What is the difference between an AGP and a PCI graphics card?

Answer: The biggest difference between AGP and PCI graphics cards is that AGP cards can access the system memory to help with complex operations such as texture mapping. PCI cards can only access the memory available on the actual card. AGP doesn't share bandwidth with other devices, whereas PCI cards do. AGP also makes pipelined requests, which means it can execute multiple instructions at one time. PCI cards are not pipelined, which means each instruction has to finish before the next one is run.

So, with all these great advantages of AGP, you'd think it would be the clear winner in performance, right? Well, not quite. Tests of similar AGP and PCI graphics cards show they perform almost the same (typically measured in frames per second). The area where AGP really shines is in high-resolution tests, where the direct access to the system memory is most beneficial.

If you're installing an AGP or PCI card in your computer, the AGP slot is usually the shortest and should be brown. The PCI slots are slightly longer and are colored white. The actual size of the cards can vary as much as a few inches, though the pins on the bottom of the card should match the correct slot.

Important: AGP and PCI slots are different sizes. Therefore, AGP cards can only be placed in AGP slots and PCI cards will only fit in PCI slots.

Published: October 7, 2004   —   Updated: July 13, 2012 — by Per Christensson