IRQ Settings for PCs

For those that haven't turned on the PC's USB port or are unsure, please contact the PC vendor. The USB does use another IRQ, so you might have to have change some hardware to free up system resources. New MBs have the USB running all of the time. With rare exception, you can't turn it off. So make sure that you account for its resources and install the drivers correctly. It usually takes IRQ 9, 10, or 11 which happen to be the only free IRQs in the PC these days.

Version 7T BIOS, 2/18/98, and later of the ABIT PII motherboard now allows the USB IRQ to be turned ON or OFF. This is a very new feature for any motherboard. Plan on the USB controller eating an IRQ.

Legacy ISA cards require you to play with the BIOS

Older ISA, non PnP, or legacy cards still need interrupts but can't use the system's IRQ manager in the BIOS. The BIOS doesn't see the card to assign any resources. Results can be tragic. Cards on the PCI bus don't work because an invisible ISA card uses the same interrupt. This still happens more than you would think. Brand new CDROM burner kits, for example,  ship with very cheesy, ISA, SCSI controller cards. These have jumpers for address and IRQ settings. The card is free, so you can't moan too much. Still the IRQ must be accounted for in the BIOS. A PCI card will try to use the same IRQ without some help. The solution, assuming you don't want to buy new hardware, is to have a MB BIOS that has some controls for MANUAL IRQ assignment.

Most good, modern MBs allow manual control of the IRQ maps. These settings are best illustrated by an example.

By default, fully automatic IRQ assignments allows the PCI bus to access IRQs, 5, 9,10,11, and 12. 12 really isn't available either, since the PS/2 mouse port takes that one. The rest are already set aside for IDE controllers and the rest of the on board I/O. Now add a legacy SCSI controller at IRQ 11.

IRQ1 Keyboard IRQ8 CMOS/Real time clock
IRQ2 Interrupt Controller IRQ9 PnP (Display Adapter)
IRQ3 COM 2 IRQ10 PnP (Ethernet Card)
IRQ4 COM1 IRQ11 PnP ( modem ) and SCSI controller with CDROM
IRQ5 PnP ( ) IRQ12 PS/2 Mouse
IRQ6 Floppy Controller IRQ14 IDE Controller ( HD)
IRQ7 Printer Port IRQ15 IDE Controller ()

With the automatic PnP BIOS both the modem and SCSI controller use IRQ11. Since both require a unique IRQ, neither will work. Notice that IRQ 15 is also wasted. The modem didn't go to IRQ 5 either because it didn't see the problem with the ISA card.

The usual IDE CDROM isn't needed. The CD is on the SCSI bus with its controller. So disable the secondary IDE controller.

Go to the PnP BIOS configuration settings in the BIOS. Now put the BIOS in manual mode, block out IRQ 11 from the PnP availability table, and add IRQ 15 so that it is available. In a 100% SCSI system you would also make IRQ 14 available to the PnP BIOS, since no IDE controllers would be needed. In either case, the results are great.

IRQ1 Keyboard IRQ8 CMOS/Real time clock
IRQ2 Interrupt Controller IRQ9 PnP (Display Adapter)
IRQ3 COM 2 IRQ10 PnP (Ethernet Card)
IRQ4 COM1 IRQ11 Manually assigned to ISA SCSI card, CDROM(s)
IRQ5 PnP ( modem ) IRQ12 PS/2 Mouse
IRQ6 Floppy Controller IRQ14 IDE Controller ( HD)
IRQ7 Printer Port IRQ15 PnP auto assigned USB controller

Tricky sound card IRQ evils

Sound cards such as the Sound Blaster family and clones often have an onboard IDE controller for a CDROM. Systems used to have no on board IDE ports. An IDE controller card added to the system drove the HD(s). The SB IDE port came in handy, since it adding a second IDE port was almost impossible.

This sound card IDE port uses an IRQ,  just as any IDE controller does. Some cards have jumpers to turn this port off, but most of the newer cards are PnP and have no jumper. New MBs have two on board, fast, UDMA33, IDE controllers now, so that sound card controller is not necessary.  In fact it gets in the way. The IDE CDROM runs from the secondary controller on the mother board. So, the SB IDE port is worthless. You think, "I'll just turn it off in WIN95 or NT. That way I won't use that precious interrupt."

Wrong.

The system BIOS assigns the IRQs during POST, Power On Self Test, before any unused hardware gets disabled in WIN95, for example. So, the system hardware uses two IRQs for the sound card - one for the sound and one for the redundant IDE port. These interrupts don't show on the start screen either, since PCI IRQs only show on startup.

If the PC has no spare interrupts, then some piece of hardware will not initialize properly. In the case of a SCSI controller which must have an IRQ to work, the SCSI BIOS won't load. Therefore ASPI devices such as the CDROM or tape will not be recognized. If the IRQ conflict happens with the ethernet card, then the network will not start.

These cases can be quite frustrating. The WIN95 or '98 resource manager will show no conflicts. You will scream, "Why doesn't this work? Nothing is wrong." Well, something is wrong, but it happened at POST. The bottom line is that you must have enough IRQs at bootup not in a particular OS.

Another common problem with older sound cards is that they will use more than one IRQ. Instead of using one IRQ for everything, the Sound Blaster uses one, and so will the MPU401 chip. Some old cards will use three if they have a controller on board! You can't afford to use more than one IRQ on a standard sound card these days. Spare IRQs are a thing of the past.  A Creative Labs, Ensoniq, PCI sound card is under $30 and works well. Getting rid of an old sound card can improve your audio quality and use system resources more efficiently at the same time.

A typical IDE PC is shown below. Notice free IRQs do not exists.

IDE System IRQ Table

IRQ1 Keyboard IRQ8 CMOS/Real time clock
IRQ2 Interrupt Controller IRQ9 Display Adapter
IRQ3 COM 2 IRQ10 Ethernet Card
IRQ4 COM1 IRQ11 USB port ? Scanner? Internal Modem?
IRQ5 Sound Card IRQ12 PS/2 Mouse
IRQ6 Floppy Controller IRQ14 IDE Controller ( HD)
IRQ7 Printer Port IRQ15 IDE Controller (CDROM, Int IDE ZIP)

From the above table, you can see that a typical PC gets busy if you add other toys to the hardware pool. Plug and Play, PnP, hardware is nice, it requires every device to have a unique IRQ. If the PC also had a SCSI controller, for a CDROM burner for example, then you are out of IRQ settings. To make maters worse, most mother boards don't allow you to disable the USB or PS/2 mouse functions. One way to help out is to disable the on board COM2 or use an external modem. Either method would save a valuable interrupt for the USB controller. This is why heavy multimedia PCs use SCSI for the HD's, CDROM, and scanners. Otherwise, they would run out of free interrupt settings.

UDMA66 Speed Complicates IDE Life

Some of the new PII/PIII motherboards now offer UDMA66 controllers on board. The ABIT BE6, for example, uses the Hot Point Technology, HPT, UDMA66 IDE controller. The HPT controller has two UDMA66 connectors on the motherboard for four UDMA66 IDE hard drives. It shows up as a SCSI controller in WIN9X or NT. It also burns another IRQ, usually IRQ11.

The good news is the HPT controller offers the new WD Expert and other UDMA66 HDs a blazing, 66MHz I/O channel, and you can now have up to eight IDE devices in the same box. The bad news is that few PCs can afford to use three interrupts for IDE controllers. Most people will end up turning off either the primary or secondary IDE controller, or perhaps the second COM port. The extra IRQ must be reclaimed.

UDMA66 IDE System IRQ Table

IRQ1 Keyboard IRQ8 CMOS/Real time clock
IRQ2 Interrupt Controller IRQ9 Display Adapter
IRQ3 COM 2 IRQ10 Ethernet Card
IRQ4 COM1 IRQ11 UDMA66 HPT IDE controller
IRQ5 Sound Card IRQ12 PS/2 Mouse
IRQ6 Floppy Controller IRQ14 USB port ? Scanner? Internal Modem?
IRQ7 Printer Port IRQ15 IDE Controller (CDROM, Int IDE ZIP)

One advantage to keeping all three IDE controllers is that speed is easy when you add CDROM burner to your system. IDE controllers do not multitask, so it is best if you can keep a CDROM burner and the source on a separate controller. Having separate controllers is very nice when it comes to making audio CD copies. Notice from the table below that the video card had to give up its IRQ in order for everything to fit properly.

UDMA66 IDE System with all controllers active IRQ Table

IRQ1 Keyboard IRQ8 CMOS/Real time clock
IRQ2 Interrupt Controller IRQ9 USB controller (no video IRQ *)
IRQ3 COM 2 IRQ10 Ethernet Card
IRQ4 COM1 IRQ11 UDMA66 HPT IDE controller
IRQ5 Sound Card IRQ12 PS/2 Mouse
IRQ6 Floppy Controller IRQ14 IDE Controller, CDROM burner
IRQ7 Printer Port IRQ15 IDE Controller (CDROM, Int IDE ZIP)

SCSI System IRQ Table

IRQ 3 COM 2, PnP UPS control IRQ9 Internal PnP, COM 4 whatever you want!
IRQ4 COM 1 (modem?) IRQ10 Ethernet Card
IRQ 5 Sound Card, SB16, etc IRQ11 Display Adapter
IRQ6 Floppy IRQ12 PS/2 Mouse
IRQ7 Printer Port, LPT1 IRQ14 SCSI Controller ( HD, CD, ZIP, Scanner, CDROM burner)
IRQ8 CMOS/Real time clock IRQ15 USB Port

Although SCSI HDs and CDROMs are expensive, sometimes it is the only way to solve the problem.

It is common practice to make a hybrid system to keep the cost down and to still free up system resources. CDROM burners don't work worth beans anyway without a good, PCI, SCSI controller. So you might as well get the most out of your system. Connect anything that you can to the SCSI bus to save those IRQs.

The example below has everything working, but all IRQs are used up. Future expansion would be limited to additional SCSI devices or removing/disabling current hardware.

Hybrid IDE/SCSI System IRQ Table

IRQ 3 COM 2, PnP UPS control IRQ9 USB Controller
IRQ4 COM 1 (modem?) IRQ10 Ethernet Card
IRQ 5 Sound Card, SB16, etc IRQ11 Display Adapter
IRQ6 Floppy IRQ12 PS/2 Mouse
IRQ7 Printer Port, LPT1 IRQ14 IDE Primary Controller, main HD and future 2nd HD
IRQ8 CMOS/Real time clock IRQ15 SCSI Controller ( CD, ZIP, Scanner, CDROM burner)

The SCSI controller used here would be an Adaptec 2910, since HD support is not necessary. The 2910 is less than half the cost of its big brother, the 2940UW. CDROM writers also work much better with the 2910, PCI, SCSI controller than the cheap unit that comes free with the kit.

The SCSI CDROM and ZIP drives are more expensive than IDE versions, but the hybrid system is much cheaper than a full SCSI system. You can get twice the storage space in IDE than you can in SCSI for the same money, for example. The system will still accommodate a second IDE HD, too.

USB to the rescue?

USB is becoming a viable alternative for some of this IRQ mess. As of 10/98 you can buy USB scanners, keyboards, mice, and high quality, V.90 56K modems. HP, for example, makes two USB scanners: a low cost $220 unit and a high end, 1200dpi $465 model which also has a SCSI port. This is the HP6250CXi. So you can burn one IRQ for the USB controller and save some headaches.

Keep in mind that USB only works on WIN98 and newer versions of WIN95, SR 2.x and later. Still this is the majority of new systems these days. WIN2000, AKA WIN NT version 5, also supports USB. It is the first version of NT to do so.You might even have a free COM port left to talk to your Palm Pilot!

In the last year USB toys have started to pop out of the woodwork. For IRQ starved PCs, you can buy USB mice, USB to Parallel, and USB to Serial converters. All of these products allow you to save IRQs for just a few dollars. It  the case of the USB to Parallel converter, you can now drive more than one printer.

Some new printers from Epson and others even have a USB input along with the conventional parallel interface. With a USB printer you can either share a printer with two PCs or save an IRQ by using the USB only and shutting off the parallel port.

IDE System with USB controller IRQ Table

IRQ 3 COM 2, PnP UPS control IRQ9 USB Controller, for Scanner, Modem, Mouse
IRQ4 COM 1 Free! IRQ10 Ethernet Card
IRQ 5 Sound Card, SB16, etc IRQ11 Display Adapter, video card (if needed *)
IRQ6 Floppy IRQ12 Free IRQ for whatever
IRQ7 Printer Port, LPT1 IRQ14 IDE Primary Controller, main HD and future 2nd HD
IRQ8 CMOS/Real time clock IRQ15 Secondary IDE for  IDE CD,  IDE ZIP

Video card may not need an IRQ*

Most of the newer video adapters can run without an IRQ. I mentioned ATI cards being famous for this years ago, but newer cards can run that way also. Your BIOS needs to support this feature, however. Go into the BIOS, under one of the advanced screens. There will be line that asks: Assign VGA IRQ ...The options are Enable/Disable/Auto. Set it to disable and free up an IRQ. DOS programs and old games need the IRQ control, but in general WIN95, '98 and 'NT don't.

Updated 8/30/99

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