The PCI Express specification allows slots to have different physical sizes, depending on the number of lanes connected to the slot. See Figure 5. This allows reducing the size of the space needed on the motherboard. For example, if a slot with an x1 connection is required, the motherboard manufacturer can use a smaller slot, saving space on the motherboard.
However, bigger slots can actually have fewer lanes than the diagram shown in Figure 5. For example, many motherboards have x16 slots that are connected to x8, x4, or even x1 lanes. With bigger slots it is important to know if their physical sizes really correspond to their speeds. Moreover, some slots may downgrade their speeds when their lanes are shared. The most common scenario is on motherboards with two or more x16 slots. With several motherboards, there are only 16 lanes connecting the first two x16 slots to the PCI Express controller. This means that when you install a single video card, it will have the x16 bandwidth available, but when two video cards are installed, each video card will have x8 bandwidth each.
The motherboard manual should supply this information. But a practical tip is to look inside the slot to see how many contacts it has. If you see that the contacts on a PCI Express x16 slot are reduced to half of what they should be, this means that even though this slot is physically an x16 slot, it actually has eight lanes (x8). If with this same slot you see that the number of contacts is reduced to a quarter of what it should have, you are seeing an x16 slot that actually has only four lanes (x4). It is important to understand that not all motherboard manufacturers follow this; some still use all contacts even though the slot is connected to a lower number of lanes. The best advice is to check the motherboard manual for the correct information.
A little-known fact is that you can install any PCI Express expansion card in any PCI Express slot. For example, you can install an x1 expansion card in any kind of PCI Express slot; it doesn’t need to be installed in an x1 slot. So, if you have an x4 expansion card but your motherboard doesn’t have an x4 PCI Express slot, no problem; simply install it in an x8 or x16 slot.
The same holds true for “bigger” cards. For example, you can install an x16 video card in a “smaller” slot. (The slot, however, must have its rear side open; otherwise, the bigger expansion card won’t fit. It is up to the motherboard manufacturer whether or not to provide slots with their rear side open.) The only disadvantage is that it will only have the maximum bandwidth provided by the slot; i.e., if you install an x16 video card in an x4 slot, it will have only x4 bandwidth available. On the other hand, this kind of installation may be useful in some situations, such as when building a computer with several video cards to have multiple displays available, and you are not worried about gaming performance.
To reach the maximum performance possible, both the expansion card and the PCI Express controller (available inside the CPU or inside the motherboard chipset, depending on your system) have to be of the same revision. If you have a PCI Express 2.0 video card and install it on a system with a PCI Express 3.0 controller, you will be limited to the PCI Express 2.0 bandwidth. The same video card installed on an old system with a PCI Express 1.0 controller will be limited to the PCI Express 1.0 bandwidth.
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Figure 5: Types of PCI Express slots
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Figure 6: Details of the PCI and PCI Express slots on a motherboard
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Figure 7: Differences on the edge contacts of PCI Express, AGP and PCI video cards