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PCI Master vs Slave

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Anonymous
a b V Motherboard
February 22, 2001 8:11:28 PM

I have a MB with 5 PCI slots. However the maual states the following: "THe 1st and 5th PCI slots share the same bus master signal. Only one of these can be used as a master at a time. If a bus master card is installed in PCI slot 1, PCI slot 5 can only accomodate a slave card, and vice versa."

From a practical point of view what does that mean? I have a NIC card in slot 1 and my Video card in Slot 5 and it all seems to work. All but Slot 3 are filled. What must I watch for if I wanted to add another PCI card, e.g. ATA100 controller.

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Anonymous
a b V Motherboard
February 23, 2001 3:31:29 AM

Hmm. PCI Video? Your lucky that and the nic are playing together nicely. Both of those boards are probably busmasters. The video for sure is, the nic most likely is. Bus-Masters, stated very simply, are the industry's way of getting around the constraints that "sharing" IRQ's put them under when the original PCI 2.1 spec was adopted. Cards like nics, video, and IO controllers (SCSI, ATA) are expected and NEEDED to run long bursts of data at a time through the bus. If they conform totally to the PCI specification, the boards end up losing a lot of bandwidth on the bus waiting for the processor to service IRQ requests. To get around this, they can be designed as busmasters that "take" control of the bus for longer periods of time than normal. There is just one problem with this grand idea. Because they like to hog the bus, they also by design generally don't "share" their IRQ's well or sometimes AT ALL. They definitely won't share with another busmaster in most cases.

Newer operating systems like Windows 2000 are able to deal with this somewhat, and newer motherboards seem to do better in Windows 98SE and ME if the BIOS is set to allow the operating system to assign resources. Problems can and do still come up, especially in a case like yours. With PCI video and with one open slot left, consider your machine a ticking time bomb. IE., that next card you buy may show you what I have tried to explain. You won't HURT anything, but putting that ATA board in that last slot might render the machine unable to turn on. It might render it unable to post, or just unable to boot into the operating system. It might boot into the operating system but you won't get an IRQ for one of the other cards anymore. Etc. Be sure to turn off any serial ports, printer ports, usb ports, etc, in the BIOS if they aren't needed. Disable DOS support for your sound card if you don't play DOS games. You might be able to free up an IRQ and stabilize the system again..........but don't count on it. It would depend on what the cards are, but a box with five PCI devices is pretty much going to be maxed out anyway you look at it.
Anonymous
a b V Motherboard
February 23, 2001 5:37:11 PM

Great info and is highly appreciated. What surprises me is that the video card does not show up using an interrupt in device manager.

What about MOdems and Sound cards? Are they Bus Masters as well.

One final question what would be a Slave PCI card?
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