The PCI (peripheral component interconnect) bus was the primary slot for many types of add-in or expansion cards for a number of years. AGP (accelerated graphics port) replaced PCI in some applications, but the latest advance, PCI-E (PCI Express) has largely replaced AGP -- and can also take the place of the standard PCI slot in many instances.
Both the AGP and PCI-E expansion slots are based on improvements to the PCI slot. Intel introduced PCI in 1993 and it became the standard expansion bus in most personal computers. The PCI slot is a versatile, general purpose expansion bus. Users can add PCI video cards, sound cards, additional USB ports and other components.
The AGP slot is a faster alternative to the PCI slot for video adapters. Even though the AGP expansion slot has a faster data transfer rate than the PCI slot, it is far less versatile. AGP slots can only host video cards, in contrast to the wide variety of cards that are compatible with PCI slots.
PCI-E is another improvement on the PCI slot. PCI-E data transfer speeds exceed those of the PCI slot and the AGP slot. PCI-E also has the versatility of its ancestor, the standard PCI slot. So PCI-E video cards are faster than AGP cards, and they are also compatible with a host of different types of expansion cards.
In addition to the speed and versatility advantages that PCI-E enjoys over AGP, there are physical differences. AGP slots sit farther away from the back of the motherboard than PCI-E slots. PCI-E slots vary in size, so some are longer than AGP slots and some are not. The Basic PCI-Express slots are x1, x2, x4, x8 and x 16. Each higher number of PCI-E slot is twice as fast as the slot in sequence before it. Higher numbered slots are also longer than lower numbered slots. PCI-E slots may be white or other colors, but AGP slots are typically brown.
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