Resolving IRQ Conflicts, Plug-and-Play BIOS

Resolving IRQ Conflicts

A conflict can occur if more than one device is assigned the same IRQ or I/O address during POST. These conflicts are resolved on older expansion cards by changing the DIP switch or jumper settings. This change causes the card to request an alternative IRQ or I/O address.

For example, if a scanner card and a network card both request IRQ 10, then there is a conflict. Startup BIOS or the operating system cannot write the memory address of the network driver in the same I/O address location associated with IRQ 10 if the memory address of the scanner driver is already written there. If this were to happen, and the scanner asked for service by sending IRQ 10 to the CPU, the CPU would respond with a network driver. In this case, probably neither the scanner nor the network card would work. If you encounter a conflict, check both cards. Most likely one of them has a jumper that you can set so that the card requests an IRQ other than 10.

With newer cards, called Plug-and-Play cards, instead of using DIP switch and jumpers settings to identify hardware information and its configuration, the Startup BIOS automatically chooses the resources (such as IRQs or I/O addresses) that are assigned to the card. Because the BIOS can select the resources, conflicts are easily resolved using Plug-and-Play cards. The BIOS must be the kind that manages Plug-and-Play devices and is called Plug-and-Play BIOS. The operating system must also be Plug-and-Play (PnP).

Plug-and-Play operating systems like Windows 9x can set IRQ or I/O addresses.

Conflicts can occur that make the cards inoperable if DIP switches or jumpers on the cards are set to select a certain resource, but Windows or BIOS does not have the resource available.

Plug-and-Play BIOS

Plug-and-Play (PnP) is a term that applies to both the Windows 9x operating system and to some ROM BIOS. It means that rather than having you reset DIP switches and jumpers, the operating system and/or the BIOS automatically configures hardware devices to reduce or eliminate conflicting requests for such system resources as I/O addresses, IRQs, DMA channels, or upper memory addresses.

Windows 9x Plug-and-Play assigns these resources to a device only if the device allows it. For example, if an older sound card requires a certain group of upper memory addresses that are hard coded into its on-board BIOS, there’s nothing that Windows 9x Plug-and-Play can do about that. (Hard coded is jargon for something being coded so that it cannot be changed.) Plug-and-Play simply tries to work around the problem as best it can.

If two non-Plug-and-Play hardware devices require the same resource and their BIOS does not provide for accepting a substitute, these two devices cannot coexist on the same PC.

Newer devices that are Plug-and-Play-compliant are more cooperative. At startup, they simply request to work and then wait for the operating system to assign the resources they need. Windows 9x and Windows 2000 try to do that whether or not the system BIOS is Plug-and-Play BIOS.

At startup, it’s the Startup BIOS that examines the hardware devices present, takes inventory, and then loads the operating system. Part of the job of Plug-and-Play BIOS is to collect information about the devices and the resources they require and later work with Windows 9x or Windows 2000 to assign the resources.

ESCD (extended system configuration data) Plug-and-Play BIOS goes even further, creating a list of all the things you have done manually to the configuration that Plug-and-

Play does not do on its own.

This ESCD list is written to the BIOS chip so that the next time you boot, the Startup BIOS can faithfully relate that information to Windows.

The BIOS chip for ESCD BIOS is a special RAM chip called Permanent RAM or PRAM that can hold data written to it without the benefit of a battery that the CMOS setup chip requires.

Most ROM BIOS made after the end of 1994 is Plug-and-Play. Windows 9x and Windows 2000 can use most, but not all, of its Plug-and-Play abilities without Plug-and-Play BIOS.