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All Aboard the PCI Express!

The PCI bus, originally developed by Intel, debuted over a decade ago as a high performance alternative to the ISA bus for desktop systems. The initial bus frequency was 33MHz with a 32-bit bus and, theoretically, a peak bandwidth of 132MB per second. This was great at the time, but as the rest of the system grew more bandwidth hungry both the bus speed and width were expanded in an effort to keep up. But even with PCI 2.2 and other revisions, PCI no longer provides enough bandwidth to support the more demanding peripheral cards.

PCI uses a shared bus topology to allow communication among the different devices on the bus. Each of the different PCI devices (i.e., a network card, a sound card, etc.) is attached to the same bus where the core logic chipset acts as a switch, or router, and routes I/O traffic among the different devices that make up the system. The shared bus topology's main advantages are that it's simple, cheap, and easy to implement. That is, as long as you are not trying to do anything too elaborate with it. Once you start demanding more performance and functionality from a shared bus, then you run into its limitations.

As it exists today, PCI does not have the ability to provide the bandwidth and features needed by current and future generations of I/O and storage devices. The utopian ideal of one bus and one bus protocol for every device will never be achieved. So enters PCI Express, a new architecture for interconnecting I/O devices.

PCI Express (PCIe) is the newest name for the technology formerly known as 3GIO and, while it was approved in July 2002, PCIe based devices have begun to hit the market this year.

PCIe's most notable advancement over PCI is its point-to-point bus topology. A shared switch replaces a shared bus as the single shared resource through which all the devices communicate. Each device has its own direct access to the bus instead of multiple components having to collectively mediate among themselves to use the bus.

Developers say PCIe will handle data-transfer bandwidths to 25 Gbits/sec/lane/direction initially and that future versions will run even faster. Experts believe it should provide ample bandwidth for scalability well into the decade and possibly beyond.

While there are obvious benefits of additional bandwidth, consumers are quick to assume that the simple answer is to buy a new PCI Express graphics card, based upon a new architecture, in order to have a “future proof” solution with the best performance possible.

The idea of spending $300 or more on the latest and greatest graphics card to play the newest games is enough to make anyone cringe. However, users are now faced with the potential need to upgrade their motherboards, as well as their processors, in order to accommodate PCIe. Who is to say that we will see any immediate benefit from PCI Express in the first generation of cards? It might be prudent to hold onto your current motherboard and processors and see how this new technology pans out before jumping in headfirst.

Experts believe that PCI Express will soon replace the conventional AGP interface and enhance the performance of PCI interfaces enabling a new generation of highly powerful 2D and 3D graphics applications. Of course, the transition from PCI to PCI Express won't be immediate. PCIe cards are already available on the market, fairly close in pricing to the AGP card, but they are still miles away from the capabilities of this architecture. Additionally, the PCI SIG is now defining a PCI-to-PCI Express bridge that will let older boards work with the new format. In the end, ordinary PCI slots are likely to be found in mid to low-end desktop systems for quite some time.

In this Tech Tip, we touched only briefly on some of the apparent benefits of using PCI Express, however, there is a vast amount of information available about PCIe and it is recommended that you research this new technology before implementing it yourself.

More about PCI Express

PCI Express Explained by John Campbell
PCI Express White Paper (in Adobe format)

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