Introduction
For the past decade, the primary interface for PC compatible computers and peripheral cards has been the venerable PCI interface. It was developed to replace the previous ISA interface that had reached its limits. Now the current PCI cards are reaching their limitations and its time for a new interface and that interface is PCI Express.
PCI-X is not PCI Express
Before moving on, it should be important to point out a common mistake made by users and the press. Often times the term PCI-X is used. This term refers to a different PCI standard that was developed for the higher bandwidth requirements of servers. This is not the same thing as PCI Express. PCI Express is designed as a general purpose interface. Of course, the confusion is often caused by the pronunciation of the terms.
It Is All About Bandwidth
The usefulness of an interface is determined by the bandwidth of data that it can provide between the system and the peripheral. The more bandwidth that is available, the more data that can be moved between devices such as a network or drive and other components. It is the limitations of the PCI standard that have lead to the development of PCI Express.
The current PCI standard allows for a 32-bit bus that has a maximum throughput of 133MB/s. This133MB/s of bandwidth must be shared between all of the PCI peripherals. Graphics quickly outstripped all of the other components in the system. To alleviate this, the AGP interface was developed to give a dedicated channel to the graphics. But now gigabit Ethernet interfaces and UltraATA/133 and SerialATA devices are quickly catching up to this 133MB/s limit.
This is where the PCI Express system is designed to alleviate problems. In its current proposed first revision, the system has the capability of allowing a single interface adapter to receive as much as 16GB/s of data bandwidth. This is achieved through the movement away from the old parallel models to the higher speed serial interconnects. Future revisions expect to have even higher bandwidth.
1x, 2x, 4x, 8x, 16x?
The PCI Express interface allows for an interface card to negotiate the number of lanes that it can communicate with the rest of the system. Each lane is comprised of 4 pins. A 1x card slot uses a single data lane, while an 8x adapter has eight total data lanes. This determines the total amount of bandwidth available to the peripheral.
Most early PCI Express systems will ship with a single 16x PCI Express card slot for a graphics card and several 1x slots. A single 1x card slot has the possible bandwidth of 500MB/s, higher than the current shared PCI bus. The 16x slot has the possible bandwidth of 8GB/s in both directions or 4GB/s in one direction. This is about twice the 2.1GB/s bandwidth of the AGP 8x standard.
Compatibility with PCI 2.2
One of the core requirements of the PCI Express standard is the ability to be compatible with the current PCI 2.2 software standards. This means that any hardware implementation of PCI Express should work with any OS that has the ability to run current PCI 2.2 interfaces. Until the actual PCI Express hardware is released, we won't know if this works as proposed.
The purpose of the compatibility is to allow for an easy upgrade path. In fact, the 1x PCI Express card slots can coexist with the 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, if the manufacturers design their motherboards appropriately. Most early prototypes are showing them on separate slots though.
Other Benefits
There are a number of other more technical benefits to PCI Express that users won't see for a while. Some of these features include:
- Hot Swappable
- Low Power Consumption
- Higher Wattage to Graphic Cards
- External PCI Express Connectors
- Quality of Service Policies
- Peer-to-Peer Communication via Switches
- Possibility of Multiple Linked Graphics Cards
In addition to the internal standards, the PCI Express group is also drawing up plans for additional PCI Express interfaces such as a replacement for the PC CardBus on notebooks and even an external PCI Express interface.
Conclusions
In order to take advantage of the speed benefits of the PCI Express interface, it is of course necessary to have a motherboard and cards that supports it. The primary benefits of the PCI Express bus will first be seen with graphics cards. Already NVIDIA and ATI are developing 16x PCI Express based cards. Other devices that will benefit will be gigabit Ethernet cards that can now operate in Full Duplex mode and RAID controllers. Most early boards will still have a few standard PCI card slots to allow for the transition, but with the potential of the PCI Express interface, once they start to be used the transition will probably be very rapid.
