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Requirements for Plug and Play

Automatically detecting and configuring hardware and software is not a simple task. To perform this work, cooperation is required from several hardware and software areas. The four "partners" that must be Plug and Play compliant in order for it to work properly are:

• System Hardware: The hardware on your system, through the system chipset and system bus controllers, must be capable of handling PnP devices. For modern PCI-based systems this is built in, as PCI was designed with PnP in mind. Most PCI-based systems also support PnP on their ISA bus, with special circuitry to link the two together and share resource information. Older PCs with ISA-only or VL-bus system buses generally do not support Plug and Play.
• Peripheral Hardware: The devices that you are adding into the system must themselves be PnP compatible. PnP is now supported for a wide variety of devices, from modems and network cards inside the box to printers and even monitors outside it. These devices must be PnP-aware so that they are capable of identifying themselves when requested, and able to accept resource assignments from the system when they are made.
• The System BIOS: The system BIOS plays a key role in making Plug and Play work. Routines built into the BIOS perform the actual work of collecting information about the different devices and determining what should use which resources. The BIOS also communicates this information to the operating system, which uses it to configure its drivers and other software to make the devices work correctly. In many cases older PCs that have an outdated BIOS but otherwise have support for PnP in hardware (PCI-based Pentiums produced between 1993 and 1995 are the prime candidates) can be made PnP-compliant through a BIOS upgrade.
• The Operating System: Finally, the operating system must be designed to work with the BIOS (and thus indirectly, with the hardware as well). The operating system sets up any low-level software (such as device drivers) that are necessary for the device to be used by applications. It also communicates with the user, notifying him or her of changes to the configuration, and allows changes to be made to resource settings if necessary. Currently, the only mainstream operating system with full PnP support is Windows 95.

As you can see, you need a lot for Plug and Play to work, and this is why the vast majority of older systems (pre-1996) do not properly support this standard.

Next: Plug and Play Operation

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