Hardware Reference
In-Depth Information
as texture maps, directly from system memory rather than having to copy the data to the
adapter memory before the processing can begin. This saves time and eliminates the need
to upgrade the video adapter memory to better support 3D functions. Although AGP ver-
sion 3.0 provides for two AGP slots, this feature has never been implemented in practice.
Systems with AGP have only one AGP slot.
Note
Although the earliest AGP cards had relatively small amounts of onboard RAM, most later
implementations use large amounts of on-card memory and use a memory aperture (a dedic-
ated memory address space above the area used for physical memory) to transfer data more
quicklytoandfromthevideocard'sownmemory.Integratedchipsetsfeaturingbuilt-inAGP
use system memory for all operations, including texture maps.
Windows 98 and later versions support AGP's Direct Memory Execute (DIME) feature.
DIME uses the main memory instead of the video adapter's memory for certain tasks to
lessen the traffic to and from the adapter. However, with the large amounts of memory
found on current AGP video cards, this feature is seldom implemented.
Four speeds of AGP are available: 1x, 2x, 4x, and 8x. (See
Table 12.7
for details.) Later
AGPvideocardssupportAGP8xandcanfallbacktoAGP4xor2xonsystemsthatdon't
support AGP 8x.
Table 12.7 AGP Speeds and Technical Specifications
AGP 3.0 was announced in 2000, but support for the standard required the development
of motherboard chipsets that were not introduced until mid-2002. Almost all motherboard
chipsets with AGP support released after that time featured AGP 8x support.
Although some systems with AGP 4x or 8x slots use a universal slot design that can
handle 3.3V or 1.5V AGP cards, others do not. If a card designed for 3.3V (2x mode) is
pluggedintoamotherboardthatsupportsonly1.5V(4xmode)signaling,themotherboard
may be damaged.
