[ The PC Guide | System Optimization and Enhancement Guide | System Optimizations and Enhancements | System Resource (IRQ, DMA, I/O, COM) Conservation and Optimization ]

Avoid Resource Conflicts By Using IRQ2/IRQ9

Many resource conflicts are tied to multiple devices fighting over the same IRQ lines. This is particularly a problem with the lower-numbered IRQ lines, from IRQ 0 to IRQ 7. Of these eight IRQs, 0 and 1 are reserved by the system, 3 and 4 are used by serial (COM) ports, 5 is normally used by the sound card, 6 is used by the floppy drive and 7 is normally reserved for the printer port. This leaves just IRQ 2, and many people are afraid to use it because of its special role as the interrupt cascade; it connects the high-numbered IRQs to the system. This confusing subject is described in detail here.

In fact, IRQ2 can be used. When the AT PC was designed and the 8 extra IRQ lines were installed, what used to be IRQ2 was rerouted to use the IRQ9 line. This means that any peripheral that is set to use IRQ2 will be seen by the system as using IRQ 9. As long as you don't set any devices to use IRQ 9, therefore, you can use IRQ 2. Just set the device to IRQ 2, and configure the software (driver, communications program, etc.) to use IRQ 9.

This can be extremely useful on a system with two COM ports, a printer, a modem and a sound card. Set the modem to use IRQ 2 and it will show up as IRQ 9 to the software. Most modems have a jumper that will let you do this.

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