Express Yourself
The Ins and Outs of PCI Express

“When I was your age, I walked 10 miles to school – each way – barefoot in the snow (OK, maybe not).”

“When I was your age, I had something called a record player, and I listened to albums, appreciating a band for more than the one song that got voted onto the top 20 on TRL.”

“When I was your age, I designed my systems using an ISA bus for my I/O interconnect. That’s how it was done. And we LIKED IT.”

It is the job of each new generation to move us forward, sometimes in small steps, sometimes in monstrous leaps. And so it goes for I/O interconnect. In the early 1990s, the first-generation Industry Standard Architecture (ISA) bus was slowly phased out in favor of the more robust Peripheral Component Interconnect (PCI) local bus architecture. PCI is a 64-bit bus, usually implemented as a 32-bit bus, running at clock speeds of 33MHz for a throughput rate of 133MB/sec. Next came PCI-X (meaning PCI-Extended). PCI-X didn’t stray far from PCI, maintaining the established parallel bus structure, but it delivered more performance, with a throughput rate up to 1 GB/sec.

High-speed peripherals like Gigabit Ethernet and USB began pushing the limits of the PCI parallel bus structure, demanding more and more bandwidth. Then along came PCI Express. Although it carries on the PCI name, it actually has very little resemblance to its forefathers. PCI Express has a layered architecture, with a software layer, a transaction layer, a data link layer, and a physical layer. It has abandoned the parallel bus structure for a two-way serial approach that carries data in packets along pairs of point-to-point lines. PCI Express bit rates are 2.5Gb/sec per lane direction, and is scalable up to 16 lanes (you do the math…).

But PCI Express isn’t a total rebel. It maintains backward compatibility with the older PCI bus structures, and core PCI attributes, including the usage model and software interfaces, are still in place. This means that existing applications and drivers are not changed. Essentially, it’s just the BMX-riding, iPod-listening, espresso-drinking, cellphone-carrying, X-Games watching member of the PCI clan – fast, smart, and a little bit complicated to understand at first. [more]