The term Legacy Plug and Play, also shortened to Legacy PnP, describes a series of specifications and

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...

features geared towards operating system configuration of devices, and some device IDs are assigned by UEFI Forum. The standards were primarily aimed at the

IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...

standard bus, later dubbed Industry Standard Architecture (ISA). Related specifications are also defined for the common external or specialist buses commonly attached via ISA at the time of development, including RS-232 and

parallel port In computing, a parallel port is a type of interface found on early computers (personal and otherwise) for connecting peripherals. The name refers to the way the data is sent; parallel ports send multiple bits of data at once ( parallel ...

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. 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. 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. ACPI is the successor to Legacy Plug and Play.

Overview

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 address Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O p ...

es, interrupt request numbers and

DMA channel Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU). Without DMA, when the CPU is using programmed input/output, it is t ...

s. The term was adopted by Microsoft in reference to their Windows 95 product. Other operating systems, such as AmigaOS

Autoconfig Autoconfig is an auto-configuration protocol of Amiga computers which is intended to automatically assign resources to expansion devices without the need for jumper settings. It is analogous to PCI configuration through ACPI. Autoconfig is i ...

and the Mac OS NuBus system, had already supported such features for some time (under various names, or no name). Even

Yggdrasil Linux Yggdrasil Linux/GNU/X, or LGX (pronounced ''igg-drah-sill''), is an early Linux distribution developed by Yggdrasil Computing, Incorporated, a company founded by Adam J. Richter in Berkeley, California. Yggdrasil was the first company to creat ...

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 reassign those 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 In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...

to support operating system-bound discovery of devices. These roles were later assumed by the ACPI standard, which also moves support for power management and configuration into the operating system, as opposed to the firmware as previously required by the "Plug and Play BIOS" and

APM APM, apm, or Apm may refer to: Technology Computer technology * Active policy management, a discipline within enterprise software *Advanced Power Management, a legacy technology in personal computers * Apple Partition Map, computer disk partit ...

specifications. The following standards compose what Microsoft describe as Legacy Plug and Play, as opposed to native Plug-and-Play specifications such as

PCI PCI may refer to: Business and economics * Payment card industry, businesses associated with debit, credit, and other payment cards ** Payment Card Industry Data Security Standard, a set of security requirements for credit card processors * Pro ...

and USB. *Plug and Play BIOS Specification *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

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. This ID code system was not integrated into the early

Industry Standard Architecture Industry Standard Architecture (ISA) is the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s. The bus was (largely) backward compatible with the 8-bit bus of the 8 ...

(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 bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...

es like

MCA MCA may refer to: Astronomy * Mars-crossing asteroid, an asteroid whose orbit crosses that of Mars Aviation * Minimum crossing altitude, a minimum obstacle crossing altitude for fixes on published airways * Medium Combat Aircraft, a 5th gene ...

, EISA and

(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 driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and ot ...

s 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.

References

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External links