Hardware Reference
In-Depth Information
See “
PCI Express
,”
p.
251
.
See “
Accelerated Graphics Port
,”
p.
253
.
AGP
The AGP, an Intel-designed dedicated video bus introduced in 1997, delivers a maximum bandwidth
up to 16 times greater than that of a comparable PCI bus. AGP was the mainstream high-speed video-
to-system interface for several years but has been replaced by the more versatile and faster PCIe
standard.
The AGP slot is essentially an enhancement to the existing PCI bus; however, it's intended for use
only with video adapters and provides them with high-speed access to the main system memory array.
This enables the adapter to process certain 3D video elements, such 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 version 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 dedicated
memory address space above the area used for physical memory) to transfer data more quickly
to and from the video card's own memory. Integrated chipsets featuring built-in AGP 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 AGP video
cards support AGP 8x and can fall back to AGP 4x or 2x on systems that don'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 plugged into a motherboard
