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Plug-and-Play BIOS

The role of the traditional BIOS was to manage the essential devices in the system: the hard drive, floppy drive, video, parallel and serial ports, and keyboard and system timer. Other devices were left to fight for the remaining IRQs and other hardware resources listed in Chapter 2, "System Components and Configuration." When Windows 95 was introduced, the role of the BIOS changed dramatically. To support Windows 95, the Plug-and-Play BIOS was introduced, changing how cards were installed and managed. Table 3.2 compares a Plug-and-Play (PnP) BIOS to a conventional BIOS.

Table 3.2 Plug-and-Play BIOS Versus Conventional BIOS


Conventional BIOS

Plug-and Play BIOS

Hardware configuration devices and video only


All PnP devices as well as motherboard devices

Configuration type

Static (fixed settings)

Dynamic (settings can be altered as various devices are installed)


Manual configuration

Manual, BIOS-assisted, or operating method system assisted

Operating system relationship to BIOS

Accepts all BIOS settings without alteration

Receives PnP device information from BIOS and can alter settings as required


A complete list of PnP device IDs is found in the Technical Reference section of the CD included with Upgrading and Repairing PCs, 13th Edition.

PnP BIOS Configuration Options

Although PnP BIOSes vary widely in their features, the following settings are typical. Use the list in Table 3.3, along with the tables that follow, to help you make configuration changes when necessary.

Resource Configuration

The Resource Configuration menu is used for configuring the memory and interrupt usage of non–Plug-and-Play (legacy) ISA bus-based devices. Table 3.3 shows the functions and options found in a typical modern BIOS.

Table 3.3 Typical Resource Configuration Menu1




Memory Reservation

C800 CBFF Available (default) | Reserved

CC00 CFFF Available (default) | Reserved

D000 D3FF Available (default) | Reserved

D400 D7FF Available (default) | Reserved

D800 DBFF Available (default) | Reserved

DC00 DFFF Available (default) | Reserved

Reserves specific upper memory blocks for use by legacy ISA devices.



IRQ 3 Available (default) | Reserved

IRQ 4 Available (default) | Reserved

IRQ 5 Available (default) | Reserved

IRQ 7 Available (default) | Reserved

IRQ 10 Available (default) | Reserved

IRQ 11 Available (default) | Reserved

Reserves specific IRQs for use by legacy ISA devices. An asterisk (*)displayed next to an IRQ indicates an IRQ conflict.

Note that these settings are only for legacy (non–Plug-and-Play) ISA devices. For all Plug-and-Play ISA devices as well as PCI devices (which are Plug-and-Play by default), these resources are instead configured by the operating system or by software that comes with the cards.

Setting these resources here does not actually control the legacy ISA device; that usually must be done by moving jumpers on the card. By setting the resource as reserved here, you are telling the Plug-and-Play operating system that the reserved resources are off-limits, so it won't accidentally set a Plug-and-Play device to use the same resource as a legacy ISA device. Reserving resources in this manner is sometimes required because the Plug-and-Play software can't detect all legacy ISA devices and therefore won't know which settings the device might be using.

In a system with no legacy devices, reserving any resources via this menu is not necessary. To enable more options for the increasing number of PCI/PnP slots and cards found on recent systems, you can disable reservation for legacy devices that you don't use. For example, if you don't use LPT 1 (IRQ 7) and COM 2 (IRQ 3) ports, disable them in the BIOS and set these IRQs as available for PCI/PnP devices.

Some boards have additional configuration options for the Plug-and-Play (PnP) BIOS features as well as the PCI bus. These features are largely chipset-dependent, but some common examples are shown in Table 3.4.

Table 3.4 Typical PnP and PCI Options1

DMA Assigned to

When resources are controlled manually, assign each system DMA channel as one of the following types, depending on the type of device using the interrupt:

Legacy ISA devices compliant with the original PC AT bus specification, requiring a specific DMA channel.

PCI/ISA PnP devices compliant with the Plug-and-Play standard, whether designed for PCI or ISA bus architecture.

PCI IRQ Activated by

Leave the IRQ trigger set at Level unless the PCI device assigned to the interrupt specifies edge-triggered interrupts.


This field enables you to select PCI IDE IRQ mapping or PC AT (ISA) interrupts. If your system does not have one or two PCI IDE connectors on the system board, select values according to the type of IDE interface(s) installed in your system (PCI or ISA). Standard ISA interrupts for IDE channels are IRQ 14 for primary and IRQ 15 for secondary.

Primary/Secondary IDE INT#

Each PCI peripheral connection is capable of activating up to four interrupts: INT# A, INT# B, INT# C, and INT# D. By default, a PCI connection is assigned INT# A. Assigning INT# B has no meaning unless the peripheral device requires two interrupt services rather than one. Because the PCI IDE interface in the chipset has two channels, it requires two interrupt services. The primary and secondary IDE INT# fields default to values appropriate for two PCI IDE channels, with the primary PCI IDE channel having a lower interrupt than the secondary.

Note that all single-function PCI cards normally use INT# A, and each of these must be assigned to a different and unique ISA interrupt request (IRQ).

Used Mem Base Addr

Select a base address for the memory area used by any peripheral that requires high memory.

Used Mem Length

Select a length for the memory area specified in the previous field. This field does not appear if no base address is specified.

Assign IRQ for USB

Select Enabled if your system has a USB controller and you have one or more USB devices connected. If you are not using your system's USB controller, select Disabled to free the IRQ resource.

When to Use the PnP BIOS Configuration Options

In an ideal situation involving PnP-aware operating systems such as Windows 9x/Me or 2000/XP, a computer with a PnP BIOS, and a PnP device, the BIOS detects the PnP device and Windows configures it without user intervention. Table 3.5 lists the circumstances under which you might need to use PnP BIOS configuration options.

Table 3.5 Solving Configuration Problems with the PnP BIOS Configuration Options




Legacy (non-PnP) card needs particular IRQ or DMA setting already in use by PnP device.

Set DMA and IRQ used by legacy card to ISA option in BIOS.

This prevents PnP devices from using the resource; verify that legacy card setting matches BIOS selections.

Windows 9x/Me/2000/XP is not detecting and configuring PnP devices not needed at boot time (such as modems, printers, and so on).

Set Plug and Play Aware Operating System option to Yes in BIOS.


PCI video card is assigned an IRQ that you need for another device.

Set Assign IRQ to VGA option to No in BIOS.

This frees up the IRQ with out ill effects in some cases; might not work if the card is used for MPEG movie playback. Do not use for AGP video cards.

New PnP device can't be detected by the system.

Set PCI Slot x IRQ Priority to desired (unused) IRQ; install card into designated PCI slot.

If setting the IRQ for the PCI slot doesn't work, remove all non-essential PnP cards, install new PnP card first, and then reinstall others.

New PnP device interferes

Check system manual to determine whether the slot that is used shares an IRQ with another slot or with an onboard device.

Using a different slot for the with existing PnP device new PnP device can avoid conflicts.

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