The first in this series of Tech Tips on expansion cards took a glance at the PCI slot, & the variety of devices that may find their home in. Graphics cards are of the lots of items that may be used in a PCI slot, but the demands of fast-paced video games need more speed & greater bandwidth than the PCI Bus can provide. Thus, the AGP slot was born, providing a dedicated interface to transfer graphics knowledge only.
The letters 'AGP' stand for Accelerated Graphics Port, & it is the term used to document a dedicated, point-to-point interface that connects a video card directly to the system's memory & processor.
AGP was first introduced by Intel in 1996, & is based off of their earlier work in developing the PCI bus. Despite being based on PCI expertise, the AGP & PCI slots on a motherboard are not interchangeable, so an AGP card cannot be installed in to a PCI slot, & vice versa.
The preliminary release of AGP saw a sizeable performance boost over PCI, & the few revisions to the standard helped increase this even more as years went by. Other than having a dedicated path to the system's memory & processor, several other design features help AGP outperform PCI when it comes to graphics performance. of the other advancements: pipelining, side band addressing & graphics address remapping table are described below.
Knowledge transfer is improved through 'pipelining', a term used to document the ability of an AGP graphics card to get, & act on, and multiple instructions simultaneously. PCI knowledge transfers need each piece of necessary information to be received separately before acting on any of it.
Something called 'side band addressing' also provides AGP with a performance boost. Fundamentally, additional lines of knowledge are included with each packet to instruct the method as to where this knowledge is to be used. PCI knowledge transfers do not have this addressing information, & the method must look at the knowledge itself in order to select its location. This is an obvious time saver, & a resource saver since the processor doesn't must analyze all knowledge to select the address.
AGP allows the operating method to store texture maps in the system's memory which allows for more space, & possibly faster access, than being limited to the use of graphics card memory only. Graphics art address remapping table, also called GART, is a term used to document a method that maps physical memory as virtual memory for the storage of texture maps. Fundamentally, GART takes the method memory it is allowed to make use of to store texture maps & re-addresses it so that the method thinks these maps are now actually being stored in the frame buffer, or virtual memory. This might not sound like anything special, but this re-addressing requires that the texture map be written to memory time & it is locked in to place right where the AGP card can find it quickly.
AGP can be broken down in to different groups based on revisions to the specification (AGP.0, AGP.0, & AGP.0), as well as by the general speeds (1x, 2x, 4x, & 8x). There is overlap between the various categories, with AGP.0 supporting 1x & 2 xs, AGP.0 supporting 1x, 2x, & 4x, & AGP.0 supporting 4x & 8x.
to be continued...
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