviceControl and DispatchInternalDeviceControl Routines
For every common type of peripheral device, the system defines a set of I/O control codes for IRP_MJ_DEVICE_CONTROL requests. New drivers for each type of device must support these requests. In most cases, these public I/O control codes for each type of device are not exported to user-mode applications.
Some of these system-defined I/O control codes are used by higher-level drivers that create IRPs for the underlying device driver by calling IoBuildDeviceIoControlRequest. Others are used by Win32 components to communicate with an underlying device driver by calling the Win32 function DeviceIoControl (described in Microsoft Windows SDK documentation) which, in turn, calls a system service. The I/O manager sets up an IRP, and stores the major function code IRP_MJ_DEVICE_CONTROL and the given I/O control code in the IO_STACK_LOCATION structure at Parameters.DeviceIoControl.IoControlCode. Then, the I/O manager calls the DispatchDeviceControl routine of the highest-level driver in the chain.
For certain system-supplied drivers designed to interoperate with and support new drivers, the operating system also defines a set of I/O control codes for IRP_MJ_INTERNAL_DEVICE_CONTROL requests. In most cases, these public I/O control codes allow add-on higher-level drivers to interoperate with an underlying device driver.
As an example, the system-supplied parallel drivers support a set of internal I/O control codes that vendor-supplied drivers send in IRP_MJ_INTERNAL_DEVICE_CONTROL requests. For more information, see Internal Device Control Requests for Parallel Ports and Internal Device Control Requests for Parallel Devices.
Almost all operations requested through system-defined I/O control codes use buffered I/O, because this type of request seldom requires the transfer of large amounts of data. That is, even drivers that set up their device objects for direct I/O are sent IRPs for device control requests with data to be transferred into or out of the buffer at Irp->AssociatedIrp.SystemBuffer (except for certain types of highest-level device drivers with closely coupled Win32 multimedia drivers).
In addition, a driver can define a set of private I/O control codes that other drivers can use to communicate with it. New public I/O control codes can be added to the system only with the cooperation of Microsoft Corporation, because public I/O control codes are built into the operating system itself.
For specific information about the set of public I/O control codes that different kinds of drivers must support and about defining private I/O control codes, see the device-specific reference sections of the Windows Driver Kit (WDK).