PCI Expressway
Issue: Volume: 27 Issue: 8 (August 2004)

PCI Expressway

Workstations and servers in the graphics community are well known for being the fastest machines on the market. Graphics professionals demand of their systems cutting-edge speed. Whereas processor, graphics, disk, and networking speeds have increased dramatically over the past decade, the PCI (Peripheral Component Interconnect) bus used to connect those devices, has remained the same in terms of speed for nearly a decade—and it's showing its age. Developed by Intel, PCI Express completely revamps PCI and provides an upgrade path fast enough for the next generation of workstations and servers.

PCI has been the standard bus for PCs for roughly a decade, eclipsing the original ISA (Industry Standard Architecture) bus in the mid-1990s. Over the course of the decade, faster graphics, networking, and hard disks have all pushed the PCI bus to its limit. To remedy the bottleneck, a number of stopgap solutions have been introduced over the years to add more bandwidth. Graphics were the first to get some relief, with the introduction of the AGP (Accelerated Graphics Port) slot now common on all PCs. PCI-X (not to be confused with PCI Express) was another solution that enhanced PCI's bandwidth, primarily on servers for Fibre Channel and Gigabit Ethernet cards.
Industry-leading manufacturers of high-end graphics accelerators have begun debuting PCI Express-enabled boards, including such innovations as (clockwise from top) ATI's FireGL 5100, 3Dlabs' Wildcat Realizm 800, and Nvidia's Quadro FX 3400.

While these solutions provided much-needed bandwidth, the wide variety of slots and bus types complicated matters significantly. Today, a PC can have as many as three or four different types of card slots, making upgrades difficult. To confuse the issue further, even the new interfaces are starting to get tapped out. High-end graphics cards already are bumping up against AGP 8x's performance limits. And PCI-X is expensive, requiring a separate controller for each slot, essentially limiting it to the server market. Over the past few years, it became clear that the PCI bus needed to be totally revamped.

Creating an entirely new interface standard is not easy, but Intel got together with a number of manufacturers a few years ago to create a robust and upgradable path for PCI. Called PCI Express, or PCIe, the new interface completely changes and unifies the way peripherals connect to the motherboard. The flexible interface not only provides increased bandwidth, but also enables devices that are normally found inside the PC case to be connected externally, opening up the possibility of new PC designs.

The first PCI Express systems and motherboards, introduced this summer by major manufacturers, look fairly standard. Dell, Hewlett-Packard, Alienware, IBM, and Boxx Technologies recently started shipping PCI Express systems. Graphics vendors, including ATI and Nvidia, also began offering PCIe cards this summer, and more vendors are sure to follow as the year progresses.

PCI Express has broad industry support and is poised to become the new standard. Vijay Sharma, marketing manager at ATI, indicates that it is likely to be the fastest ramp-up of a new bus in history. He estimates that 40 to 60 percent of all new PCs will be PCI Express-compliant by the end of 2004, and nearly 100 percent of the machines will be compliant by the end of 2005.

PCI Express dispenses with the parallel interface that has been part of the PC bus since its inception. Instead, PCIe is based on serial communications. Serial interfaces suffer from much less interference than parallel connections, and they can be scaled for much higher speeds. In PCIe, a single serial connection can handle up to 500mb/second of data transfer, which is more than quadruple that of the PCI interface. Jeff Wood, director of workstation marketing for HP, notes that PCI cannot take full advantage of Gigabit Ethernet's bandwidth, whereas a single channel of PCI Express can.

To add even more bandwidth, PCI Express enables multiple serial connections to a single device. Called lanes, these connections determine the upper limit of a card's speed. A graphics card will accommodate up to 16 lanes, for example, while a network card may accept one or two.

This fact actually introduces a little bit of confusion into the equation. The ability to have multiple lanes means a PCI Express machine may have several different types of PCIe card slots. All systems will have a 16x slot for the graphics card, but manufacturers may divide up the remaining lanes as they see fit. Initial PCI Express chipsets provide for 24 lanes, so you will see a 16x slot for the graphics card, with other lanes filled by an 8x slot or any combination of 4x and 1x slots.

While this may be a little confusing, it is heartening to know that PCIe allows any card to fit into any slot, just as long as the slot has enough lanes. A 1x card can fit into any available slot, for example, while a 4x card requires a 4x slot or higher to work. The 1x PCI Express card slots can coexist with a standard PCI slot. The connector sits in front of the PCI slot, allowing for a slot to be used either as a PCI or PCI Express slot. This is, of course, if OEMs design the motherboard to take advantage of this feature.

HP's new line of PCI Express workstations, for example, includes a variety of configurations. The low-end machines have several 1x slots, and the higher-end workstations have 4x slots, along with a 16x graphics slot and a handful of normal PCI slots. As for original PCI slots disappearing, HP's Wood says, "I doubt we'll see PCI go away completely anytime soon. It still provides sufficient bandwidth for many applications, and some customers will want to hang on to their older cards."

Because PCI Express is serial-based, it allows for devices to be attached to the system via a serial cable, much like a FireWire or USB connector. While it's not set to compete directly with these standards soon, devices such as disk arrays or video decks could easily incorporate PCI Express to get increased bandwidth and speed.

In the future, PCI Express could cause the entire architecture of the PC to be re-imagined. Intel has shown interest in making servers, and even workstations, modular, placing devices where they are needed.

This modularity could streamline the case of the PC, making it smaller and quieter, and also improving its upgradability. A network card could be integrated into a router or the graphics card could be integrated into the monitor, for example. Nobody has plans for breaking apart the PC just yet, and these sorts of configurations will need support from a wide range of vendors.

The first companies to take advantage of PCI Express will be the graphics vendors, which need the added bandwidth. PCI Express will quickly eclipse AGP as the graphics slot on most new workstations, and vendors such as 3Dlabs, ATI, and Nvidia have already announced support for the standard across their product lines.

3Dlabs has jumped into the PCI-Express world with the new Wildcat Realizm 800, which is expected to be a strong contender for the fastest card on the market. With such features as 512mb of video RAM, dual GPUs, and 128mb of geometry memory, along with genlock capabilities, the Wildcat Realizm 800 promises to be a robust and ultra-high-end card.

Nvidia's latest offerings include the Quadro FX 3400, FX 130, and FX 330. To maximize bandwidth, Nvidia announced the Scalable Link Interface (SLI), which enables multiple graphics cards in a PCI Express machine to work as one.

ATI has implemented PCI Express across its entire product line, from entry-level cards all the way up to the workstation-class FireGL series. ATI's Sharma notes that all new ATI cards will be PCI Express-based, demonstrating the strong commitment vendors have to this new technology.

PCI Express offers immense bandwidth gains, as much as 15 times that of AGP. According to Jeff Brown, general manager of professional graphics at Nvidia, one of the biggest benefits is that it is equally fast in both directions, enabling data to be read as fast as it is written. In addition to enabling new applications, this capability will allow the powerful graphics processors on graphics cards to be used almost as a second processor. "PCI Express enables high-end applications, like real-time HD editing," says Sharma.

PCI Express is, without a doubt, the future of workstations. Those shopping for workstations will want to add PCI Express to the list of requirements, as it will be the standard within the next 12 months. The added performance and flexibility it affords will be of great benefit to anyone working in high-end graphics computing.

George Maestri is president of Rubberbug, a Los Angeles-based animation studio specializing in character animation.