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PC Architecture. A book by Michael B. Karbo

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At the same time, the <strong>PC</strong>I system is spared from the large amount of graphic data traffic to and from the<br />

video card. It can now focus on the other intensive transfer tasks, such as transfer to and from the network<br />

adapter and disk drive.<br />

Figure 154. AGP Video card from ATI.<br />

Technical details<br />

AGP consists of a number of different technical elements, of which I will highlight two:<br />

● A bus structure built around a double-clocked <strong>PC</strong>I bus.<br />

● The ability to use the motherboard RAM as a texture cache.<br />

The texture cache is used <strong>by</strong> games, and <strong>by</strong> giving access to the motherboard RAM, less RAM is needed on<br />

the cards.<br />

The AGP bus is actually a 64-bit variant of the <strong>PC</strong>I bus. You can also see that on the surface, the<br />

motherboard AGP slot looks a fair bit like a <strong>PC</strong>I slot. But it is placed in a different position on the<br />

motherboard, to avoid confusion (see Fig. 156). The slot also has a different colour.<br />

The first version of AGP was 1X, with a bandwidth of 254 MB/sec. But AGP was quickly released in a new<br />

mode, called 2X, with 508 MB/sec.<br />

Later came 4X and 8X, which are the standards today. This involves a clock doubling, just as we have seen,<br />

for example, with DDR RAM. Two or four data packets are sent for each clock pulse. In this way, a bandwidth<br />

of 2,032 MB/sec has been reached.<br />

Texture cache and RAMDAC

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