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-conflicting IO addresses, interrupt request numbers and DMA channels. The term was adopted by Microsoft in reference to their Windows 95 product. Other operating systems, such as AmigaOS Autoconfig and the Mac OS NuBus system, had already supported such features for some time (under various names, or no name). Even Yggdrasil Linux advertised itself as "Plug and Play Linux" at least two years before Windows 95. But the term plug 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". ==Specifications==

Legacy Plug and Play Specification was defined by Microsoft and Intel, which proposed changes to legacy hardware, as well as the BIOS to support operating system-bound discovery of devices. These roles were later assumed by the ACPI standard, • Plug and Play ISA Specification • Plug and Play Design Specification for IEEE 1394 • Plug and Play External COM Device Specification • Plug and Play Parallel Port Device Specification • Plug and Play ATA Specification • Plug and Play SCSI Specification • Legacy Plug and Play Guidelines Windows Vista requires an ACPI-compliant BIOS, and the ISAPnP is disabled by default. ==Requirements==

To use Plug and Play, three requirements have to be met: • The OS must be compatible with Plug and Play. • The BIOS must support Plug and Play. • The device to be installed must be a Plug and Play compliant device. == Hardware identification ==

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). In addition, a PnP device may have Class Code and Subsystem ID. This ID code system was not integrated into the early Industry 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", since I/O addresses and IRQ lines were often set incorrectly in the early days. Later computer buses like MCA, EISA and PCI (which was becoming the industry standard at that time) integrated this functionality. Finally, the operating system of the computer needs to be able to handle these changes. Typically, this means looking for interrupts 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 appropriate device 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 the Internet and install it automatically. ==See also==