How PCIe Works

By: Tracy V. Wilson  | 
Diagram showing PCIe card connectors inside a computer.
PCI Express is a high-speed serial connection that operates more like a network than a bus. Learn how PCI Express can speed up a computer and replace the AGP and view PCI Express pictures.
Photo courtesy Consumer Guide Products

Peripheral Component Interconnect Express, commonly known as PCIe, stands as a pivotal technology in the realm of computer hardware. It provides a high-speed interface for connecting peripheral devices (such as keyboards, sound cards, or external hard drives, to name a few) to a computer's motherboard. From its inception to its current iterations, PCIe has continually evolved to meet the demands of modern computing.

Whether you're a computer enthusiast keen on building your own system or simply curious about the mechanics behind your device's operations, you're in luck. In this article, we'll examine what makes PCIe different from PCI. We'll also look at how PCI Express makes a computer faster, can potentially add graphics performance, and can replace the Accelerated Graphics Port (AGP) slot.


The Need for a PCIe Interface

Peripheral Component Interconnect (PCI) slots are such an integral part of a computer's architecture that most people take them for granted. For years, PCI has been a versatile, functional way to connect sound, video and network cards to a motherboard.

But PCI has some shortcomings. As processors, video cards, sound cards and networks have gotten faster and more powerful, PCI has stayed the same. It has a fixed width of 32 bits and can handle only 5 devices at a time. The newer, 64-bit PCI-X bus provides more bandwidth, but its greater width compounds some of PCI's other issues.


Fortunately, PCI Express (PCIe) eliminates a lot of these shortcomings, provides more bandwidth, and is compatible with existing operating systems.

High-Speed Serial Connection

In the early days of computing, a vast amount of data moved over serial connections. Computers separated data into packets and then moved the packets from one place to another one at a time. Serial connections were reliable but slow, so manufacturers began using parallel connections to send multiple pieces of data simultaneously.

It turns out that parallel connections have their own problems as speeds get higher and higher — for example, wires can interfere with each other electromagnetically — so now the pendulum is swinging back toward highly-optimized serial connections. Improvements to hardware and to the process of dividing, labeling and reassembling packets have led to much faster serial connections, such as USB 2.0 and FireWire.


The Advantage of PCIe Devices

PCI Express is a serial connection that operates more like a network than a bus. Instead of one bus that handles data from multiple sources, PCIe has a switch that controls several point-to-point serial connections. (See How LAN Switches Work for details.)

These connections fan out from the switch, leading directly to the devices where the data needs to go. Every device has its own dedicated connection, so devices no longer share bandwidth like they do on a normal bus. We'll look at how this happens in the next section.


The Data Transfer Rate of PCIe Lanes

A diagram showing how PCIe lanes increase the data transfer rate in a computer.

When the computer starts up, PCI express slots determine which devices are plugged into the motherboard. It then identifies the links between the devices, creating a map of where traffic will go and negotiating the width of each link. This identification of devices and connections is the same protocol PCI uses, so PCIe does not require any changes to software or operating systems.

Each PCIe lane contains two pairs of wires — one to send and one to receive. Packets of data move across the lane at a rate of one bit per cycle. A x1 connection, the smallest PCIe link, has one lane made up of four wires. It carries one bit per cycle in each direction. A x2 link contains eight wires and transmits two bits at once, a x4 link transmits four bits, and so on. Other configurations are x12, x16 and x32.


Peripheral Component Interconnect Express is available for desktop and laptop PCs. Its use may lead to lower cost of motherboard production, since its connections contain fewer pins than PCI connections do. It also has the potential to support many devices, including Ethernet cards, USB 2 and video cards.

But how can one serial connection be faster than the 32 wires of PCI or the 64 wires of PCIx? In the next section, we'll look at how PCIe is able to provide a vast amount of bandwidth in a serial format.


PCI Express Connection Speeds

Diagram showing how each device using PCI Express has its own connection to the computer switch.
Devices using PCI share a common bus, but each device using PCI Express has its own dedicated connection to the switch.

The 32-bit PCI bus has a maximum speed of 33 MHz, which allows a maximum of 133 MB of data to pass through the bus per second. The 64-bit PCI-X bus has twice the bus width of PCI. Different PCI-X specifications allow different rates of data transfer, anywhere from 512 MB to 1 GB of data per second.

A single PCIe lane, however, can handle 200 MB of traffic in each direction per second. A x16 PCIe connector can move an amazing 6.4 GB of data per second in each direction. At these speeds, a x1 connection can easily handle a gigabit Ethernet connection as well as audio and storage applications. A x16 connection can easily handle powerful graphics adapters.


How is this possible? A few simple advances have contributed to this massive jump in serial connection speed:

  • Prioritization of data, which allows the system to move the most important data first and helps prevent bottlenecks
  • Time-dependent (real-time) data transfers
  • Improvements in the physical materials used to make the connections
  • Better handshaking and error detection
  • Better methods for breaking data into packets and putting the packets together again. Also, since each device has its own dedicated, point-to-point connection to the switch, signals from multiple sources no longer have to work their way through the same bus.


PCIe Card and Advanced Graphics

SLI link card
NVIDIA SLI link card
Photo courtesy NVIDIA

We've established that the data link layer of PCIe can eliminate the need for an AGP connection. A x16 PCIe slot can accommodate far more data per second than current AGP 8x connections allow.

In addition, a x16 PCIe slot can supply 75 watts of power to the video card, as opposed to the 25watt/42 watt AGP 8x connection. But PCIe has even more impressive potential in store for the future of graphics technology.


Manufacturers Leveraging External PCI Express

With the right hardware, a motherboard with two x16 PCIe connections can support two graphics adapters at the same time. Several manufacturers are developing and releasing systems to take advantage of this feature:

NVIDIA Scalable Link Interface (SLI): With an SLI-certified motherboard, two SLI graphics cards and an SLI connector, a user can put two video cards into the same system. The cards work together by splitting the screen in half. Each card controls half of the screen, and the connector makes sure that everything stays synchronized.


ATI CrossFire: Two ATI Radeon video cards, one with a "compositing engine" chip, plug into a compatible motherboard. ATI's technology focuses on image quality and does not require identical video cards, although high-performance systems must have identical cards. Crossfire divides up the work of rendering in one of three ways:

  • splitting the screen in half and assigning one half to each card (called "scissoring")
  • dividing up the screen into tiles (like a checkerboard) and having one card render the "white" tiles and the other render the "black" tiles
  • having each card render alternate frames

Alienware Video Array: Two off-the-shelf video cards combine with a Video Merger Hub and proprietary software. This system will use specialized cooling and power systems to handle all the extra heat and energy from the video cards. Alienware's technology may eventually support as many as four video cards.


The PCI Express Standard

Since PCI, PCI-X, and PCI Express are all compatible, all three can coexist indefinitely. So far, video cards have made the fastest transition to the PCIe format.

Network and sound adapters, as well as other peripherals, have been slower in development. But since PCIe is compatible with current operating systems and can transmit data at faster speeds to multiple devices, it is likely that it will eventually replace PCI as a PC standard. Gradually, PCI-based cards will become obsolete.


For more information about PCI Express and related topics, check out the links on the next page.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.


PCIe Slots FAQs

What is a PCI express slot?
A PCIe or PCI express slot is the connection between a PC's motherboard and peripheral components.
Is PCI compatible with PCI express?
They are not compatible because they have different configurations. There are motherboards, however, that host a combination of the two.
Does the graphics card go into the PCIe slot?
It should go into the first available PCI Express x16 slot. That isn't to say, however, that lower slots are incapable of running a graphics card.
Is PCI the same as PCIe?
They are not the same. PCIe is able to connect graphics cards that PCI can not, and it is also faster by comparison.
What is the main difference between PCI and PCIe?
PCIe is a serial interface whereas PCI is a parallel interface.

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More Great Links

  • "Advanced Switching for PCI Express Architecture." Intel. AdvancedSwitching.pdf
  • Bhat, Ajay V. "What Is PCI Express?" Desktop Architecture Labs, Intel. WhatisPCIExpress.pdf
  • Campbell, John. "PCI Express Explained." CoolTechZone, August 19, 2004. view&id=506&Itemid=0
  • Fisher, Ken. "PCI Express launched in time for the future." Ars Technica, June 24, 2004.
  • Fisher, Ken. "Alienware Announces Dual PCI-Express Graphics Subsystem." Ars Technica, May 12, 2004.
  • "Introduction to PCI Express." PC Stats.
  • PCI Express FAQ. PCI-SIG.
  • "PCI Express Overview." Intel, October, 2003. PCI-Express-Overview-Oct2003.pdf
  • Tong, Terren. "A Look at PCI Express." Neoseeker, April 21, 2004.
  • Wasson, Scott. "NVIDIA's SLI Resurrects GPU Teaming." The Tech Report, June 28, 2004."