Tech Primer

Peripheral Component Interconnect Express

What is PCI Express (PCIe)?

PCI (Peripheral Component Interconnect) was originally developed by Microsoft, Dell, IBM, Intel and several other vendors. PCIe is a serial interconnect technology that currently operates at 2.5 GHz using just 0.8v. It is backward-compatible with PCI, enabling PCIe equipped motherboards to also provide PCI slots for legacy cards. It is the industry’s attempt to unify all current different types of I/O bus technologies into a single “future-proof” standard. This is used to distribute I/O messages on a peer-to-peer basis. This means that if one PCI-Express device wants to send data to another, it doesn’t have to go through the chipset. This reduces the number of messages that the chipset has to process by itself. This technology was designed to regain the balance between raw CPU speed and overall system speed.

Why do we need PCI Express?

The PCI 2.2 bus (the earlier version and other revisions) did not provide enough bandwidth to support the ever increasing demands of peripheral cards. Hard drive controllers and networking cards just aren’t capable of handling the throughput of newer hard drives as PCI offers a throughput (maximum theoretical bandwidth) of 1.056Gbps (gigabits per second) and the latest drives can saturate that.

Are there multiple formats of PCIe?

PCI Express comes in five formats: x1, x2, x4, x8, x12 and x16. Each lane comprises 4 pins; x1 has one lane, x2 has two lanes, x4 has four lanes, and so on. PCI Express can transmit 100MB per second per pin. x1 has one lane of traffic which is divided into input and output.

Each link can contain many ‘lanes’ making each device link individually scaleable, in turn, adding more bandwidth with the addition of each lane. Standard add-in cards may be 1x (low bandwidth) while graphics cards may be 16x (very high bandwidth). Eventually we might see x32 and possibly x64 slots being made available for PCI Express. Those slots are likely to be used for graphics cards. For now, desktop users will most likely see x1, x2, and x16 slots alongside conventional PCI slots on their motherboards.

What are PCIe’s advantages over PCI?

PCIe allows each device to use its full bandwidth capabilities without having to compete for the maximum bandwidth offered by a single shared bus. PCIe is a 64 bit interface in a 32 bit package. The PCIe bus can deliver the same throughput on a 32 bit interface that other parts of the machine deliver through a 64 bit path.

It is potentially cost-efficient to manufacture when compared to PCI and AGP slots with much higher scalability when compared to the legacy PCI bus. PCIe is advantageous for hard drive controllers, gigabit LAN cards, and other bandwidth intensive devices. Though PCI Express is still in its infancy, drivers and revisions will be upgraded periodically.

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