Hardware installation and IRQ assignment

The PCI cards of the DSP24 series are using a modern busmaster I/O interface. This allows you to record and playback multiple channels of audio with extremly low CPU load and ultra low latency. This however means that conflicts with other busmaster devices and PCI cards need to be avoided or you may get drop-outs when you record or playback audio signals with low latency.

Basically every PCI device in your system is using an IRQ to allow access to the hardware. On critical configurations it is best if your DSP24 PCI card us using a unique IRQ that has not been assigned to any other device in the same system. While the card usually works with a shared IRQ it still could affect the performance. On certain configurations (e.g. if you are using a mainboard based on a VIA chipset), we strongly recommend to install the card with an unshared IRQ.

In some PCs, it is not that simple to assign a unique IRQ to a certain PCI card. The reason for that is that the PCI hardware itself and its driver are not affecting the IRQ assignment at all (if the hardware follows the PCI 2.1 specifications as the DSP24 series of cards). The IRQ is assigned by the Plug and Play section of the BIOS on your mainboard. Every mainboard works a little bit different but basically IRQs are assigned depending on the PCI slot you use for the PCI-card. This means that the actual PCI-slot position in your PC can be important to improve the performance of the hardware installation.

PCI device listing

On most modern PCs, the BIOS displays the so-called PCI device listing table before Windows gets loaded. Maybe you have never really noticed the list because it apears only for seconds or less. That's no problem as you can hold the display using the PAUSE/BREAK key on your keyboard (upper right corner). After you turn on the power of your PC, press the PAUSE/BREAK key. If you see the screen on the picture below, your timing was excellent. If not, just press any other key (e.g. Enter) and then hit PAUSE/BREAK again. Repeat this process until you finally see the PCI device listing table. If you will see the title screen of Windows 9x/Me or the boot manager of Window NT/2000/XP, you somehow missed the correct moment and you can repeat the process (first boot the operating system and perform a normal shutdown). Note that a few(!) PCs do not display this table (see below).

The list in the example screen the IDE Controller (usually onboard), the USB controller (Serial bus), two Multimedia devices (usally soundcards, but also for example TV-cards) and the graphic card (Display controller). In each line, the assigned IRQ is displayed in the last column. Your DSP24 series card can be identified as a Multimedia Device with the Vendor ID 1412 and Device ID 1712. In the screen above, you can see that the DSP24 uses IRQ 10 (in this example). If this table is displayed but there is no entry with this Vendor and Device IDs, the card is probably not installed correctly (double check that!) in the PCI slot or maybe even defective.

How to avoid IRQ sharing

If the same IRQ number that has been assigned to the DSP24 PCI card is shown also on other entries in the PCI device listing table, the two (or more) devices are sharing one IRQ. This can affect the performance and in some situations, depending on the other devices with the same IRQ, even create major stability problems. For example, if you use a graphic card with nVidia chipset, you should avoid IRQ sharing (especially under Windows 2000/XP) with the DSP24 card.

As mentioned before, the IRQs are assigned depending on the PCI-slot. This means that you need to install the card into a different PCI-slot when you want to assign a different IRQ that is not shared. Some mainboard manufacturers (e.g. ASUS) have lists in the printed documentation that are showing the PCI slots / IRQ assignments. If you do not have that, you will need to try it. On most mainboards with VIA or AMD chipset, the third PCI slot is the best one to use, on most mainboards with Intel chipset, the second PCI slot is optimal. However, this is not a general rule as every mainboard can be different.

Please note that the IRQ number itself (e.g. 9, 10, 11, etc.) is not important. The DSP24 card will work on any of them. Also: if your BIOS allows you to assign certain IRQs to certain PCI slots, you can try this function before you review the PCI device listing table. In most cases however, all devices that share the same IRQ will use the assigned one (shared) after you change that setting. This would mean that the card has to be installed into a different PCI-slot as described above.

No PCI device listing table?

Some PCs do not display the PCI device listing table. This may have several reasons:

The BIOS of your mainboard is very old (e.g. it does not fully support the PCI 2.1 specs).
Please check with your mainboard vendor if there is a newer version and install it.
This usually only happens with very old mainboards (older than about three years).
The BIOS of your mainboard simply does not display this information.
This happens for example if your mainboard BIOS is based on an older version of a Phoenix or AMI BIOS.
Currently most generic PC mainboards are using BIOS software from Award.
Your PC does not display anything or at least not any configuration information before Windows gets loaded.
This happens on some PCs from Compaq, Dell or IBM for example.

Also keep in mind, that the PCI device listing table looks a bit different in some cases (e.g. if you have an AMI BIOS). The information in it is usually always the same.

It is a small limitation of your mainboard if its BIOS does not display the PCI device listing table. It means that you will have to check the IRQ assignment under Windows after the driver installation. If you already have problems with driver installation (e.g. Windows does not detect the card automatically), the most important diagnostic tool is not available with the missing PCI device listing table. Under Windows, you can can check the IRQ assignment of the devices in the Device Manager. Except the so-called 'IRQ-Holder', nothing should be displayed with the same IRQ number if you want to have an unshared, unique IRQ for the DSP24 card.

Windows 2000/XP

As described in our special Windows 2000 / XP article [1], Windows 2000/XP can change the IRQ assignment of your BIOS when you have ACPI enabled. That is one reason why it can be good in some situations to disable ACPI. On many mainboards, if ACPI is enabled under Windows 2000/XP, all devices will use the same IRQ (usually 9, sometimes 11) and that will be displayed in the Windows Device Manager. The real IRQ assignment still is made by the BIOS and displayed in the PCI device listing table however. You should check this list even if you have ACPI enabled under Windows 2000/XP when you want to optimize your system.

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last updated: 07/11/2002 author: Claus Riethmüller

References to other documents or external websites
[1] Optimizing Windows 2000 / XP, ST Audio Knowledge Base

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