Chris Cant - Writing Windows WDM Device Drivers

Click Next two times, select "No, I want to select hardware from a list" and click Next. Select "Other devices" and click Next. Click "Have Disk…" and browse to the path of the Wdm1 driver (e.g., C:\WDMBook\Wdm1\Sys) and click "OK".

Two models are listed from the found installation INF files, one for the Wdm1 checked build and one for the free build. Select the model you want to install and select Next. Select "Finish" to complete installation.

The installation process does whatever the INF file specifies. The Wdm1 INF files copy the relevant driver to the Windows system32\drivers directory, adds registry settings, etc.

Windows should now have created a Wdm1 device for you. Check that it appears in the Device Manager "Other devices" category (e.g., named "WDM Book: WDM1 Example, free build"). The next chapter describes a Win32 user mode program that you can use to test that the driver is working.

In the Wdm1 code, the DriverEntry routine has been called along with various other Plug and Play callback routines, as described later.

You may be interested to know precisely what happens as a result of installing the Wdm1 driver and one Wdm1 device.

Windows 98

Windows 98 copies the INF file to the Windows INF\OTHER directory. The INF file is renamed, after the manufacturer name, to "WDM BookWDM1.INF".

Windows 98 remembers that the INF file has been installed in the registry. The HKLM\ Software\Microsoft\Windows\CurrentVersion\Setup\SetupX\INF\OFM Name key has a value C:\W98\INF\OTHER\WDM BookWDM1.INF set to "WDM BookWDM1.INF". This entry is not deleted if you remove the Wdm1 device.

Windows 98 also keeps a note of the most recently used install locations in the registry.

Windows 2000

Windows 2000 copies the INF file and renames it to the next available OEM filename in the Windows INF directory. For example, WDM1.INF might be copied and renamed as C:\WINNT\INF\OEM1.INF.

Windows 2000 also remembers install locations in the registry.

The device interface for Wdm1 devices has a registry key at HKLM\System\CurrentControlSet\ Control\DeviceClasses\{C0CF0640-5F6E-11d2-B677-00C0DFE4C1F3) along with a series of subkeys for each device and their associated symbolic link(s). This key includes the WDM1_GUID {C0CF0640…}.

Windows 98

The HKLM\Enum\Root\Unknown\0000 key is the entry for the Wdm1 device. The digits 0000 will increment for each Unknown device. The HKLM\System\CurrentControlSet\Services\Class\Unknown\0000 key is the entry for the Wdm1 driver. This key is passed to DriverEntry in the RegistryPath string.

These entries are removed if you remove the Wdm1 device.

The ClassGUID value in the Wdm1 device key is {4D36E97E…} for "Unknown" type devices. This has a registry entry at HKLM\System\CurrentControlSet\Services\Class\{4D36E97E…}.

Windows 2000

The HKLM\System\CurrentControlSet\Control\Class\{4D36E97E…}\0000 key is the entry for the Wdm1 device. {4D36E97E…} is the GUID for "Unknown" type devices.

The driver service entry key is HKLM\System\CurrentControlSet\Services\Wdm1. This key is passed to DriverEntry in the RegistryPath string. The Enum subkey has values named 0 onwards such as Root\UNKNOWN\0000 for each device instance.

The winobj program lets you see what Windows 2000 objects are present for the driver and the device.

The driver has an entry \driver\wdm1.

There is a symbolic link object \??\Root#UNKNOWN#0000#{C0CF0640…}. If you double-click this, you will see that this links to one of the listed devices (e.g., to \device\004059). Symbolic links are covered in the next chapter.

Having installed Wdm1, there are various device and driver management jobs that you can perform.

You can open another device with the same driver using the Add New Hardware wizard again. Windows made a copy of the INF file so you can select the device from the list without having to specify your driver location. Additional entries are made in the registry for the second Wdm1 device and the device interface to it.

In the Device Manager, select the device you want to remove. Click Uninstall or Remove.

Most of the device and driver registry entries are removed if the Wdm1 device is removed. However, the registry entry for the device interface and the Windows 2000 Services entry persist after a device is removed.

When you remove a device, the INF file is not removed from the Windows INF directory structure. Similarly, the Wdm1.sys driver file remains in place in the System32\Drivers directory. This makes it easy to reinstall a Wdm1 device.

If all the driver's devices have been removed, the driver is unloaded from memory. At this stage, if you update the driver in the Windows System32\drivers directory and reinstall the device, the new driver is used.

The simplest way to update a driver is to use the Update/Change driver option in the Device Manager properties for a device. The book software projects always make a copy of the latest build of a driver in the Windows System32\Drivers directory so you can reinstall a new driver without having to select the Have disk option. W2000 uses the files in this directory while W98 requires you to specify the location of the new files.

Alternatively, remove all devices and invoke the Add New Hardware wizard again.

NT style (non-Plug and Play) drivers must use a special installation process, as described in Chapter 11.

Updating an NT style driver happens in a different way, as well. First, you must get your driver into the Windows System32\drivers directory.

In NT 3.51 and NT 4, run the Control Panel Devices applet. Find your driver, stop it, and start it again. In Windows 2000 you must opt to show hidden devices in the Device Manager before you can start or stop NT style drivers. Alternatively, you can run the book software Servicer program; type in the name of your driver; press Stop and then Start. If you run an NT style driver in W98, you must reboot the computer to use an updated driver.

This chapter has shown how to set up a development computer for device driver development. A very basic WDM driver has been written and installed in Windows 98 and Windows 2000.

The following chapters explain how to access this driver from a user program and enhance this driver to implement the correct Plug and Play and Power Management handling.

Listing 4.7 Wdm1.h

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//Copyright © 1998 Chris Cant, PHD Computer Consultants Ltd

//WDM Book for R&D Books, Miller Freeman Inc

//

//Wdm1 example

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//wdm1.hCommon header

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//Version history

//27-Apr-991.0.0CCcreation

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//Include WDM standard header with C linkage

#ifdef __cplusplus

extern "C" {