How PCI Works

Twenty or 30 years ago, the processors were so slow that the processor and the bus were synchronized -- the bus ran at the same speed as the processor, and there was one bus in the machine. Today, the processors run so fast that most computers have two or more buses. Each bus specializes in a certain type of traffic.

A typical desktop PC today has two main buses:

Technically there are other buses as well. For example, the Universal Serial Bus (USB) is a way of connecting things like cameras, scanners and printers to your computer. It uses a thin wire to connect to the devices, and many devices can share that wire simultaneously. Firewire is another bus, used today mostly for video cameras and external hard drives.

Next, learn about the history of PCI buses.

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:

­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.

Learn more about the development of the PCI bus and PCI card on the next page.

The frontside bus is a physical connection that actually connects the processor to most of the other components in the computer, including main memory (RAM), hard drives and the PCI slots. These days, the frontside bus usually operates at 400-MHz, with newer systems running at 800-MHz.

The backside bus is a separate connection between the processor and the Level 2 cache. This bus operates at a faster speed than the frontside bus, usually at the same speed as the processor, so all that caching works as efficiently as possible. Backside buses have evolved over the years. In the 1990s, the backside bus was a wire that connected the main processor to an off-chip cache. This cache was actually a separate chip that required expensive memory. Since then, the Level 2 cache has been integrated into the main processor, making processors smaller and cheaper. Since the cache is now on the processor itself, in some ways the backside bus isn't really a bus anymore.

PCI can connect more devices than VL-Bus, up to five external components. Each of the five connectors for an external component can be replaced with two fixed devices on the motherboard. Also, you can have more than one PCI bus on the same computer, although this is rarely done. The PCI bridge chip regulates the speed of the PCI bus independently of the CPU's speed. This provides a higher degree of reliability and ensures that PCI-hardware manufacturers know exactly what to design for.

PCI originally operated at 33 MHz using a 32-bit-wide path. Revisions to the standard include increasing the speed from 33 MHz to 66 MHz and doubling the bit count to 64. Currently, PCI-X provides for 64-bit transfers at a speed of 133 MHz for an amazing 1-GBps (gigabyte per second) transfer rate!

­ PCI cards use 47 pins to connect (49 pins for a mastering card, which can control the PCI bus without CPU intervention). The PCI bus is able to work with so few pins because of hardware multiplexing, which means that the device sends more than one signal over a single pin. Also, PCI supports devices that use either 5 volts or 3.3 volts.

Although Intel proposed the PCI standard in 1991, it did not achieve popularity until the arrival of Windows 95 (in 1995). This sudden interest in PCI was due to the fact that Windows 95 supported a feature called Plug and Play (PnP), which we'll talk about in the next section.

Plug and Play (PnP) means that you can connect a device or insert a card into your computer and it is automatically recognized and configured to work in your system. PnP is a simple concept, but it took a concerted effort on the part of the computer industry to make it happen. Intel created the PnP standard and incorporated it into the design for PCI. But it wasn't until several years later that a mainstream operating system, Windows 95, provided system-level support for PnP. The introduction of PnP accelerated the demand for computers with PCI, very quickly supplanting ISA as the bus of choice.

To be fully implemented, PnP requires three things:

PnP BIOS - The core utility that enables PnP and detects PnP devices. The BIOS also reads the ESCD for configuration information on existing PnP devices.

Extended System Configuration Data (ESCD) - A file that contains information about installed PnP devices.

PnP operating system - Any operating system, such as Windows XP, that supports PnP. PnP handlers in the operating system complete the configuration process started by the BIOS for each PnP device. PnP automates several key tasks that were typically done either manually or with an installation utility provided by the hardware manufacturer. These tasks include the setting of:

While PnP makes it much easier to add devices to your computer, it is not infallible.

Variations in the software routines used by PnP BIOS developers, PCI device manufacturers and Microsoft have led many to refer to PnP as "Plug and Pray." But the overall effect of PnP has been to greatly simplify the process of upgrading your computer to add new devices or replace existing ones.

Let's say that you have just added a new PCI-based sound card to your Windows XP computer. Here's an example of how it would work.

As processor speeds steadily climb in the GHz range, many companies are working feverishly to develop a next-generation bus standard. Many feel that PCI, like ISA before it, is fast approaching the upper limit of what it can do.

All of the proposed new standards have something in common. They propose doing away with the shared-bus technology used in PCI and moving to a point-to-point switching connection. This means that a direct connection between two devices (nodes) on the bus is established while they are communicating with each other. Basically, while these two nodes are talking, no other device can access that path. By providing multiple direct links, such a bus can allow several devices to communicate with no chance of slowing each other down.

HyperTransport, a standard proposed by Advanced Micro Devices, Inc. (AMD), is touted by AMD as the natural progression from PCI. For each session between nodes, it provides two point-to-point links. Each link can be anywhere from 2 bits to 32 bits wide, supporting a maximum transfer rate of 6.4 GB per second. HyperTransport is designed specifically for connecting internal computer components to each other, not for connecting external devices such as removable drives. The development of bridge chips will enable PCI devices to access the HyperTransport bus.

PCI-Express, developed by Intel (and formerly know as 3GIO or 3rd Generation I/O), looks to be the "next big thing" in bus technology. At first, faster buses were developed for high-end servers. These were called PCI-X and PCI-X 2.0, but they weren't suitable for the home computer market, because it was very expensive to build motherboards with PCI-X.

PCI-Express is a completely different beast - it is aimed at the home computer market, and could revolutionize not only the performance of computers, but also the very shape and form of home computer systems. This new bus isn't just faster and capable of handling more bandwidth than PCI. PCI-Express is a point-to-point system, which allows for better performance and might even make the manufacturing of motherboards cheaper. PCI-Express slots will also accept older PCI cards, which will help them become popular more quickly than they would if everyone's PCI components were suddenly useless.

It's also scalable. A basic PCI-Express slot will be a 1x connection. This will provide enough bandwidth for high-speed Internet connections and other peripherals. The 1x means that there is one lane to carry data. If a component requires more bandwidth, PCI-Express 2x, 4x, 8x, and 16x slots can be built into motherboards, adding more lanes and allowing the system to carry more data through the connection. In fact, PCI-Express 16x slots are already available in place of the AGP graphics card slot on some motherboards. PCI-Express 16x video cards are at the cutting edge right now, costing more than $500. As prices come down and motherboards built to handle the newer cards become more common, AGP could fade into history.

For more information on PCI and related topics, check out the links on the following page.