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Sneak Preview: Intel Alderwood/Grantsdale Chipsets

PCI Express: Up To 8 GB/s

Take a good look at this logo - it will be with us for the years to come.

The fastest and the slowest: x16 PCI Express (top) and x1.

The PCI bus and the AGP interface will be replaced by PCI Express. As opposed to PCI, this involves a serial protocol that can operate all connected devices with the full bandwidth. PCI and PCI-X, by contrast, are parallel busses whose bandwidths are shared by all the devices. In the transition period, PCI Express has the advantage is that it is completely software-compatible to PCI. Apart from this, however, we have determined quite clearly that PCI Express cannot be considered a replacement for PCI-X in the long run. More on this later on.

A single serial connection with PCI Express works with two circuit pairs and a clock speed of 2.5 GHz. Here, a total of 10 bits are transferred for 8 bit data, which results in a total bandwidth of 2 GBit/s and 256 MB/s. Depending on the source line, which is called link or lane, this connection works in full-duplex mode and can therefore send and receive the data simultaneously, and this results in 512 MB/s.

Performance Through Lines

PCI Express was designed to be a scalable interface, so it's not surprising that its faster version is based on multiple lines. The specification is as follows:

PCI Express LinesBandwidth per StreamBandwidth, duplex
1256 MB/s512 MB/s
2512 MB/s1 GB/s
41 GB/s2 GB/s
82 GB/s4 GB/s
164 GB/s8 GB/s
32*8 GB/s16 GB/s

* Not specified for desktop use

Above is a typical PCI slot, below is a x16 PCI Express slot.