Complex Configurations and IRQ Information for AMD Processors

As your computer configuration becomes more complicated and/or sophisticated, sometimes the system becomes difficult to configure and run properly. This problem is often caused by IRQ conflicts. IRQs (Interrupt ReQuest) are unique signals that allow the cards or components to communicate with the system. Many current accessory cards still require an IRQ dedicated to their specific use. About fifteen years ago, when the architecture for the IBM PC-AT with the 16-bit bus was first defined, it was thought that a total of fifteen useable IRQs would be sufficient for a system's needs. It is doubtful that anyone back then expected these systems to grow to the complexity that they have reached. (For a more complete understanding of the IRQs and what it means, read the "What is an IRQ?" explanation near the end of this section.

Recommendation: Use Current Cards and Proper Installation Sequences

In most cases, the difficulties in configuring your system can be avoided by using current cards (PCI) that allow IRQ sharing. The proper card installation sequence can also help. Changing the installation sequence often helps because many of these cards can use only one of a predefined selection of IRQ values (specific to each card). By using the proper sequence, the system can often install the cards in such a way as to keep the proper IRQ available for the next card, thereby eliminating IRQ conflicts that occur when two boards try to use one IRQ.

Following is the sequence that our testing lab has found that works for the system configuration examples listed below (this sequence is very dependent on board/BIOS version).

Installation Sequence for Third Party Manufacturers' Components

For BCM and Microstar motherboards utilizing Award BIOS W6167MS v1.1b9 082299 5:0052

Microstar Motherboard

​BCM Motherboard
​One (1) AGP slot​One (1) AGP slot
​Six (6) usable card slots, consisting of:​Six (6) usable card slots, consisting of:
​Five (5) PCI slots, (one shared)​Four (4) PCI slots, (one shared)
​Two (2) ISA slots, (one shared)​Three (3) ISA slots, (one shared)


CMOS/PnP Requirements

PnP Operating System is "ENABLED" for Windows® 95 and Windows 98
PnP Operating System is "DISABLED" for Windows NT®

Initial System Startup

The operating system (Windows 95/98 or Windows NT) should be loaded with the graphics card installed on the motherboard (AMD recommends using an AGP graphics card). You should configure your system with only the graphics card (normally AGP, but could be PCI), RAM and the storage drives (FD, HD, ATAPI CD-ROM or DVD to allow for the easiest startup with a lower possibility of system conflicts. (If you are using SCSI drives, the PCI SCSI card must be installed also.)

Note: If multiple nonessential cards (devices) are included in the initial load of the operating system in addition to the graphics card, the possibility of system conflicts are more likely.

Installing Additional Components after System Startup

After the basic system has been configured (and after you have successfully loaded your operating system) you should add each additional component one at a time in the order listed below. This will require that you properly install each card, configure the BIOS and restart your computer each time you install a new card.

After the installation of each component into your computer, it is advised that you note the IRQ setting on the splash screen (this is the screen that shows up momentarily when you start your system). To freeze this screen so you can easily read it, press the Pause Break key (located at the top of your keyboard) as soon as you see the screen that lists IRQ usage.

Note: It may take you more than one try to press the button at just the right moment. Or, you can inspect the IRQ settings found in your Windows 95/98 Control Panel under System, then Device Manager. Always record the IRQ settings and the devices to which they are assigned each time you install a new device or component that requires an IRQ address. This information can be utilized to help resolve a system conflict or crash as the result of an IRQ conflict.

After each successful installation of a component from this list, you will need to exit and shut down your system and unplug it before installing the next component.

Note: In case you are not planning to use one of the devices listed below, just skip to the next device in the sequence of installation. Generally, ISA based components are less flexible; therefore, if you must use any ISA based components, you may decide to install these first. The sequence below is only a suggested guideline to follow.

Recommended Loading Sequence for Additional Cards

  1. Internal modem (PCI)
  2. Sound card (PCI or ISA)
  3. Network card, a.k.a. NIC (PCI or ISA)
  4. DVD devices requiring Dxr3 decoder board (PCI) - Note: Some video decoder boards require two (2) IRQs for complete functionality. Check with your video decoder card manufacturer for video decoder support and requirements.
  5. SCSI adapter (PCI) (This assumes that the system does not have an SCSI hard drive.)
  6. Any others


Special Notes on Installation

External Modem: May be installed any time after the operating system has been successfully installed. This includes regular v.90 modems as well as ISDN modems.

Diamond Monster 3D Voodoo I or Voodoo II Cards: Since these cards do not require an IRQ address, it is recommended that they be installed after the successful installation of your operating system. They do require a PCI slot.

Other Third Party Manufacturer Component Cards: Many other third party manufacturer component cards may require an IRQ. Be sure to check with the manufacturer of your product to find out the exact installation requirements before you attempt to install it into your computer system. It is recommended that these component cards be installed after all of your other primary cards are installed if you have unused IRQ addresses available.

As you install the next device into your computer, you may experience a system conflict. You have to change the installation sequence for various components (i.e., which comes first, second, third, etc.).

IRQ Conflicts During Installation and Configuration

It would be ideal to be able to have a system with an unlimited number of slots and IRQ addresses. Unfortunately, the way PC systems were designed many years ago causes limitations today.

When the basic design concepts of the PC architecture were defined, there were a specific number of slots allowed and a limited number of IRQs. With the limited number of IRQs available in today's systems, if you are using old-style cards that do not allow IRQ sharing, you may not be able to install all of the devices you that you desire using PCI-based components without experiencing a system conflict. You may consider using ISA-based components as well as an external modem to optimize the use of assigned IRQs for cards or components that require a dedicated IRQ.

You should carefully investigate and choose the types of devices that require a dedicated IRQ for your system. The more devices you want to install, the harder it will be to find an available IRQ that the device can use or is capable of using. This becomes even more of a challenge for devices that require two IRQ addresses (specific sound cards, DVD devices, etc.).

Tactics to Overcome IRQ Conflicts

Although the procedure listed above has worked in a testing lab environment, you may need to experiment to determine the best sequence and method of setting up your devices for proper computer operation. Check the manufacturer's literature for each component used and list the IRQs with which each is compatible. List all of the IRQs and determine the path that allows the devices to use available IRQs. To install all of the devices you wish to use, you may have to exchange one of your devices for another type (i.e., change from a PCI sound card to an ISA sound card or vice versa) for proper configuration. Check with the manufacturer to determine if there is an upgraded version of the card. Sometimes a newer version or a different brand of the component may solve the problem. Systems cannot properly function if the number of cards and/or devices installed requires more IRQs than are actually available within the computer.

What is an IRQ?

The easiest way to explain an IRQ is to think of it as a unique identifier. The IRQ process is similar to an auction. The CPU is the auctioneer and the peripherals are the bidders. Each bidder has a flag or paddle to get the auctioneer's attention. If more than one bidder has the same flag, there is chaos. Almost all primary components, such as graphics, sound, modem, etc., require at least one IRQ, regardless of whether they are cards or built onto the system boards. Many older devices require their own unique IRQ (their unique flag) with respect to all of the other devices. Newer cards can often share IRQs, but older cards usually cannot share with each other because they treat their IRQ as their own. When two of these older devices try to use the same IRQ, one or more of the devices will not function properly. This is called an IRQ conflict. Such conflicts can cause improper performance, system lock-ups and crashes.

Most newer components, however, can share an IRQ. Sharing an IRQ is the modern way to allow more cards to be added to a system. Note that some components (like video cards) cannot share IRQs. This is because the video card's IRQ is usually 10, 11 or 12. These are low priority and may get delayed (or lost) due to higher priority requests. This can cause undesirable visual effects. Cards that fully conform to the intent of the PCI Plug-n-Play standard allow IRQ sharing.

Typical IRQ Assignments

There are 16 IRQs (15 usable) in a computer system. Here is a typical assignment of these IRQs:

IRQ 0 System- System Timer
IRQ 1 System- Keyboard
IRQ 2 System- Cascadeable PIC (programmable interrupt controller), controls IRQ 8-15
IRQ 3 System- Serial Port (COM 2 and COM4)
IRQ 4 System - Serial Port (COM 1 and COM3)
IRQ 5 Available- General Adapter Use
IRQ 6 System- Diskette Controller
IRQ 7 System- Printer 1
IRQ 8 System- CMOS Real-time Clock
IRQ 9 Available- General Adapter Use
IRQ 10 Available- General Adapter Use
IRQ 11 Available- General Adapter Use
IRQ 12 System- Mouse Port
IRQ 13 System- Math Co-processor (even though this is built into the CPU, it still uses an IRQ)
IRQ 14 System- Hard Disk Controller
IRQ 15 Available- General Adapter Use

As you can see, there are five (5) IRQs that are not assigned by the system design. Of these, one usually goes to the graphics card, one to the USB ports and one to the modem. That leaves two (2) available IRQs for everything else. With care (and use of the proper cards), this is normally sufficient.

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Please visit the System Building and Compatibility page for available documents that will guide you through the configuration of a stable AMD processor-based platform.