Taking the Express

By Jerry Pournelle

May 26, 2003

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Column 274 (Continued from the Previous Week)

PCI Express: The Future of the PCI Bus

WinHEC isn't just about Windows. At what may have been the first WinHEC I ever attended, one of the speakers asked the 4500 or so engineers present "How many of you would like to take the ISA bus out in the parking lot and shoot it?" He got rousing cheers.

Since then the ISA bus has indeed gone away, and the PCI bus that replaced it is on the way out.

As computers have gotten faster, bus speeds have become more of a bottleneck. CPU-to-memory bandwidth has increased from 800 MB/s with PC100 memory four years ago, to the 6.4 GB/s of the Intel Canterwood (875P) chipset with dual-channel DDR400 memory today. Incidentally, while I won't have a lot of room to talk about it this month, I continue to marvel at the performance of my Canterwood systems (see the April column). Everything just works, smoothly and efficiently, and they are FAST.

While clock speeds have gone to 3 GHz, CPU bus speeds haven't kept up with clock speeds, but they have increased more quickly than peripheral bus speeds: the PCI bus. Yes, high-end PCs have begun to ship with 64-bit buses, and the very latest 64-bit slots also run at 66 MHz. That's four times the throughput of the usual 32-bit, 33 MHz slots we're all familiar with. The latest slots (mostly in servers today), PCI-X, double that again. Still, that's only 8 times faster than last year. By the time Longhorn ships, mid-market PCs will be moving enough data around that you'll actually feel the pinch.

In anticipation of this bottleneck, at last year's WinHEC, the PCI SIG announced PCI Express, a new, serial bus designed to supplant both PCI and PCI-X. Despite the unfortunately similar name to PCI-X, PCI Express (or just "Express Bus") is a completely different hardware standard: Where all previous PCI buses were completely parallel, all Express connections are serial, and point-to-point.

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