Back in the Jurassic days of Windows 3.x, the process of installing a new modem, network interface, or other expansion card was at best hit-or-miss. You plugged your card into an expansion slot and waited to see if its IRQ, or interrupt request line, would happily coexist with the ones for all the other hardware in your PC. If it didn't, readjusting the settings was a nightmare.

Nowadays, adding a new card is much easier thanks to Windows 9x's Plug and Play technology. But in a way, today's PCs are victims of their own success. Many currently available systems come with so many internal devices (each preconfigured with its own IRQ) that you may actually run out of free IRQs before you install a new card or device. If you find yourself without a free resource when installing a new card or device, here's what you need to know.

Many of your PC's hardware components communicate with your system through an IRQ. Whenever a device has a task for the CPU, it sends a signal to the CPU via an IRQ. When the CPU sees this signal, it interrupts its current task and gives its attention to the device. For example, each time you type a letter on your keyboard, the keyboard sends a request on its IRQ line alerting the CPU that a character needs to be processed.

No room for add-ons. Before you buy a new card or peripheral, find out whether it needs an IRQ. Most cards that plug into expansion slots, whether PCI or ISA, require an IRQ. Significant exceptions include most video cards, USB devices, and EIDE or SCSI devices like CD-ROM drives or hard disks. (The IRQs that these devices use are automatically assigned to the USB, EIDE, or SCSI bus rather than to the device itself.)

If your new hardware does need an IRQ, figure out how many free IRQs your computer has. Unfortunately, most systems come from the factory with only a few unused IRQs. A typical PC has a total of 16 IRQs, numbered 0 through 15. To see how many IRQs are free on your PC, go to the Windows 98's System Information utility at Start-Programs-Accessories-System Tools-System Information, click Hardware Resources, and then IRQs.

Windows 95 users can obtain a list of IRQ designations by right-clicking the My Computer icon on the desktop, selecting Properties and then the Device Manager tab, and double-clicking the Computer icon at the top of the hardware tree.

If you have one or more unused IRQs, your PC and Windows should automatically match any new hardware with a free IRQ during installation. But even if all your system's IRQs appear to be in use, you may be able to work around the deficiency.

Fix faulty steering. If you're installing a PCI card (check the box it came in if you're not sure), then it might not need its own IRQ. PCI slots were designed to share IRQs, and if you're lucky, your card will install without requiring a new one. But if that's not the case, try the following tweaks.

First, enter your PCs CMOS setup program and find the PCI setup screen. What you see there will vary depending on your BIOS's make and version, but the choices and labels should generally be the same. Look for a setting that controls IRQ sharing and make sure it's enabled. If you're given a choice of ISA/Legacy, PCI, or Automatic setting for each IRQ, specify Automatic (usually the default) for each. If that doesn't help, try selecting PCI.

If you're using Windows 98 or the OSR2 version of Windows 95 (version 4.00.950B or 4.00.950C as listed in the System Properties dialogue box in My Computer), you can enable its PCI steering feature, which also facilitates the sharing of IRQs on the PCI bus.

In Device Manager, double-click System devices, then PCI bus. Under the IRQ Steering tab, make sure the Use IRQ Steering box is checked. Also make sure that all the boxes below the Use IRQ Steering box are checked except for 'Get IRQ table from Protected Mode PCIBIOS 2.1 call'. If you still can't get your PCI card to install properly, try enabling that box as well.

An IRQ for an IRQ. Another way to free up an IRQ is to take it from another device. Start by looking through your CMOS setup program for settings that let you disable devices. For example, many CMOS programs will let you disable your mouse port and clear its IRQ ù usually IRQ 12 ù for use by another device. That's an option if you're using a serial mouse, not one attached to the dedicated mouse port.

Most CMOS programs will also allow you to free up an IRQ by disabling your unused USB, serial, or parallel ports. And if you're really desperate, you can try disabling one of the two EIDE channels on your motherboard, assuming that they're not being used by a hard disk, CD-ROM drive, or other device. Check your CMOS setup program to be sure.