How PCI Works

The power and speed of computer components has increased at a steady rate since desktop computers were first developed decades ago. Software makers create new applications capable of utilizing the latest advances in processor speed and hard drive capacity, while hardware makers rush to improve components and design new technologies to keep up with the demands of high-end software. There's one element, however, that often escapes notice - the bus. Essentially, a bus is a channel or path between the components in a computer. Having a high-speed bus is as important as having a good transmission in a car. If you have a 700-horsepower engine combined with a cheap transmission, you can't get all that power to the road. There are many different types of buses. The idea of a bus is simple -- it lets you connect components to the computer's processor. Some of the components that you might want to connect include hard disks, memory, sound systems, video systems and so on. For example, to see what your computer is doing, you normally use a CRT or LCD screen. You need special hardware to drive the screen, so the screen is driven by a graphics card. A graphics card is a small printed circuit board designed to plug into the bus. The graphics card talks to the processor using the computer's bus as a communication path. The advantage of a bus is that it makes parts more interchangeable. If you want to get a better graphics card, you simply unplug the old card from the bus and plug in a new one. If you want two monitors on your computer, you plug two graphics cards into the bus. And so on. In this article, you will learn about some of those buses. We will concentrate on the bus known as the Peripheral Component Interconnect (PCI). We'll talk about what PCI is, how it operates and how it is used, and we'll look into the future of bus technology.

PCI History

The original PC bus in the original IBM PC (circa 1982) was 16 bits wide and operated at 4.77 MHz. It officially became known as the ISA bus. This bus design is capable of passing along data at a rate of up to 9 MBps (megabytes per second) or so, fast enough even for many of today's applications.

Several years ago, the ISA bus was still used on many computers. That bus accepted computer cards developed for the original IBM PC in the early 1980s. The ISA bus remained in use even after more advanced technologies were available to replace it.

There were a couple of key reasons for its longevity:

• Long-term compatibility with a large number of hardware manufacturers.
• Before the rise of multimedia, few hardware peripherals fully utilized the speed of the newer bus.

As technology advanced and the ISA bus failed to keep up, other buses were developed. Key among these were Extended Industry Standard Architecture (EISA) -- which was 32 bits at 8 MHz-- and Vesa Local Bus (VL-Bus). The cool thing about VL-Bus (named after VESA, the Video Electronics Standards Association, which created the standard) is that it was 32 bits wide and operated at the speed of the local bus, which was normally the speed of the processor itself. The VL-Bus essentially tied directly into the CPU. This worked okay for a single device, or maybe even two. But connecting more than two devices to the VL-Bus introduced the possibility of interference with the performance of the CPU. Because of this, the VL-Bus was typically used only for connecting a graphics card, a component that really benefits from high-speed access to the CPU.

During the early 1990s, Intel introduced a new bus standard for consideration, the Peripheral Component Interconnect (PCI) bus. PCI presents a hybrid of sorts between ISA and VL-Bus. It provides direct access to system memory for connected devices, but uses a bridge to connect to the frontside bus and therefore to the CPU. Basically, this means that it is capable of even higher performance than VL-Bus while eliminating the potential for interference with the CPU.