Legacy Plug and Play
A set of standards for automatic device configuration on ISA-based systems.
Legacy Plug and Play, often called Legacy PnP, refers to a set of specifications and Microsoft Windows features that allow an operating system to configure hardware devices. These standards were mainly developed for the IBM PC standard bus, which later became known as the Industry Standard Architecture (ISA). Similar specifications were also created for common external or specialized buses that connected through ISA at the time, such as RS-232 and parallel port devices. In the context of Windows, Plug and Play describes the operating system's ability to support the connection, setup, and management of devices that are designed to work with this system.
Plug and Play can detect devices without requiring the user to do anything, and it can sometimes handle minor configuration tasks like setting I/O ports and device memory maps. It is important to note that PnP is a specific set of standards, not the same as the general term "plug and play," which refers to any hardware design that avoids manual user configuration of resources. The Advanced Configuration and Power Interface (ACPI) is the technology that replaced Legacy Plug and Play.
Under the Plug and Play standard, device configuration is managed by the PnP BIOS, which gives the operating system information about how resources are allocated. This process happens when the computer starts up. At boot time, compatible devices are found and given non-conflicting I/O addresses, interrupt request numbers, and DMA channels.
Microsoft adopted the term for its Windows 95 product. Other operating systems, like AmigaOS with its Autoconfig system and the Mac OS with its NuBus system, had already supported similar features for years, sometimes without a specific name. Yggdrasil Linux also advertised itself as "Plug and Play Linux" at least two years before Windows 95. However, the term became widely used because of Windows' global popularity.
Devices that are not PnP-compatible usually need to be listed in the computer's BIOS setup. This prevents the PnP system from assigning those devices' resources to other hardware. Problems between older non-PnP devices and the PnP system can cause failures, which led to the nickname "plug and pray."
The Legacy Plug and Play Specification was created by Microsoft and Intel. It proposed changes to older hardware and the BIOS to allow the operating system to discover devices.
- Defined by
- Microsoft and Intel
- Successor
- ACPI
- Related buses
- ISA, RS-232, parallel port
- Hardware id forms
- 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)
- Os requirement for vista
- Windows Vista requires an ACPI-compliant BIOS, and ISAPnP is disabled by default
Lore & Background
The Plug and Play standard requires configuration of devices to be handled by the PnP BIOS, which then provides details of resource 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. 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'. The 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, 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 specifications.
Reader's Guide
Legacy Plug and Play represents a transitional phase in PC hardware configuration, moving from manual jumper settings to automated resource assignment. Its significance lies in establishing the principle that the operating system, rather than the user, should manage device resources such as I/O ports, interrupt requests, and DMA channels. The standard required three components to work together: a compatible operating system, a PnP BIOS, and a PnP-compliant device. However, its implementation on the ISA bus proved problematic, as ISA hardware lacked integrated identification mechanisms. This led to frequent resource conflicts and the coining of the term 'plug and pray'. The ID code system used by PnP hardware—either a 3-byte manufacturer ID plus 2-byte hex number or a 4-byte manufacturer ID plus 2-byte hex number—was not integrated into early ISA hardware, contributing to unreliability. Later buses like MCA, EISA, and PCI integrated this functionality, making Plug and Play more reliable. The legacy of Legacy Plug and Play is its role as a precursor to ACPI, which subsumed its configuration and power management responsibilities into the operating system. Windows Vista ultimately required an ACPI-compliant BIOS and disabled ISAPnP by default, marking the end of the legacy standard's active use.
Did You Know?
- The term 'Plug and Play' was adopted by Microsoft in reference to Windows 95.
- Problems with non-PnP devices interacting with the PnP system led to the derisive term 'plug and pray'.
- ACPI is the successor to Legacy Plug and Play.
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