Introduction

So you want to know about PCI Express? PCI Express is a recent feature addition to many new motherboards. PCI Express support can have a big impact on your hardware choices both now and in the future. This article will explain the topic in plain english withoutboring you with useless information.

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 back to the early 90's and no longer fits our needs in terms of speed/performance. 2) AGP also is in a similar position as PCI now, and chipset manufacturers are killing AGP motherboard support in favor of the much faster PCI Express interface. This means you are looking at a forced transition in the 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 latest chipsets 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 and bandwidth we were comfortable with in the 90's. PCI as we know it is holding us back - it is a bottle neck - 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 as being 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 sub systems. On most systems the bus speed of the PCI bus is 33MHz. If you go higher than that, then cards, drives, and other devices can have problems. The exception to this is found in servers. In some 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.

Now in my previous article I mentioned a special type of PCI (64 bit). The reason 64 bit helps is that it improves the bandwidth of the PCI Bus. Bandwidth, normally expressed in MB per second, is basically 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 then you probably have a good idea of what's going on in the modern PCI Bus. You've got too many cars (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 than basic PCI.

* Note 1 - Since PCI Express is a serial based technology, data can be sent over the bus in two directions at once. Normal PCI is Parallel, and as such all data goes in one direction around the loop. 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 in both directions. Also please note that in PCI Express bandwidth is not shared the same way as in PCI, so there is less congestion on the bus.

Update: Table above contains speeds for PCI Express 1.0 bus. For version 2.0, multiply all bandwidths by 2. For example a PCI Express 2.0 16x slot has a max bandwidth of 8000 MB/s one way or 16000 MB/s both ways.

* Note 2 - Firewire 800 has a bandwidth of 786.432 Mbit/s this converts over to between 98 and 99 MB/s.

Increased bandwidth can be equated into increased system performance. We've long known that to get the most out of your processor you need to get as much information into it as possible, as quickly as possible. Chipset designers have consistently addressed this by increasing Front Side Bus 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 that needs to get to the memory or CPU like drives, network traffic, video, etc. PCI Express addresses this 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

Currently, the most common use for PCI Express is Video. On the graphic at right you can see the physical differences between the cards.

The connector on PCI Express video cards will always start out with a small piece not directly joined with the rest of the slot, and then a notched piece not directly joined with the rest of the slot. This notch goes into the 1x portion of the 16x slot. The characteristic notch makes it easy to tell the difference between PCI-E (PCI Express) and an AGP Video card. As you can imagine, a PCI Express Video card will not fit into an AGP card slot, and a AGP Video card will not fit into a 16x PCI Express card slot.

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

PCI Express 1x / 4x cards also have the physical difference in regard to bracket distance. They are both a good deal smaller than standard PCI. At this time this type of card is still quite rare so at this point there is little chance of confusing them with something else. The PCI Express 1x connector does however bare some minor resemblance to an AMR slot, so it is important not to confuse the two. No motherboard on the market today currently has both the older AMR slot and PCI Express.

Q&A Common Questions about 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 to PCI Video the difference is enormous: PCI Express 16x video is over 29x faster than PCI Video.

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 the connection 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 feature that isn't supported in hardware.

So, what this means is that in terms of real world performance there may not be a huge difference between AGP and PCI Express if you are talking about identical chipsets. Unfortunately this is very hard to prove because graphics chipsets are designed either for PCI Express or AGP. If you have a card that is available in both forms then you have a graphics chipset that was designed for PCI Express and has a special bridge chip installed to let it communicate with the AGP bus. The short of this is: if two cards of the same chipset are available in AGP and PCI-E then the PCI-E one will always be faster. On PCI-E you don't have the overhead of the bridge chip so it's faster, and you have the better bandwidth so in intense situations such as high resolution gaming you'll come out 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 just to get PCI-E video right now. The fastest AGP card to ever come out is likely to be the nVidia 6800GT. If you are at a point where that is too slow then by all means it makes sense to make a complete switch. If your happy with your AGP graphics options, wait until you are ready to upgrade the processor or other components before making the PCI-E switch. For more information on 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 based graphics cards *that support SLI* and a motherboard *that supports SLI* to achieve a very high level of video performance. SLI works by splitting the rendering of the screen between the two cards- one card renders half, the other card renders the other half. This technique is extremely effective. For instance two 6600GT cards in SLI can do vastly better than a 6800GT or X800 card even though the price is lower for two 6600GT Cards. The downside to this is that SLI is still new and is limited to systems based on AMD 64 / AMD FX Socket 939 processors.

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-E cards don't currently use it. This means that you should only probably worry about this if you are buying bleeding edge PCI Express parts. Card based on the ATI X600, ATI X700, ATI X300, ATI X1300, 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't have one you can always just use a PCI Express Power Adapter that converts a 4-pin molex connector to PCI-E Power.

Conclusion

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