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PCI Express x1 Bus

Besides the new PCI Express x16 graphics bus, Intel also introduced a new bus for regular expansion cards: PCI Express x1. However, unlike PCI Express x16, which doesn’t have any alternatives in the new i925/i915 chipsets, the announcement of the PCI Express x1 support doesn’t imply that the current PCI standard will sink into oblivion. ICH6 South Bridges, which will go with the new i925/i915 solutions will still support up to 6 PCI Master devices. They will simply acquire up to PCI Express x1 devices support. As a result, mainboards based on the new chipsets from Intel will offer different combinations of PCI and PCI Express x1 slots at the same time.

PCI Express slots are laid out on the mainboard PCB instead of the regular PCI slots, however, you will be able to distinguish between them easily: the 36-pin connector of the serial PCI Express x1 bus is much shorter than the standard PCI slot.

What are the advantages of the PCI Express x1? First of all, it is higher bus bandwidth. Unlike the regular 32bit 33MHz PCI bus, the bandwidth of the new PCI Express x1 is much higher and reaches 500MB/sec. Moreover, PCI Express x1 features point-to-point topology like any serial bus. As a result, each PCI Express x1 device receives a dedicated bandwidth of 500MB/sec while all devices connected to a parallel bus share the 133MB/sec bandwidth. Besides that, some of the PCI Express x1 advantages are coming from its architecture. Here I am talking about such things as pipelines reading or lower latencies.

Of course, those devices that feel pretty limited by the today’s PCI interface should very soon move to the new bus. Among them are gigabit network controllers, high-performance RAID controllers, etc. However, unlike graphics card manufacturers, the peripherals developers didn’t respond with the same excitement and activity, that is why the only PCI Express x1 device available today is a gigabit network controller – Marvell Yukon 88E8050.

I have to point out that the mainboard manufacturers gave this controller a very warm welcome and today you can see it integrated onto the whole bunch of mainboards based on i925X Express and i915 Express chipsets.

Since we had an i925X Express mainboard with this controller onboard at our disposal, we decided to check how fast it actually was. Let’s find out if connecting this controller to the high-performance PCI Express x1 bus is efficient at all and how the controller performance corresponds to that of the Intel 82547EI controller connected along the CSA bus with 266MB/sec bandwidth in i875/i865 systems. We ran the tests in a system with Intel Pentium 4 3.4E CPU. The performance was measured with the help of PassMark Advanced Network Test utility.

As we see, the use of PCI Express x1 bus does provide certain benefits when we have a gigabit network controller connected to it. At least Marvell Yukon 88E8050 works faster with PCI Express x1 bus than a similar chip with the PCI interface. However, controller for the dedicated CSA bus introduced by Intel for gigabit network implementation in i875/i865 chipsets does work much faster still. Despite this fact Intel gave up the CSA bus in the new i925/i915, because the network controller makers didn’t welcome it that warmly.

 
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