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What is PCI Express? A Layman's guide to high speed PCI-E technology

by Lee Penrod

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Introduction

So you want to know about PCI Express? PCI Express is a recent feature addition to many newmotherboards. PCI Express support can have a big impact on your hardware choices both now andin the future. This article will explain the topic in plain english without boring you with uselessinformation.

Why should I care about PCI-Express?

There are two main reasons to care about PCI-Express: 1) PCI is now an old standard dating backto the early 90's and no longer fits our needs in terms of speed/performance. 2) AGP also is in asimilar position as PCI now, and chipset manufacturers are killing AGP motherboard support infavor of the much faster PCI Express interface. This means you are looking at a forced transition inthe graphic sector, thus you really don't have a lot of choice in the coming years. At this point(August, 2008) choices in AGP cards represent less than 5% of available cards, and the latestchipsets can not be found in the form factor.

While we've spent plenty of time and energy improving the speed of processors, memory, and other parts of the PC we've done virtually nothing with the main connection betweern many devices-PCI.

As such we are stuck with a technology in our PCs and Servers that still runs at the speeds andbandwidth we were comfortable with in the 90's. PCI as we know it is holding us back - it is a bottleneck - a limitation to the maximum performance of our systems.

We all want the most from our PC. To get the most out of our PC we must remove all bottlenecks(obstacles to performance). To that end we must turn to the next best alternative: PCI Express.

Easing into It - All about the Why?

If you have read my guide to the Front Side Bus then you are familiar with the analogy of a PC asbeing like a city with many road (buses) in it that move cars (data) to any number of destinations.Let's quickly revisit my explanation of PCI from that article:

PCI Bus - The PCI bus connects your expansion cards and drives to your processor and other subsystems. On most systems the bus speed of the PCI bus is 33MHz. If you go higher than that, thencards, drives, and other devices can have problems. The exception to this is found in servers. Insome servers you have a special 64-bit (extra wide) 66MHz PCI slots that can accept special high-speed cards. Think of this as a double sized passing lane on a major road that allows higher speed cars to go through.

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Now in my previous article I mentioned a special type of PCI (64 bit). The reason 64 bit helps is thatit improves the bandwidth of the PCI Bus. Bandwidth, normally expressed in MB per second, isbasically a measure of the amount of data that can be pushed through something at one time.

If you ever have sat in your car looking at the back bumper of another car during rush hour thenyou probably have a good idea of what's going on in the modern PCI Bus. You've got too manycars (data) going through too narrow and too slow a road (pci bus) at one time.

Bandwidth

PCI Express in all it's flavors: 1x, 2x, 4x, 8x, 16x and 32x all have much greater bandwidth thanbasic PCI.

Common Buses and their Max Bandwidth

PCI 132 MB/sAGP 8X 2,100 MB/sPCI Express 1x 250 [500]* MB/sPCI Express 2x 500 [1000]* MB/sPCI Express 4x 1000 [2000]* MB/sPCI Express 8x 2000 [4000]* MB/sPCI Express 16x 4000 [8000]* MB/sPCI Express 32x 8000 [16000]* MB/sIDE (ATA100) 100 MB/sIDE (ATA133) 133 MB/sSATA 150 MB/sGigabit Ethernet 125 MB/sIEEE1394B [Firewire] 100 MB/s

* Note - Since PCI Express is a serial based technology, data can be sent over the bus in twodirections at once. Normal PCI is Parallel, and as such all data goes in one direction around theloop. Each 1x lane in PCI Express can transmit in both directions at once. In the table the first number is the bandwidth in one direction and the second number is the combined bandwidth inboth directions. Also please note that in PCI Express bandwidth is not shared the same way as inPCI, so there is less congestion on the bus.

Increased bandwidth can be equated into increased system performance. We've long known that toget the most out of your processor you need to get as much information into it as possible, asquickly as possible. Chipset designers have consistently addressed this by increasing Front SideBus speeds. The problem with this is that front side bus speed increases the speed of transfer between the memory and CPU but often you've got data that's coming from other sources thatneeds to get to the memory or CPU like drives, network traffic, video, etc. PCI Express addressesthis problem head on by making it much faster and easier for data to get around the system.

Physical Differences: PCI Express [ PCI-E ] vs AGP vs PCI

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Currently, the most common use for PCIExpress is Video. On the graphic at rightyou can see the physical differencesbetween the cards.

The connector on PCI Express videocards will always start out with a smallpiece not directly joined with the rest of the slot, and then a notched piece notdirectly joined with the rest of the slot.This notch goes into the 1x portion of the16x slot. The characteristic notch makesit easy to tell the difference between PCI-E (PCI Express) and an AGP Video card.

As you can imagine, a PCI ExpressVideo card will not fit into an AGP cardslot, and a AGP Video card will not fitinto a 16x PCI Express card slot.

Another physical difference between PCIExpress Cards, AGP, and PCI is thedistance between the card's bracket andthe start of the connector. On PCIExpress cards, there is very littledistance between the metal bracket andthe start of the connector. On both PCIand AGP the distance is much longer.

PCI Express 1x / 4x cards also have thephysical difference in regard to bracketdistance. They are both a good deal

smaller than standard PCI. At this timethis type of card is still quite rare so atthis point there is little chance of confusing them with something else. ThePCI Express 1x connector does however bare some minor resemblance to an

AMR slot, so it is important not toconfuse the two. No motherboard on themarket today currently has both the older

AMR slot and PCI Express.

Q&A Common Questionsabout PCI Express

Q:Is PCI Express Faster Than PCI? A:PCI Express is much faster than PCI.For 1x Cards it at least 118% faster .When you compare PCI Express video toPCI Video the difference is enormous:PCI Express 16x video is over 29x faster than PCI Video.

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Q:Is PCI Express Video Faster than AGP Video?A:Yes and No. A 16x PCI Express connection is at least 190% Faster than AGP 8x but this is theconnection between the system and the video card. You use the connection the most when your video card is low on memory or when the game you are using uses a Direct X or Open GL featurethat isn't supported in hardware.

So, what this means is that in terms of real world performance there may not be a huge differencebetween AGP and PCI Express if you are talking about identical chipsets. Unfortunately this is veryhard to prove because graphics chipsets are designed either for PCI Express or AGP. If you have acard that is available in both forms then you have a graphics chipset that was designed for PCIExpress and has a special bridge chip installed to let it communicate with the AGP bus. The shortof this is: if two cards of the same chipset are available in AGP and PCI-E then the PCI-E onewill always be faster . On PCI-E you don't have the overhead of the bridge chip so it's faster, andyou have the better bandwidth so in intense situations such as high resolution gaming you'll comeout on top every time.

The main point here is: If you have a system with AGP on it, it doesn't make sense to upgrade justto get PCI-E video right now. The fastest AGP card to ever come out is likely to be the nVidia6800GT. If you are at a point where that is too slow then by all means it makes sense to make acomplete switch. If your happy with you're AGP graphics options, wait until you are ready toupgrade the processor or other components before making the PCI-E switch. For more informationon AGP and PCI please see the general FSB guide .

Q:What is SLI?A:SLI or Scalable Link Interface is a technology that lets you take two identical nVidia basedgraphics cards *that support SLI* and a motherboard *that supports SLI* to achieve a very highlevel of video performance. SLI works by splitting the rendering of the screen between the twocards- one card renders half, the other card renders the other half. This technique is extremelyeffective. For instance two 6600GT cards in SLI can do vastly better than a 6800GT or X800 cardeven though the price is lower for two 6600GT Cards. The downside to this is that SLI is still newand is limited to systems based on AMD 64 / AMD FX Socket 939 processors. Needless to saythough, there are some good solutions like this one from Asus .

Q:Do I need a special power supply for PCI-E [PCI Express] .A:Yes and no. Although the PCI-E spec calls for a PCI Express power connector, most PCI-Ecards don't currently use it. This means that you should only probably worry about this if you arebuying bleeding edge PCI Express parts. Card based on the ATI X600, ATI X700, ATI X300, ATIX1300, Nvidia 6600, Nvidia 7600 or Nvidia 7300 series graphic chipsets rarely use the connector. If you are in a situation where you need a PCI Express power connector but the power supply doesn'thave one you can always just use a PCI Express Power Adapter that converts a 4-pin molexconnector to PCI-E Power.

Conclusion

PCI Express is an exciting advance in the area of computers. Although AGP is now starting to dierapidly, standard PCI is taking/will take longer to die off. Expect to see at least 1 or 2 standard PCIslots along side PCI-E in all motherboards for at least the next 2 years. By that time there will bePCI-E replacements for all common devices such as modems, network cards, raid cards, i/o andmore.

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Last Updated : 8/13/2008