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I could use an extra PCI slot since my motherboard only has one. Many online shops offer "dual PCI riser cards" such as this one:

Dual PCI riser card

Those riser cards are usually meant for compact cases like rack servers, but I can't figure out if any regular PCI slot will support it. Is support for this kind of riser cards a special feature of certain motherboards, or is it part of the PCI specification (like the USB standard)?

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No special extra motherboard hardware is required to use those devices, what those devices are is called a "PCI to PCI bridge".

PCI actually allows for 256 separate "buses" to exist at once, each bus can contain up to 32 devices. While the motherboard is performing its startup enumeration if the PCI device it is communicating with responds that it is a PCI to PCI bridge the motherboard will increment its "Bus" counter then enumerate each device on this new bus. If it finds another PCI to PCI bridge on this new bus it just keeps repeating the process until it finds the lowest level devices. After it enumerates all the devices on the new bus it walks backwards out continuing on where it left off enumerating the next device on the higher level bus.

Wikipedia has a very good article that explains this process in much more detail than I can.

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