Specifications
Video Display Adapters
909
√√ See “Accelerated Graphics Port,” p. 364.
√√ See “The PCI Bus,” p. 358.
√√ See “PCI–Express,” p. 362.
The Accelerated Graphics Port (AGP), an Intel-designed dedicated video bus introduced in 1997, delivers
a maximum bandwidth up to 16 times larger than that of a comparable PCI bus. AGP has been the
mainstream high-speed video standard for several years. The AGP slot is essentially an enhancement to
the existing PCI bus; it’s intended for use with only 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 recent and all current implementa-
tions of card-based AGP 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 do use system memory for all operations, including texture maps.
Ironically, the memory aperture used by AGP cards can actually cause out-of-memory errors with Windows 9x and
Windows Me on systems with more than 512MB of RAM. See Microsoft Knowledge Base document #253912 for
details.
All recent nonintegrated motherboard chipsets from major vendors (Intel, Acer Labs, VIA Technologies,
and SiS) for Pentium II and newer Intel processors and AMD Athlon and Duron processors support some
level of AGP. However, some of the early integrated chipsets don’t support an AGP slot.
Even with the proper chipset, however, you can’t take full advantage of AGP’s capabilities without the
proper operating system support. AGP’s Direct Memory Execute (DIME) feature uses main memory
instead of the video adapter’s memory for certain tasks to lessen the traffic to and from the adapter.
Windows 98/Me and Windows 2000/XP support this feature, but Windows 95 and Windows NT 4 do
not. 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 15.14 for details).
Table 15.14 AGP Speeds and Technical Specifications
AGP Speed AGP Specification Clock Speed Transfer Rate Slot Voltages
1x 1.0 66MHz 266MBps 3.3V
2x 1.0 133MHz 533MBps 3.3V, 1.5V
1
4x 2.0 266MHz 1,066MBps 1.5V
8x 3.0 533MHz 2,132GBps 1.5V
2
1. Varies with card implementation.
2. Uses 0.8V internal signaling.
Because of the bandwidth AGP 3.0 requires, systems featuring this version of AGP also support DDR333
or faster memory, which is significantly faster than DDR266 (also known as PC2100 memory). 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. Virtually all current motherboard chipsets and AGP
Chapter 15
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