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Free Up an IRQ by Combining IDE channels and Disabling Unused Ones

Most newer motherboards come with two IDE/ATA channel controllers built in, for using IDE hard disks, CD-ROMs and similar devices. Each of these controllers uses an IRQ; the normal allocations are IRQ14 for the primary IDE controller, and IRQ15 for the secondary channel.  In many cases, it is possible to recover at least one of these IRQs if you need them for other devices such as a SCSI host adapter, network card or tape accelerator card.

Here are some ideas that you can use:

• Disable Unused Motherboard IDE Channels: If you are only using the primary IDE channel on a system with a motherboard that contains two built-in controllers (and most do), then you should disable the secondary controller to free up the resources it uses. This is easy: simply go into the BIOS setup and look for the settings that control the IDE controllers. These are usually found in the "Integrated Peripherals" grouping (and not where the hard disk parameters are entered, mind you.) Disable the secondary IDE controller; how exactly you do this depends on how the settings are programmed on your PC.
• Disable All IDE If Using SCSI Only: If you are using only SCSI devices and no IDE at all, disable all the IDE controllers on the system. This is done in a similar way to disabling the secondary controller, as described immediately above.
• Disable Unused IDE Ports on Sound Cards: Many sound cards today come with integrated IDE ports on them, intended for supporting CD-ROM drives as part of multimedia kits. These often are set up to establish themselves as tertiary IDE controllers, using IRQ11 or IRQ12. Most people don't use them because their motherboards have two IDE controllers already. If you aren't using yours, disable the IDE controller using whatever means the sound card provides: usually this is either a jumper on the sound card, or a setting in a special setup program.
• Rearrange IDE Channel Usage: Each IDE channel can handle two hard disks, CD-ROM drives or other devices. For optimal performance, though, many people (including myself) recommend using only one device per channel if you have two IDE devices. If on the other hand saving an IRQ is more of a priority for you than optimal performance (and the difference in performance in some cases isn't really large) then you may want to rearrange your setup and put both devices on the primary channel, and then disable the secondary channel as above. This section in the Reference Guide discusses IDE device configuration and lays out all the options.

Next: Use an Add-in 16-Bit Card I/O Card for Multiple Serial Ports

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