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Understanding PCI Express

PCI Express is here to stay.  Is it a quick demise for AGP?  Should you upgrade now to PCI Express or wait a while?  This guide should give you some good info and tips on your way to PCI Express.

Getting Started

PCI Express isn’t just about new graphics cards.  It’s about new architecture affecting not only your graphics but your chipsets, your processor, peripheral connectivity, and in turn your motherboard.  It’s about greater bandwidth and faster throughput.

It's important to understand that PCI, though effective back in the early 90’s when it debuted, is now subject to bottlenecks that are exacerbated by the latest generation of Gigabit Ethernet adapters, high-definition audio devices, and multidrive RAID arrays. Based on a shared bus topology, individual devices populating the bus are forced to contend for its 133MBps of bandwidth. And because PCI transfers data in a parallel manner, multiple streams at a time, it doesn't allow much in the way of scalability.

How Does PCI Express Work?

PCI Express replaces PCI's parallel, shared bus with a serial, point-to-point design. This means that instead of sending multiple streams of data, PCI Express, in its most basic form, employs one path for sending and another path for receiving data, effectively  achieving 250MBps. The combination is called a lane. Up to 32 lanes may be aggregated to create an 8GBps link between the PCI Express device and its host switch. Because the two are linked directly, bandwidth is dedicated, not shared, and equal in both directions at the same time. It almost goes without saying that the throughput improvements alone make PCI Express superior to PCI and its derivatives.

Of course, PCI Express also boasts advanced power management.  Because bus traffic is arbitrated by a switch, PCI Express supports QoS (Quality of Service) functionality, the prioritizing of time-sensitive data packets. Perhaps you've already heard of QoS related to wireless networking. Similarly, appropriately flagged packets of information will fly over the PCI Express bus first, ensuring real-time audio and video streams don't cut out or drop frames.  PCI Express indeed offers a number of benefits over AGP 8X with regard to graphics and the standard parallel PCI bus for peripheral connectivity.

Graphics

Many of you want to know if you should wait for PCI Express X16 video cards or go ahead and purchase another AGP card. The easy answer is, “Some of you should, and some of you shouldn’t.” First, we need to look at the differences between AGP in its 8X incarnation, which is, of course, the mainstay of today’s enthusiast video cards, and PCI Express X16, the future of video cards.

Generally speaking, there are only two real differences. First, just to be clear, AGP and PCI Express cards will not work in the same slot. The second difference is the supported bandwidth across each interface. AGP 8X supports up to about 2.1GBps in one direction out to the video card. PCI Express X16 supports 16 lanes of data that will allow for 4GBps in each direction, or 8GBps. Approximately 25% of that bandwidth is consumed by other information needed to make the bus work, effectively giving you 3GBps of “real” data each way. Most of the marketing you’ll see shows PCI Express at 8GBps compared to AGP 8X at 2GBps, but in today’s reality, all that 8GB of bandwidth simply is not used by games. Also take into account that if you benchmark the same video card using an AGP 4X (1GBps) bus and then an AGP 8X bus, you’ll likely see very little performance increase and none that would reflect a better real-world gameplay experience. All in all, today’s AGP 8X isn’t even being taken advantage of by game-content developers.

What Else is New?

DDR memory has been the standard for some time across all platforms. However, Intel has decided that now is the time to step up to DDR2. It’s still in the early days, but DDR2 does function at faster clock speeds. There are other advantages such as a lower operating voltage (1.8V instead of 2.5V), a larger prefetch (four instead of two), and on-die determination for better signal integrity.

Faster clock speeds are one thing, but at this stage the latency increase means that currently memory sticks work with a CAS 4 latency, which isn’t the same as the CAS 2 we’re used to in the performance world with DDR. This situation will change in the future, but it certainly hampers overall performance of the 9xx series.

High-definition audio is another new feature (code-named Azalia), which is a considerable improvement over the AC’97 standard. Intel’s HD audio is able to do eight-channel, 192KHz, 24-bit audio streams. Multistreaming support allows for the routing of different audio sources to multiple output devices.

Serial ATA isn’t new, but for the 9xx series of chipsets, Intel has dubbed its storage capabilities as “Intel Matrix Storage Architecture.” The company has upped the number of SATA ports to four and now added RAID 1 functionality. Matrix Raid gives you the performance advantages associated with striping RAID 0 but with the added backup of using RAID 1 with only two drives. Intel’s software is extremely easy to use and should help bring RAID even to the beginner's PC.

Currently only Intel has released chipsets that support PCI Express.  The 915 (a.k.a.  Grantsdale), 925X (a.k.a. Alderwood), and the soon to be released 925XE are showing up now on motherboards.  The 915 has two flavors, the G and P.  The 915G has integrated graphics options where the 915P does not.  The only real difference between the two chipsets is the 915 supports both DDR and newer DDR2 memory.  The 925X only supports DDR2.  Intel's 925XE chipset which is just beginning to roll out, is accompanied by Pentium 4 processors designed to run on a 1066MHz frontside bus instead of the standard 800MHz frontside bus. The new bus speed should establish parity between itself and the memory bus, both of which transfer 8.5GBps of data.

This means that currently only Intel based boards offer PCI Express architecture.  But don’t worry, AMD expects to have incorporated chipsets most likely from nVidia by later this year or early 2005.

Also attached to PCI Express is a new CPU Socket.  Intel’s socket 478 is replaced with a new socket, the LGA (land grid array) 775. The extra pins do serve up more power but require a bit of extra care when mounting your CPU. Instead of the (bendable) pins being on the processor itself, they are now on the motherboard socket with the CPU having flat contact points.

This new socket means new LGA 775 processors. The Prescott P4 is up to 3.6GHz, dubbed the 560, and there is a P4 3.4GHz Extreme Edition. A 3XX chipset series will also show up, denoting the re-release of a budget “Celeron” line of processors.  To go along with the 925XE chipset is a P4 3.46 Extreme Edition processor that will run on the 1066MHz front side bus.

Adding It All UP

PCI Express offers excellent benefits and future benefits that we are limited with on current PCI architecture.  With Intel promoting this change heavily and AMD coming on board soon, there is not doubt PCI Express is here to stay.  It does not mean AGP is dieing a quick death.  So should you upgrade now to PCI Express?   Yes...and no.

If the video card you want to purchase will soon be offered in PCI Express and you’re looking to upgrade your motherboard at the same time, there’s no reason not to go with PCI Express.  Looking at the big picture, if you just bought a $500 AGP video card today, you’ll likely have enough system to carry you through the life of the video card and you can upgrade to PCI express on your next upgrade go around.

Stay tuned for some PCI Express hardware reviews and Buying Guide!!

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