KarbosGuide.com. Module 5b7.

About the interface AGP

The contents:

  • Introducing AGP?
  • The technology
  • Next page
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  • What is AGP?

    A new bus was introduced in 1997. It is called AGP (Accelerated Graphics Port), and it is exclusively designed for video cards.


    AGP was designed with two purposes:

  • To relieve the PCI bus of work with graphics data. Hence, it can concentrate on other demanding transport duties, like transfer to and from network adapter and disk drives.

  • To have better bandwidth within the video system.

    AGP was introduced with the Pentium II processor and Intel 82440LX chip set. Intel hoped to lift more of the CPU market away from the Socket 7 compatible CPUs by designing a completely new motherboard layout and including a new powerful bus for the graphics card. However both Ali and VIA soon introduced chip sets for Socket 7 motherboards including AGP. So today AGP is found on most motherboards.

    The technology

    AGP includes several techniques:


    The PCI bus in version 2.1 is the basis. It runs with 66 MHz bus frequency. That gives a doubling of transfer speed compared to traditional PCI, from 133 to 266 MB/sec.


    A kind of clock doubling in the 2X mode, where the bandwidth is expanded to 533 MB/sec.

    Texture cache

    This is a method to utilize system board RAM for texture cache. A technology that expands the memory used by the graphics card, utilizing the ordinary RAM in the PC.

    The data for texture (backgrounds) need not be processed by the graphics controller, so it is just being loaded from the RAM. This technology is called DIME by Intel (for Direct Memory Execute).

    Here you see the architecture involving chip set, main memory and AGP:

    The socket

    Below you see the AGP socket at the bottom. It looks like a PCI socket, but it has been placed in a different position on the board.

    In the top you see two (black) ISA sockets. Then four (white) PCI sockets, and then the brown AGP socket:

    At a first hand glance, the AGP socket very much look like a PCI socket. But it is placed in a different position, so you cannot plug an AGP card into a PCI socket.

    AGP 4X and above

    In 2000 we saw first AGP4X chip sets (like i820) and motherboards implementing this feature, which many graphics controllers already have been made for. With AGP the bandwidth to the video subsystem is up at 1066 MB per second.

    Late in 2001 the AGP 8X standard is expected to hit the market. It will increase the bandwidth to more than 2 GB/sec. To use this power, we have to have more powerfull chipsets with high-speed RAM. Intels i850 with dual Rambus channels will deliver the necessary RAM bandwidth.

    Try www.agpforum.org for more info.

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    To learn more

    Read more the boot process and system bus in Module 2b

    Read about file systems in module 6a

    Read about I/O buses in module 2c

    Read about the motherboard chip set in module 2d

    Read about RAM in module 2e

    Read Module 5c2 about USB.

    Read module 7a about monitors, and 7b on graphics card.

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