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The termLegacy Plug and Play,[1] also shortened toLegacy PnP,[2] describes a series of specifications andMicrosoft Windows features geared towards operating system configuration of devices, and some device IDs are assigned byUEFI Forum.[3] The standards were primarily aimed at theIBM PC standard bus, later dubbedIndustry Standard Architecture (ISA). Related specifications are also defined for the common external or specialist buses commonly attached via ISA at the time of development, includingRS-232 andparallel port devices.
As a Windows feature, Plug and Play refers to operating system functionality that supports connectivity, configuration and management with native plug and play devices.[4] Originally considered part of the same feature set as the specifications, Plug and Play in this context refers primarily to the responsibilities and interfaces associated with Windows driver development.[5]
Plug and Play allows for detection of devices without user intervention, and occasionally for minor configuration of device resources, such as I/O ports and device memory maps. PnP is a specific set of standards, not be confused with the generic term plug and play, which describes any hardware specification that alleviates the need for user configuration of device resources.[6]
ACPI is the successor to Legacy Plug and Play.
The Plug and Play standard requires configuration of devices to be handled by the PnP BIOS, which then provides details of resources allocations to the operating system. The process is invoked at boot time. When the computer is first turned on, compatible devices are identified and assigned non-conflictingIO addresses,interrupt request numbers andDMA channels.
The term was adopted by Microsoft in reference to theirWindows 95 product. Otheroperating systems, such asAmigaOSAutoconfig and theMac OSNuBus system, had already supported such features for some time (under various names, or no name).[7] EvenYggdrasil Linux advertised itself as "Plug and Play Linux" at least two years before Windows 95. But the termplug and play gradually became universal due to worldwide acceptance of Windows.
Typically, non-PnP devices need to be identified in the computer's BIOS setup so that the PnP system will not assign other devices the resources in use by the non-PnP devices. Problems in the interactions between legacy non-PnP devices and the PnP system can cause it to fail, leading to this technology having historically been referred to as "plug and pray".[2]
Legacy Plug and Play Specification was defined byMicrosoft andIntel, which proposed changes to legacy hardware, as well as theBIOS to support operating system-bound discovery of devices. These roles were later assumed by theACPI standard,[1] which also moves support forpower management and configuration into the operating system, as opposed to thefirmware as previously required by the "Plug and Play BIOS" andAPM specifications. The following standards compose what Microsoft describe as Legacy Plug and Play, as opposed to native Plug-and-Play specifications such asPCI andUSB.
Windows Vista requires an ACPI-compliant BIOS, and the ISAPnP is disabled by default.[11]
To use Plug and Play, three requirements have to be met:
Plug-and-play hardware typically also requires some sort of ID code that it can supply, in order for the computer software to correctly identify it. The Plug-and-play ID can have two form: 3-byte manufacturer ID plus 2-byte hex number (e.g. PNP0A08), or 4-byte manufacturer ID plus 2-byte hex number (e.g. MSFT0101).[12] In addition, a PnP device may have Class Code and Subsystem ID.[13]
This ID code system was not integrated into the earlyIndustry Standard Architecture (ISA) hardware common in PCs when Plug and Play was first introduced. ISA Plug and Play caused some of the greatest difficulties that made PnP initially very unreliable. This led to the derisive term "Plug and Pray", sinceI/O addresses andIRQ lines were often set incorrectly in the early days. Latercomputer buses likeMCA,EISA andPCI (which was becoming the industry standard at that time) integrated this functionality.
Finally, theoperating system of the computer needs to be able to handle these changes. Typically, this means looking forinterrupts from the bus saying that the configuration has changed, and then reading the information from the bus to locate what happened. Older bus designs often required the entire system to be read in order to locate these changes, which can be time-consuming for many devices. More modern designs use some sort of system to either reduce or eliminate this "hunt"; for example, USB uses a hub system for this purpose.
When the change is located, the OS then examines the information in the device to figure out what it is. It then has to load up the appropriatedevice drivers in order to make it work. In the past, this was an all-or-nothing affair, but modern operating systems often include the ability to find the proper driver on theInternet and install it automatically.