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PCI Express Primer PCI Express Primer: Still sitting on the fence about PCI Express? We review the technology and tell you the advantages and disadvantages over AGP.
Date: April 7, 2005
Written By:

In July of 2002 PCI-Express was approved, and in mid 2004 we started seeing deployment on Intel based motherboards, with the nForce4 and K8T890 adding AMD support later in the year. Why PCI-Express you ask? Wasn't PCI/AGP enough bandwidth for our ever increasing desire to move things along faster? Well, PCI Express is here to alleviate some short comings in the PCI world and to satisfy our ever increasing need for BANDWIDTH...

PCI (Peripheral Component Interconnect) was originally developed by Intel Corporation, but is now slowly coming to an end, by slow I mean it wont be discarded for a couple of years yet. PCI 2.2 bus and other revisions/iterations just don't provide us with enough bandwidth to support the increasingly bandwidth hungry and demanding peripheral cards. Hard drive controllers and networking cards just aren't provided enough bandwidth that some hard drives have the potential to offer.

The reason is because PCI only offers a throughput (maximum theoretical bandwidth) of 1.056Gbps while Serial-ATA hard drives can offer a maximum of 1.5Gbps (3.0Gbps with SATA II); however, if a SATA controller can only have a transfer rate of 1.056Gbps, the SATA drive will have a transfer rate of only 132MBps (megabytes per second), which is roughly equivalent to ATA-133 technology. This is true if there are no other peripherals requesting use of the bandwidth. PCI shares that 132MBps bandwidth as it only uses one PCI bus which is connected in parallel. Although there are other PCI versions available, most motherboards and components use the 32-bit PCI which operates at 33MHz.

Now for all of you with the Gigabit LAN cards must realize that at 1000Mbps you are using approximately 95% of your available PCI bus' bandwidth. This leaves little room for any other peripherals to use bandwidth. Currently the primary user will be using the Sound Card, Hard Drive and LAN on the PCI Bus, this could cause transfer issues, sound clipping and HD transfer interrupts. While most of us probably find PCI to be satisfying, we are rapidly approaching the point where PCI just won't be enough for our needs. Therefore, I present to you PCI-Express.

Bandwidth: AGP vs. PCI vs. PCI Express

PCI-Express should provide ample bandwidth for scalability well into the decade and perhaps even further. Currently, PCI-Express comes in five formats:

x1, x2, x4, x8, x12, x16. x4, x8, and x12 are likely to be reserved only for the server market while x1, x2, and x16 remain for the consumer for now.

Note: Each lane is comprised of 4 pins; x1 has one lane, x2 has two lanes, x4 has four lanes, and so forth. PCI-Express can transmit 100MB per second per pin.

Input/Output Bus Systems: [ISA = Industry Standard Architecture, EISA = Extended ISA, VLB = Video Electronics Standards Association (VESA) Local Bus, PCI = Peripheral Component Interconnect, AGP = Accelerated Graphics Port]

Graphics Standards
Possible Bandwidth
16-bit ISA
16MB/s
EISA
32MB/s
VLB
132MB/s
PCI
132MB/s
AGP 1x
264MB/s
AGP 2x
528MB/s
AGP 4x
1056MB/s
AGP 8x
2112MB/s
PCIe x1
500MB/s (Single Data Lane - Both Directions)
PCIe x2
1000MB/s (Dual Data Lane - Both Directions)
PCIe x4
2000MB/s (Quadruple Data Lane - Both Directions)
PCIe x8
4000MB/s (Eight Data Lane - Both Directions)
PCIe x12
6000MB/s (Twelve Data Lane - Both Directions)
PCIe x16
8000MB/s (4000MB/s Per Direction (Two Directions))

This is what the PCI-Express slots look like (excluding x2). These slots are attached to a switch (yes similar to that switch in your network) that controls the data flow. This is an improvement over a shared bus because each device pretty much has its own direct access to the bus instead of multiple components having to share the bus. This allows each device to use its full bandwidth capabilities without having to compete for the maximum bandwidth offered by a single shared bus. Then you add in the lanes of traffic that each device has access to then one can truly control much more bandwidth than previous PCI technologies. As mentioned before, x1 has one lane of traffic which is divided into input and output, obviously. Each lane is capable of providing approximately 500MB/s of bandwidth in both directions.

Eventually, x32 and possibly x64 slots will be realized for PCI-Express, but those probably won't be seen for a good while. Those slots are likely to be used for highly-demanding graphics cards. For now, desktop users will most likely see x1, x2, and x16 slots along with previous PCI slots on their motherboards. The following is a summary of PCI-Express:

" Much higher scalability over PCI bus
" Initially advantageous for hard drive controllers, gigabit LAN cards, and other bandwidth intensive devices
" Not necessary for a graphics upgrade as AGP 4x and 8x provide ample bandwidth for today's and tomorrow's Games. Games requiring the large amount of bandwidth PCI-Express x16 offers will likely be released in 2006.
" Not to be the ultimate deciding factor of a current system build as PCI and AGP should provide enough performance for your system unless you require gigabit LAN and other bandwidth-demanding peripherals.
" PCI-Express is still in its infancy and drivers and revisions will periodically be upgraded.

Final Words

For those who are looking for an upgrade to their system, PCIe (short for PCI Express) is a great technology to take advantage of. However, if you are looking to have the best possible graphics, there is no reason to splurge on a motherboard just with PCIe as AGP 4x and 8x should provide ample bandwidth for any intensive games you run, including Doom III / Half Life 2 and other soon-to-be released titles. In other words, pick what motherboard you want, don't just select one that utilizes PCIe because it uses PCIe. Presently, PCI and AGP will handle most jobs adequately, but for the future ... well, that's a different story.

If you have any comments, be sure to hit us up in our forums.

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