Windows® Internals, Sixth Edition, Part 2

Loading appropriate drivers is another responsibility of the PnP manager. The PnP manager determines, based on the identification of a device, whether a driver capable of managing the device is installed on the system, and if one is, it instructs the I/O manager to load it. If a suitable driver isn’t installed, the kernel-mode PnP manager communicates with the user-mode PnP manager to install the device, possibly requesting the user’s assistance in locating a suitable set of drivers.

The PnP manager also implements application and driver mechanisms for the detection of hardware configuration changes. Applications or drivers sometimes require a specific hardware device to function, so Windows includes a means for them to request notification of the presence, addition, or removal of devices.

It also provides a place for storage device state, and it participates in system setup, upgrade, migration, and offline image management.

In addition, it supports network connected devices, such as network projectors and printers, by allowing specialized bus drivers to detect the network as a bus and create device nodes for the devices running on it.

Level of Plug and Play Support

Windows aims to provide full support for Plug and Play, but the level of support possible depends on the attached devices and installed drivers. If a single device or driver doesn’t support Plug and Play, the extent of Plug and Play support for the system can be compromised. In addition, a driver that doesn’t support Plug and Play might prevent other devices from being usable by the system. Table 8-7 shows the outcome of various combinations of devices and drivers that can and can’t support Plug and Play.

Table 8-7. Device and Driver Plug and Play Capability

 Type of Driver

Type of Device

Plug and Play

Non–Plug and Play

Plug and Play

Full Plug and Play

No Plug and Play

Non–Plug and Play

Possible partial Plug and Play

No Plug and Play

A device that isn’t Plug and Play–compatible is one that doesn’t support automatic detection, such as a legacy ISA sound card. Because the operating system doesn’t know where the hardware physically lies, certain operations—such as laptop undocking, sleep, and hibernation—are disallowed. However, if a Plug and Play driver is manually installed for the device, the driver can at least implement PnP manager–directed resource assignment for the device.

Drivers that aren’t Plug and Play–compatible include legacy drivers, such as those that ran on Windows NT 4. Although these drivers might continue to function on later versions of Windows, the PnP manager can’t reconfigure the resources assigned to such devices in the event that resource reallocation is necessary to accommodate the needs of a dynamically added device. For example, a device might be able to use I/O memory ranges A and B, and during the boot the PnP manager assigns it range A. If a device that can use only A is attached to the system later, the PnP manager can’t direct the first device’s driver to reconfigure itself to use range B. This prevents the second device from obtaining required resources, which results in the device being unavailable for use by the system. Legacy drivers also impair a machine’s ability to sleep or hibernate. (See the section The Power Manager later in this chapter for more details.)

Driver Support for Plug and Play

To support Plug and Play, a driver must implement a Plug and Play dispatch routine, a power management dispatch routine (described in the section The Power Manager later in this chapter), and an add-device routine. Bus drivers must support different types of Plug and Play requests than function or filter drivers do, however. For example, when the PnP manager is guiding device enumeration during the system boot (described in detail later in this chapter), it asks bus drivers for a description of the devices that they find on their respective buses. The description includes data that uniquely identifies each device as well as the resource requirements of the devices. The PnP manager takes this information and loads any function or filter drivers that have been installed for the detected devices. It then calls the add-device routine of each driver for every installed device the drivers are responsible for.