s will take advantage of AGP, BYTE found no significant difference in performance between the PCI and AGP versions of a popular graphics card when running 3-D modeling and visualization programs on a 300-MHz Pentium II PC.
AGP is designed to improve the graphics performance of Pentium II systems by providing a direct link between a PC's graphics card processor and system RAM through the core chip set. This gets the graphics card off the slower (133-MBps) PCI bus and onto its own dedicated channel. Intel claims AGP will speed graphics operations by allowing texture maps and other graphics data to be moved through a 66-MHz channel directly to main memory. The first implementation of AGP (called AGP-1x) will result in an effective doubling of graphics bandwidth over today's PCI to 266 MBps. Some vendors of graphics accelerator chips and boards will initially support AGP-1x, while others will support AGP-2x, which will deliver about 533 MBps. In 1999, AGP-4x will offer
bandwidth of more than 1 GBps.
Intel officials say a PCI graphics card in a Socket 7 system doesn't provide enough bandwidth for high-end graphics, and that AGP solves this problem by letting graphics and other data run in parallel over separate channels. But some vendors say the problem with PCI isn't one of bandwidth but one of contention. "The limitations of PCI affect graphics only when your SCSI, network, and graphics cards are contending for resources at the same time," says Phil Parker, director of corporate communications at Number Nine Visual Technology. In most cases, he says, a slow graphics processor, not the PCI bus, is the bottleneck.
BYTE polled Intel and numerous graphics accelerator vendors, and none could provide the name of a single application currently suffering from a bandwidth limitation when using a PCI graphics card. (However, Intel officials say this is partly due to developers who write applications, such as games, so they don't exceed the PC's available bandwidth.) Des
pite this, many vendors, including Number Nine, Matrox, STB, and ATI, are already fielding AGP-based graphics cards.
Another advantage of AGP touted by Intel is that it reduces the amount of video memory that must be present on a graphics card. AGP allows the use of system memory as a virtual extension of a graphics card's memory, so that a system vendor can include a 4-MB video card instead of an 8-MB one.
But board vendors and software developers dismiss this idea as not meriting serious consideration for high-performance graphics. "Graphics memory is cheap," Number Nine's Parker says. "We see the AGP bus as being a very large pipeline that feeds our graphics technology. 3-D applications will benefit with our AGP implementation by being able to send large textures across the high-speed AGP bus a single time and caching those textures in our processor's 8-KB internal texture cache and on the board's local memory, up to 16 MB. Once the texture is on-board, the on-board graphics engine can manipula
te those textures at speeds of up to 1.6 GBps [which is faster than AGP]. In this case, additional memory on the host is the secondary cache." John Heap, spokesman for U.K.-based Rage Software, whose forthcoming game
Incoming
will take special advantage of AGP by using highly detailed, large textures, agrees. "It is more beneficial to use the local RAM [on a video card] and then use AGP and system memory as an overdraft on local texture memory."
Several factors are contributing to AGP's lukewarm reception -- with the lack of currently bottlenecked applications heading the list. Some manufacturers point out that the increase in the speed of the PCI bus from 33.3 MHz to 66.6 MHz will allow it to shoulder more of a load when servicing graphics cards and postpone any real need for AGP. IT managers also face an additional support headache: Those who embrace AGP will have to support two different styles of video boards -- something we thought we left behind with VLB on the 486.
Fin
ally, there's the question of what to do with AGP when you get it. Currently, OS support is minimal. Although touted as a technology for high-end workstations, AGP won't be supported in NT until version 5. For Windows 95, an Intel-written VxD is currently available, but native support for AGP isn't planned until the release of Windows 98.
So who needs AGP? Applications that specifically exploit it are not available today, but AGP puts the foundation in place to provide better support for developers of games and other programs that can benefit from smoother play and more realistic images. And, AGP also provides an immediate benefit by freeing up the PCI slot from having to carry graphics traffic, giving more headroom to high-speed PCI networking peripherals and hard drives. So if you buy a new AGP system, you might as well buy an AGP board, especially since it will likely cost no more than the PCI version of the same card. In the meantime, as we wait for applications that really show off AGP's benefits,
it will be interesting to see if some vendor finds a new way to use this contention-free, high-bandwidth channel that's different than what Intel originally envisioned.
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Applications that take specific advantage of AGP, like Rage's Incoming game, can deliver improved realism and smoother operation. But don't expect these programs to arrive until later this year.
AGP: Who Needs It?
