www.cpuplanet.com/features/article.php/2117941
March 19, 2003 The Latest Thing, or the Last Hurrah?Latest and greatest PC technologies are often touted as some kind of Second Coming, but the latest example is a third coming -- AGP 3.0, the standard behind the new AGP 8X graphics cards. This new version of the Intel-created, royalty-free-licensed specification improves on AGP 4X, promising unprecedented bandwidth and faster graphics performance, especially for 3D game maniacs. But does it really deliver? Is it worth changing your buying plans today? And can it be counted on for tomorrow? The AGP 1.0 and 2.0 LegacyThe original Accelerated Graphics Port 1.0 specification was introduced in 1996 and signaled a real shift in the way computer graphics were handled. The AGP interface provides a dedicated, high-bandwidth connection between a PC's core logic chipset and its graphics controller. It lets 3D textures stored in system memory be delivered directly to the frame buffer memory on a graphics card, bypassing the PCI bus so 3D and video traffic needn't compete with other hardware for that bus's 132MB/sec bandwidth.  The original AGP 1.0 design built on PCI bus technology, while enhancing it through a dynamic link to system memory, lower latencies, and increased speeds. The AGP bus is 32 bits wide (transferring 4 bytes per clock cycle); AGP 1.0 allowed for 1X and 2X modes, which supplied 266MB/sec and 533MB/sec of bandwidth respectively. These 1X and 2X numbers denote the operational speed (multiples of the standard 66MHz bus speed) used. In addition to the 32 lines for addresses and data, an additional 8 lines provide what's called sideband addressing, letting the graphics controller issue new requests while continuing to receive data from previous ones. AGP 1.0 was supposed to usher in a new era of 3D power, supplying bandwidth galore for cutting-edge graphics chips. In fact, it was a bumpy transition for many vendors and users, as the AGP and PCI interfaces are incompatible, and hardware upgrades were required to make use of the new specification. Soon, however, AGP quickly became a standard feature, enjoying industry-wide acceptance and virtually eliminating both PCI graphics cards and older architectures such as VL-Bus. Unfortunately, many of the first wave of AGP cards (such as the 3dfx Voodoo boards) were little more than PCI/66 clones that really didn't make use of the AGP bus for other than marketing reasons. Others made use of the AGP 2X bus, but simply didn't have the onboard hardware to keep it full -- their graphics chips didn't process data quickly enough to need the new bandwidth. In 1998, Intel stepped up to AGP 2.0, which defined an AGP 4X mode (1.066GB/sec) using the same physical interface with lower-voltage signaling -- 1.5V, down from 3.3V, with backward-compatible 1X and 2X modes available at both voltages. At this point, the AGP landscape started to become clearer, and many graphics pretenders were weeded out. This created a much higher performance bar for AGP 4X graphics cards, arguably led by Nvidia, whose GeForce series really helped usher in the AGP 4X technology that's common today. (Intel also introduced the AGP Pro specification, but this had no performance impact; it was simply a way to provide an additional power rail to higher-end video cards.) |
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