Resources Conflicts On Plug-and-Play Systems

Plug and Play (PnP) represents a major revolution in interface technology. PnP first came on the market in 1995, and most motherboards and adapter cards since 1996 take advantage of it. Prior to that, PC users were forced to muddle through a nightmare of DIP switches and jumpers every time they wanted to add new devices to their systems.

The results, all too often, were system resource conflicts and nonfunctioning cards. PnP was not an entirely new concept. It was a key design feature of MCA and EISA interfaces that preceded it by almost 10 years, but the limited appeal of MCA and EISA meant that they never became true de facto industry standards.

Therefore, mainstream PC users still had to worry about I/O addresses, DMA channels, and IRQ settings. Early PCI-based systems also used a form of PnP configuration, but because there was no provision for managing conflicts between PCI and ISA cards, many users still had configuration problems.

But now that PnP has become prevalent, worry-free hardware setup is available to all computer buyers. For PnP to work, the following components are desired:

  • PnP hardware

  • PnP BIOS

  • PnP operating system

Each of these components needs to be PnP-compatible, meaning that it complies with the PnP specifications.

Hardware Component

The hardware component refers to both computer systems and adapter cards. The term does not mean, however, that you can't use your older ISA adapter cards (referred to as legacy cards) in a PnP system. You can use these cards; in fact, your PnP BIOS automatically reassigns PnP-compatible cards around existing legacy components.

Also, many late-model ISA cards can be switched into PnP-compatible mode. PnP adapter cards communicate with the system BIOS and the operating system to convey information about which system resources are necessary.

The BIOS and operating system, in turn, resolve conflicts (wherever possible) and inform the adapter card which specific resources it should use. The adapter card then can modify its configuration to use the specified resources.

BIOS Component

The BIOS component means that most users of pre-1996 PCs need to update their BIOSs or purchase new machines that have PnP BIOSs. For a BIOS to be compatible, it must support 13 additional system function calls, which can be used by the OS component of a PnP system. The PnP BIOS specification was developed jointly by Compaq, Intel, and Phoenix Technologies.

The PnP features of the BIOS are implemented through an expanded POST. The BIOS is responsible for identification, isolation, and possible configuration of PnP adapter cards. The BIOS accomplishes these tasks by performing the following steps:

  1. Disables any configurable devices on the motherboard or on adapter cards

  2. Identifies any PnP PCI or ISA devices

  3. Compiles an initial resource-allocation map for ports, IRQs, DMAs, and memory

  4. Enables I/O devices

  5. Scans the ROMs of ISA devices

  6. Configures initial program-load (IPL) devices, which are used later to boot the system

  7. Enables configurable devices by informing them which resources have been assigned to them

  8. Starts the bootstrap loader

  9. Transfers control to the operating system

Operating System Component

The operating system component is found in most modern operating systems, such as Windows 9x/Me/2000/XP. In some cases system manufacturers have provided extensions to the operating system for their specific hardware. Such is especially true for notebook systems, for example. Be sure you load these extensions if they are required by your system.

It is the responsibility of the operating system to inform users of conflicts that can't be resolved by the BIOS. Depending on the sophistication of the operating system, the user then could configure the offending cards manually (onscreen) or turn off the system and set switches on the physical cards.

When the system is restarted, the system is checked for remaining (or new) conflicts, any of which are brought to the user's attention. Through this repetitive process, all system conflicts are resolved.