WTF? Slot 4/5 IRQ? [Archive]

KlumpDud

03-19-2003, 05:20 PM

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I have 5 PCI slots on my mobo. Slot 1 is taken up by my NIC. The other day, I went to install a firewire card. For no particular reason, I put it in the 4th slot. I booted up and the firewire card was not detected. After some initial frustrations, I decided to try putting it in slot 3. Bingo! Everything is dandy.

However, my curiosity was piqued so I went into the BIOS, looked under PCI configuration, and I see this:

Slot 1     IRQ   [Auto]
Slot 2     IRQ   [Auto]
Slot 3     IRQ   [Auto]
Slot 4/5  IRQ   [Auto]

Notice the slot 4/5? I know nothing about IRQ's, but I'm assuming that this had something to do with why my firewire card wasn't being detected in slot 4.

In the future, I may need all the slots. Can someone please explain this to me?

Thanks.

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reboot

03-19-2003, 05:29 PM

Slot 4 and 5 share the same IRQ, so you can't (theoretically) put two cards in there. It's just the way things are built.
If you disable com 2 in BIOS, and manually assign IRQ's, things get a little more complicated, but usually works.

KlumpDud

03-19-2003, 05:31 PM

What exactly is an IRQ? Are you telling me that my mobo actually has 4 slots? I got ripped off!!

KlumpDud

03-19-2003, 05:34 PM

And why did it not work when I put it in slot 4 (slot 5 is empty)???

Nuclear Krusader

03-20-2003, 01:41 AM

IRQ = Interrupt Request.

Basically, it means the device "interrupts" the CPU at some time. It demands the CPU attention to help the device perform its tasks. I can be a little wrong but that is the main idea of it.

In some mobos the PCI 1 shares the same IRQ with the AGP, so you musn't use the PCI 1 slot. In other mobos, like yours, the 4 and 4 PCI slots share the same IRQ.

What OS are you running? Do you have APIC enabled on the BIOS?

Charles

03-20-2003, 03:35 AM

Not wrong at all.

IRQ's are 1 of 4 System Resources. Memory Addresses, I\O Port Addresses and DMA's(Direct Memory Access) being the other 3.
The classic analogy for IRQ's is "students in a classroom". All students(IRQ's) have their hands up, but the teacher(CPU) can only listen to one at a time according to priorities. Win95B (0SR2) and later support a function known as PCI IRQ steering. For this to work both your BIOS and Windows have to support it. Older system BIOSes and Win95 or 95A do not support this feature. If you have this feature enabled BIOS initially assigns IRQ's to PCI devices. To eliminate conflicts Windows will assign changes when needed. If there are insufficient free IRQ's , IRQ steering allows Windows to assign multiple PCI devices to a single IRQ, therefore enabling all devices to function properly. Without IRQ steering, windows begins to disable devices after it runs out of free IRQ's.

Make a printout of your computers IRQ's, follow reboots advice and confirm IRQ steering is enabled in your BIOS and in your OS (95B or later), especially if you plan to use the other PCIs.

glc

03-20-2003, 01:50 PM

Your sig shows Win2K - which with ACPI will share IRQ very nicely. Slot 1 will share with the AGP (NOT a good idea) and Slot 4 will share with Slot 5. ONE of the other slots will share with the USB - and if you have onboard sound, one of the other slots will share with it.

If your bios has "APIC" (not the same as ACPI) enable it, and Win2K will use IRQ's higher than 15.