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PCRECORDING.COM - Windows got you IRQ'd?

One of the most common installation problems DAW users encounter under Win9x is an IRQ conflict. More often than not, this occurs at the most inopportune time. In reality, Windows' Plug and Play (PnP) installation usually works well with newer systems. Most hardware components are now PnP compliant and do not result in IRQ problems. However, there are times when it seems all but impossible to figure it out. Unfortunately, most of us have run into this difficulty at one time or another. Relax, there are a few things you can try before you throw in the towel and/or your system. I get ahead of myself however.

Plug and Play (aka Plug and Pray)

What is an interrupt? There are two ways that a computer talks to a device. First, the computer can "ask" a device, such as a soundcard, if it is ready. This is known as polling. Since the computer does not know the device is ready for a data transfer until it asks, the computer will keep going back and asking. This is wasteful, using up considerable PC resources.

Second, the other way is to have the device talk to the computer when it is ready to transfer data. The computer is busy communicating with its other components but the device can "interrupt" this dialog to announce it is ready to transfer data. The computer has special interrupt wiring attached to each slot that accommodates this. A standard PC has 16 available interrupts, of which at least five are dedicated for system resources such as the timer and the keyboard, etc. Interruption is much more efficient because the computer's resources will not be used up "polling" the device constantly. The computer will dedicate resources to the device only when it is "interrupted." So, standard PCs assign the 16 interrupts to the various devices installed on a system.

This is all well and good but problems can arise when you have older ISA (Industry Standard Architecture) devices in your computer. Some of these may not be PnP compliant. The ISA connectors in your system are wired to 11 of the available 16 interrupts. These are known as "legacy" cards. You must select which interrupt the device will use by setting jumpers on the device or the device is hard-wired to work with a particular interrupt. Newer ISA cards are PnP compliant and allow the system's BIOS or Windows to select the interrupt for them. However, only one ISA device can occupy an interrupt at a time.

PCI (Peripheral Connect Inteface) devices are PnP compliant by design. PCI connectors have only four interrupts wired to them but they can be shared with other PCI connector slots. In addition, each connectors interrupt can be "steered" to one of the above 11 interrupts. This is done either with the system BIOS or by Windows 95/98. The PCI device can share an interrupt only with another PCI device, not an ISA device.

Boot up problems

When you boot your system, the BIOS checks each connector for PnP compliant devices. Each PnP compliant device contains a list of resource needs it passes on to the system. This list includes how many interrupts it needs and whether they need to be specific interrupts. The system BIOS configures each device and will assign interrupts as needed. This occurs for both ISA and PCI devices that are PnP compliant.

Problems arise though when older "legacy" devices are in the system. This occurs because the the device is not PnP compliant and has either hard-wired or jumper-configured its interrupt settings. The BIOS and/or Windows 95/98 are unable to automatically detect these devices and usually just ignore them. For instance, you may have an old scanner card inserted in an ISA slot that is jumper-configured to IRQ 5. When the BIOS fails to detect this card, it assumes the IRQ 5 is free and may assign that interrupt to your soundcard. When that happens, neither device will work correctly.

Secondly, Windows 95/98 oftentimes will "reassign" IRQs for PnP compliant devices. This can be a problem when you have a previously installed, functional non-PnP device which must use a particular interrupt. Windows may reassign a previously installed, non-conflicting device, to your dedicated interrupt, when you install the new device. Thus, an IRQ conflict arises but it may not seem to be related to the new device because the conflict is now between the two older devices.

Suggested Solutions

Search your manual. Ideally, you will have the manuals for your legacy devices and will be able to ascertain which interrupts they need. If so, you will be able to remedy the problem, which will be discussed below.

If you do not have the manuals then you will need to burn some elbow grease. Take out all of your devices and reinstall them one by one. If and when your system stops functioning properly, make a note of the last device you installed because it is the interrupt assignment surrounding this device that will need your attention.

If you are using Windows 95/98, you can try to reserve the needed IRQ for the "legacy" device. Click on Start>Settings>Control Panel>System>Device Manager. The window should show the "computer" is highlighted. If so, click on Properties and select the tab for Reserve Resources. Ensure that the "interrupt request" radio button is selected. Click on Add and enter the IRQ that is reserved for your legacy device. It is likely that Windows will tell you it is reserved elsewhere. If so, click on Details to find out which device has that IRQ reserved. It will likely be the one that is conflicting with your legacy device. If it is, go ahead and reserve the card for your legacy device. Reboot and when Windows restarts it will not assign that IRQ to the PnP device. Windows should then reassign the PnP device to another IRQ, perhaps sharing with another PnP device.

Some BIOS systems let you reserve an IRQ for use by a legacy device. Boot your computer, press whichever key (delete or F1 usually) after the memory test. This will get you into the BIOS settings window. In the BIOS settings window check for a manual configuration option for IRQs. If it is there, you should be able to reserve an IRQ for "legacy ISA" devices only. If so, the BIOS will not assign that to a PnP device.

Lastly, there are a few other considerations that need to be mentioned before I close. First, with older cards and PCs there will be hardware devices that simply will not get along in the same computer ever. No matter what you do there may be a conflict, some of which may be explainable, some not. For instance, if you have a scanner card that is hardwired to IRQ 5 and an old Soundblaster that is hardwired for IRQ 5, there is nothing you can do. In that situation just about the only thing you can do is upgrade your system and/or your devices.

Secondly, ISA slots are getting rarer on new motherboards anyway. Since most of these IRQ problems arise due to leftover "legacy" devices, these issues may get rarer too. Just a thought. As we all know, progress has a way of dragging us along the upgrade path whether we like it or not. Well that is about it. I hope this helps. Keep on recording!