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Grafting AGP onto a PCI-Express Motherboard

The best way to achieve a motherboard with both AGP and PCI-Express functions is through the motherboard chipset. As I have covered in detail over the years in my book Upgrading and Repairing PCs, the North Bridge or Memory Controller Hub chip in a typical two-chip chipset is responsible for connecting external video slots such as AGP or PCI-Express to the remainder of the system architecture. Unfortunately, the first generation of PCI-Express-compatible chipsets from Intel, VIA Technology and SiS supported only PCI-Express video.

As an alternative, Elitegroup (ECS), Albatron and Foxconn have created their own solutions for Pentium 4 users by connecting an AGP video card connector to the PCI bus. Elitegroup's so-called "AGP Express" solution connects an AGP 8x-compatible slot to the PCI bus on systems which feature PCI-Express external graphics. ECS's implementation permits both graphics cards to work at the same time, a feature known as Dual Graphics Engines (DGE). Albatron refers to its similar solution as "AGP Extension" (AGPe). AGPe also supports simultaneous use of PCI-Express and AGP 8x video cards. Foxconn refers to their AGP extension as Foxconn Graphics Extension (FGE).

Table 1 lists current motherboards using an AGP on PCI approach.

Table 1 – Motherboards Providing AGP Support via the PCI Bus

Vendor

Model

Chipset

Website

Albatron

PX915P-AGPe

Intel 915P

http://www.albatron.com.tw/english/it/mb/specification.asp?pro_id=136

 

PX915G-AGPe

Intel 915G

http://www.albatron.com.tw/english/it/mb/specification.asp?pro_id=137

ECS*

915P-A

Intel 915P

http://www.ecsusa.com/products/915p-a.html

 

915G-A

Intel 915G

http://www.ecsusa.com/products/915g-a.html

Foxconn

915PL7AE-8S

915PL

http://www.foxconnchannel.com/products_motherboard_2.cfm?pName=915PL7AE-8S

 

915PL7AE-8EKRS

915PL

http://www.foxconnchannel.com/products_motherboard_2.cfm?pName=915PL7AE-8EKRS

*Elitegroup (ECS) has announced other motherboards using various chipsets with AGP Express. Check the ECS website at www.ecsusa.com or www.ecs.com.tw for updates.

AGP 8x Through the PCI Bus – What Do You Lose In Performance?

Although the approach Elitegroup, Albatron and Foxconn use allows you to re-use your existing AGP 8x cards on a motherboard which permits an upgrade to PCI-Express in the future, you sacrifice a lot of performance by using the PCI bus for your AGP card. Here's why: The PCI bus connects to the slower I/O Controller Hub (South Bridge) chip instead of the Memory Controller Hub (North Bridge), and unlike the AGP bus, which provides dedicated bandwidth for the video card, the PCI bus services several PCI slots as well as PCI motherboard devices such as ATA/IDE and Serial ATA host adapters, USB ports and the like at the same time. If you're looking for full AGP 8x performance in the present, along with the ability to move to PCI-Express later, this compromise approach will disappoint you.

VIA's PT880 Pro – True AGP 8x and PCI-Express in One Chipset

Although the first generation of PCI-Express chipsets from all vendors abandoned AGP 8x entirely, the new VIA Technology PT880 Pro chipset provides for true hardware support for both AGP 8x and PCI-Express graphics.

VIA refers to its AGP and PCI-Express graphics technology as Universal Graphics Interface (UGI). The PT880 Pro also supports simultaneous display on both AGP and PCI-Express (DualGFX Express), and supports DDR and DDR2 system memory (a feature referred to as StepUp technology).

As with previous AGP/PCI-Express solutions, there is a compromise, but this time PCI-Express is making the sacrifice. AGP 8x cards plug into a full-speed AGP 8x slot, but the PCI-Express video slot on the PT880 Pro is a 4x rather than the 16x slot used in PCI-Express-only implementations. As this article is written, only evaluation motherboards based on the PT880 Pro chipset have been available. However, it's likely that a number of system makers will use this board by mid-2005 and beyond.

If you are planning to move up to PCI-Express with a motherboard based on this chipset, you should check reviews of shipping products at websites such as Tom's Hardware (www.tomshardware.com and Anandtech (www.anandtech.com) to determine how much of a performance impact PCI-Express 4x support versus 16x actually has with current graphics cards.

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