It should come as no surprise to anyone that the gaming industry is quite capable of forcefully driving the need for innovation - often found in the form of faster processors, more powerful 3D graphics adapters, and improvements in data exchange protocols. One such specification, PCI Express (PCI-E), was prompted for development when system engineers and integrators acknowledged that the interconnect demands of emerging video, communications, and computing platforms far exceeded the capabilities of traditional parallel buses, such as PCI. Although PCI Express is really nothing more than a general purpose, scalable I/O signaling interconnect, it has quickly become the platform of choice for industry segments that desperately need the high-performance, low latency, scalable bandwidth that it consistently provides. Graphics card makers have been using PCI Express technology for more than a generation now and today's choice of PCI Express-enabled devices is becoming larger by the minute.

Unlike older parallel bus technologies such as PCI, PCI Express adopts a point-to-point serial interface. This architecture provides dedicated host to client bandwidth, meaning each installed device no longer must contend for shared bus resources. This also removes a lot of the signal integrity issues such as reflections and excessive signal jitter associated with longer, multi-drop buses. Cleaner signals mean tighter timing tolerances, reduced latencies, and faster, more efficient data transfers. To the gamer, of course, only one thing really matters: more frames per second and better image quality. While PCI-E doesn't directly provide for that relative to AGP, it has enabled some improvements along with the return of multi-card graphics solutions like SLI and CrossFire. For these reasons, it is no wonder that PCI Express is the interconnect of choice on modern motherboards.

The typical Intel chipset solution provides a host of PCI Express resources. Connections to the Memory Controller Hub (MCH) are usually reserved for devices that need direct, unfettered, low-latency access to the main system memory while those that are much less sensitive to data transfer latency effects connect to the I/O Controller Hub (ICH). This approach ensures that the correct priority is given to those components that need it the most (usually graphics controllers). When it comes to Intel chipsets, a large portion of the market segment distinction comes from the type and quantity of these available connections. Later we will look at some of these differences and discuss some of the performance implications associated with each.

In late 2006, PCI-SIG (Special Interest Group) released the 2.0 update to the PCI Express Base Specification to members for review and comment. Along with the introduction of a host of new features comes the most predominant change of all, an increase in the signaling rate to 5.0GT/s (double that of the PCI Express 1.x specification of 2.5GT/s). This increase effectively doubles the maximum theoretical bandwidth of PCI Express and creates the additional data throughput capabilities that tomorrow's demanding systems will need for peak performance.

Both ATI/AMD and NVIDIA have released their first generation of PCI Express 2.0 capable video cards. ATI has the complete Radeon HD 3000 series while NVIDIA offers the new 8800 GT as well as a 512MB version of the 8800 GTS (G92) built using 65nm node technology. Last month we took an in-depth look at these new NVIDIA cards - our testing, comments, and conclusion can be found here. We reviewed the ATI models a little earlier in November - the results are interesting indeed, especially when compared to NVIDIA's newest offerings. Take a moment to review these cards if you have not already and then come read about PCI Express 2.0, what it offers, what has changed, and what it means to you.

PCI Express Link Speeds and Bandwidth Capabilities


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  • kjboughton - Sunday, January 6, 2008 - link

    Are you sure this isn't fiber or optical? Any supporting information you can provide would be great. Reply
  • Hulk - Saturday, January 5, 2008 - link

    First of all great article. Great writing. You should be proud of that article.

    I see that currently the Southbridge can transmit data to the Northbridge at 2GB/sec max. In real world situations about how much bandwidth would the Southbridge require assuming a light, medium, and heavy loading situation?
  • kjboughton - Monday, January 7, 2008 - link

    I can help you with some of the base information needed to calculate this yourself (since every system is different based on attached peripherals as well as their type) and we'll leave the rest to you as an exercise.

    For example, a 1Gbps Ethernet connection to the ICH would have a maximum theoretical sustained data transfer rate of 125MB/s (1Gbps x 1 byte/8 bits). A single SATA 3.0Gbps drive would be limited by the interface to three times this number, or about 375MB/s (although the disk to bus/cache transfer rate is much less, somewhere on the order of 120-140MB/s sustained) - but nevertheless, burst read speeds could easily saturate the bus in one direction (1Gbps). Then there's USB devices, possibly a sound card or other onboard solution...going through the numbers, adding up the maximum possible bandwidth for all your attached devices you should be able to get an idea for what would be "light, medium and heavy" loading for your system. Again, this is something that varies from system to system. Hope this helps.
  • LTG - Saturday, January 5, 2008 - link

    Excellent article, good tech level.

    Would you believe "simple ecards" benefit from PCI-E 2.0 right now?

    At most sites when you send an ecard it just e-mails a link to a flash animation to someone.

    However when you send an ecard at the site below, it's rendering and compositing custom photos and messages into a 3d scene on the fly for each card sent.

    Because this is a web site all of this runs on the server side for many users at once.

    PCI-E 3.0 will be welcome :).">
    or (sorry, link buttons not working)
  • JarredWalton - Saturday, January 5, 2008 - link

    I don't know that e-cards would really benefit much - especially right now. The FSB and memory bandwidth aren't much more than what an x16 PCI-E 2.0 slot can provide in one direction (8GB/s). I would imagine memory capacity and the storage subsystem - not to mention network bandwidth - are larger factors than the PCI-E bus.

    Are you affiliated with that site at all? If so, I'd be very interested to see a performance comparison with a single 8800 GTX vs. an 8800 GT on a PCI-E 2.0 capable motherboard. The 8800 GTX even has a memory and performance advantage, but if as you say the bottleneck is the PCI-E bus, it should still see a performance increase from the 8800 GT.
  • LTG - Sunday, January 6, 2008 - link

    Hi Jared, yes I'm a developer on the site - (pls don't think of my post as spam, i've been a reader at AT forever and it just seemed relevant :)

    You could be right, we are just now starting to test pci2.0 so the benchmark you mention will definitely shed some light.

    The network is not a bottleneck because cards are rendered and compressed on a given server node.

    The disk IO is a 6 drive RAID0 array (no data is at risk because the nodes just render jobs) with the Segate 7200.11 drives max out at 100MB/sec transfer rate each, which is less than 600MB/sec total, however I have "heard" that the effective PCI-E video card bandwidth is much less than the theoretical limit.

    I wish there were a utility to easily measure PCI-E bandwidth but currently I only know of indirect experiments as you mention.

    Thanks again for the nice article.

  • PizzaPops - Saturday, January 5, 2008 - link

    I can't help but be amazed by the speed at which hardware is improving. I remember when we were stuck with just PCI and AGP for what seemed like forever. Now the speeds are getting ridiculous. Can't wait to see what the future has in store.

    Very informative article and not too difficult for the average person to understand either. Now I know why my X38 gets so hot.
  • Spoelie - Saturday, January 5, 2008 - link

    Why does the 790FX stays so cool then?

    Besides, there hasn't been a review of that one yet on AT.
  • Gary Key - Saturday, January 5, 2008 - link

    The 790FX does not have the memory controller on-board among other items, so the additional power required for PCI-E 2.0 is minimal at best as are resulting thermal increases. AMD also took a very elegant approach on the 790FX in regards to PCI-E 2.0 (they had time to ensure proper integration, Intel's is fine, just they had a lot to cram into the chipset this time around ;) ) that we will cover shortly.

    NVIDIA's current approach on the 780i is to use a bridge chipset that is creating a few problems for us right now when overclocking both the bus and video card. We will have a complete 790FX roundup the week of the 14th along with a "how to" guide on getting the most out of Phenom on these boards.
  • Comdrpopnfresh - Monday, January 7, 2008 - link

    So by bridge chip, some intermediate chip slows things? Or creates asymmetric latencies leading to unbalanced clocks (like initial SATA drives implementing connections and features like NCQ natively on PATA with a cross-over to SATA)?

    If so, I read an article dealing with a similiar problem with raid-spanning of SSDs somewhere... have to dig up the link...

    The problem began with one dive on the Intel ICH..9 (I understand there is a workaround now). So the tester switched to an add-in discrete raid-handling card. When they began adding more and more drives (believe they went 1-2-3-4), these mucho-expensive raid cards were zapping throughput one after another (with successively higher prices of course) because the companies shaved on the onboard processing power because before these SSDs, the throughput on a RAID spanning standard HDDs just wasn't nearly as great. When they got to four drives, and something like a > $900 add-in card they stopped- one hell of an expensive review!

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