PCI and PCI-Express: What You Need to Know

If you have opened a computer since the mid-nineties, you’ve seen a Peripheral Component Interconnect slot (pictured at right). What USB did for things outside your computer, PCI did for things inside it. Instead of having to make sure your motherboard could accept one of several, often bizarre, standards to plug in what it was you were buying, just about every motherboard has PCI slots and almost any expansion card would fit in it.

That was even more important back then, because motherboards didn’t come with all kinds of stuff built in, we currently take for granted. Network connections, and before that, even hard disk controllers, were not built into motherboards. Forget about built in sound. You had to drop in a card, and it always using the same kind of slot was great.

Sorry if I sound like an old man who has taken leave of his senses: “We walked 3 miles to school; in the snow; uphill; both ways. One year we didn’t have enough coal to run the modem…”

Anyways, at its zenith, the PCI bus could handle 133 MB/s. That is enough for even today’s sound and network cards… if you only have one. The problem is that all the PCI traffic goes back to the same bus and shares that 133 limit. A single Gig-Ethernet card needs almost all of it just to itself.

A larger slotted 64-bit version of PCI never got that popular for a pair of reasons. Things that really needed the bandwidth, like hard drive controllers and network connections, were by now being included on the motherboard (or the connected device), eliminating them from the PCI bus ridership. Also, the only thing that needed the bandwidth that wasn’t being moved to the motherboard (at least not universally, today’s integrated graphics still can’t handle many applications) was a video card. And those were using AGP slots.

AGP solved the bandwidth question for graphics cards in two ways: it was not only faster; but got the video card off the shared PCI bus. AGP gave the GPU its own 266 MB/s connection.

This was then double pumped to offer 533 MB/s per second, and called AGP 2x. Double pumping means moving data twice instead of once on each clock cycle, like Double Data Rate (DDR) memory. It was further pumped through 4x and 8x, the final 8x version delivering 2133 MB/s.

That was more bandwidth than a graphics card could use in 2004, when PCI Express showed up with even more speed. For a time, deciding on a motherboard and video card was tricky, as you could go with a time honored AGP slot or a less proven PCI-E solution that offered more bandwidth than the card could use.

In hindsight, PCI-E was the way to go. Speeds picked up, and midrange graphics cards can now use about half of a PCI version 1.1 x16’s bandwidth (x16, pronounced “by 16” refers to the number of lanes). High end cards use it all, justifying the recent PCI-E 2.0 standard, which doubles available bandwidth per lane. We will get into lanes and further into PCI-E versions in the next article.

AGP could still run a low-end card without ham-stringing it, but there is no upgrade potential. Newegg offers 34 AGP, but over 400 PCI-E, graphics cards. Four motherboards there still offer AGP, to the over 500 PCI-E boards listed at the site. AGP is gone, but the older, slower PCI is still everywhere.

Because you might have an old PCI card you still want to use, and it is still easier and cheaper to get a PCI than PCI-E expansion card, it is a good thing motherboards still have a PCI slot. I say a PCI slot because the shared bus means using more than one with a modern card will likely reduce performance.

As a rule of thumb, go ahead and use a PCI slot if it is cheaper or easier for you. If you are already using one PCI slot and want to add another expansion card (you already have a PCI sound card and want to add a network card, for instance), look for a PCI Express one. Even adding USB ports via a PCI card can be a bad idea, as a single USB port can use 60 of the 133 MB/s available. We explain, with pictures, how to install a PCI card here.

So forget about AGP, graphics cards need a 16 lane PCI-E slot, and you might need a smaller x1, x4, or x8 slot for other cards. In the concluding part of this series we explain the physical and bandwidth differences between types of PCI-E card, complexities of multi-GPU setups, as well as the increased bandwidth of PCI-E 2 and if you need it?