Plug and Play

Each time the system starts, new plug-and-play devices are detected and added to the Device Manager tree. Use the Control Panel Add/Remove Hardware Wizard to install devices that are not automatically detected. The wizard allows the addition of devices manually or it will scan for both legacy and plug-and-plan devices.

Drivers

Windows comes with over 50 MB of drivers stored in %SystemRoot%\Driver Cache\i386\driver.cab.

Legacy devices can have their power managed and resources allocated but they cannot be automatically detected dynamically. Some legacy drivers work but without power management nor plug-and-play support.

Troubleshooting

If a device fails to respond, start Device Manager and check the device. Use the View menu to Show hidden devices for legacy devices.

Resources

Device hardware and device driver software communicate with each other with mechanisms that are referred to as resources in Device Manager. It is important that devices do not conflict with the resource usage and the automatic plug and play mechanism will avoid conflicts. Device resource assignments can be view in Device Manager as shown below. The resources are

• IRQ - Interrupt Requests
• Input/Output IO Ports
• Memory - Addresses reference memory on the device
• DMA Direct Memory Access by the device for transfers to main memory without processor involvement

The different types of resources provide different ways to communicate and transfer data between the hardware device and the software device driver. DMA is used to transfer blocks of data as in the case of a disk controller transferring a disk read into working memory. This is in contrast to IRQs which are only used to interrupt the processor. Devices may conflict over resource usage. If a device is not working, try disabling the device that is causing the conflict and then rescan for new devices. Work through each device, one at a time.

Driver Signing

Because drivers run in kernel mode, a bad driver can cause your system to crash with a stop error, i.e. the blue screen of death. Microsoft digitally signs drivers that have passed the WHQL Windows Hardware Quality Lab test. Through System properties you can configure your system to allow or disallow the installation of unsigned drivers as shown in the following dialog. The last dialog shows how this can be done with a system policy.

Phantom

When a device is physically removed from a machine, the driver becomes a phantom and is no longer visible in Device Manager. Normally this is desirable, but can be a problem if you wish to remove the device driver. If the phantom device driver is not visible, the option to remove it is also not available. Making phantoms visible is not obvious, but the steps are as follows.

1. Selecting the option Show hidden devices is required but not sufficient.
2. In addition, you must have an environment variable set. To see phantom devices you must
       SET DEVMGR_SHOW_NONPRESENT_DEVICES=1.
   Remember that environment variables can be set by using the System applet in the Control Panel.

Keyboard Exercise

Start Device Manager and try the different views.

Use the Control Panel System icon to set the environment variable DEVMGR_SHOW_NONPRESENT_DEVICES=1 and then start Device Manager and check for hidden phatom devices. Remember that you need to Show hidden devices.