PCI Express (PCIe)

Like the transition from PCI to AGP video cards, PCI Express brings in a new form factor and renders AGP cards inoperable in new systems. The spotlight on PCIe has been on the graphics slot with people quick to point out at the initial lack of high end video cards being available hurts the case for PCIe which really misses the point. PCIe updates a sorely outdated system bus that has been essentially stagnant since the latter days of the 486.

As mentioned in a previous article, PCIe brings the following benefits

• lower implementation cost
• more bandwidth per pin
• scalable performance
• point to point architecture

The key points are point to point architecture and scalability. The traditional PCI bus operated on what is essentially a broadcast network model where bandwidth is shared by all components on the PCI bus - theoretically it had 133MB/s of bandwidth (33Mhz * 32 bits * 1 byte/8bits ). This sounds fine and all until the requirements of various devices are taken into account. Gigabit ethernet requires more bandwidth than the entire PCI bus can provide. Firewire can eat up to 50 MB/s, Firewire B, 100 MB/s, sound cards, video capture cards. The list of bandwidth hungry devices competing for this 133 MB/s of bandwidth go on and on. In the PCI Express architecture, each device has dedicated bandwidth to the processor and memory so there is no resource contention. Furthermore, the x1 slot already provides 250MB/s in each direction (500 Mb/s in full duplex mode), meaning there is roughly 3.75x more bandwidth available than the total PCI bus.

An x16 PCIe graphics slot alongside a x1 PCIe slot

Scalability is the next issue to come into play. If the 250MB/s of the x1 slot is not enough, more lanes can be dedicated to the slot. Graphics have been allocated 16 of the usual 20 lanes that are seen on this first generation of PCI Express boards. PCI Express is about much more than just a new graphics slot although there is a heavy emphasis on this for another reason, the return of dual video card processing being pushed by both Alienware and NVIDIA. On the Intel side, Intel is the only one that has a platform that supports the NVIDIA and Alienware implementations in its Tumwater chipset. While there is a lot of emphasis on the video applications of PCI Express, on a more practical level, there are a lot of additional devices that can take advantage of the bandwidth provided by PCI Express.

Socket-T (LGA-775)

I know people who have done incredibly stupid things with processors. There was a person I knew and he had a pretty old computer; an old Cyrix or something abysmal. A friend of mine donated an old Socket A CPU while another friend coughed up an old motherboard. The recipient was excited when he received both. He proceeded to install the processor into the board. We hear back from him a few minutes later - "it's a little hard to get the processor in." Turns out he did not lift the locking mechanism required to install the processor and ended up bending a lot of the pins and destroying the processor. I do not think he ever got off that Cyrix.

The Socket-T packaging used on the 915 and 925x manages to avoid the nightmare scenario described above - pins are no longer located on the processor itself but have moved onto the motherboard.

The LGA-775 Socket. Pins are located on the motherboard. The retention mechanism is processor independent

This several advantages, there is truly zero insertion force required to install a processor as it is simply placed on the bed of pins in a motherboard. A second advantage is the retention mechanism no longer relies the processor is used to clamp down the processor. This is an important point; with pin based sockets the locking mechanisms rely on the processor pins which is not an ideal situation. I have had thermal grease form enough suction between the processor and heatsink to pull the processor from the socket while trying to remove the heatsink. Not a pleasant experience. This has changed with a clamshell format and the metal bar that takes the force off of the processor and places it on the socket and board instead.