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A PCIe card will fit into a slot of its physical size or bigger, but
may not fit into a smaller PCIe slot. Some slots use open-ended sockets
to permit physically longer cards and will negotiate the best available
electrical connection. The number of lanes actually connected to a slot
may also be less than the number supported by the physical slot size.
An example is a x8 slot that actually only runs at ×1; these slots will
allow any ×1, ×2, ×4 or ×8 card to be used, though only running at the
×1 speed. This type of socket is described as a ×8 (×1 mode)
slot, meaning it physically accepts up to ×8 cards but only runs at ×1
speed. The advantage gained is that a larger range of PCIe cards can
still be used without requiring the motherboard hardware to support the
full transfer rate—in so doing keeping design and implementation costs
down.
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- a PCIe card will physically fit (and work correctly) in any slot
that is at least as large as it is (e.g., an ×1 sized card will work in
any sized slot); - a slot of a large physical size (e.g., ×16) can be wired
electrically with fewer lanes (e.g., ×1, ×4, or ×8) as long as it
provides the power and ground connections required by the larger
physical slot size.
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