Chapter 3: AGP Data Transfer Modes While the PCI bus supports a maximum of 132 MBytes/s, AGP at 66 MHz runs at 533 MBytes/s peak. It gets this speed increase by transferring data on both the rising and falling edges of the 66 MHz clock and through the use of data transfer modes that are more efficient. (Actual throughput will vary among various systems and applications, but usually they obtain about 50-80% of peak values in sustainable real-world transfers.) AGP provides two modes for the graphics controller to directly access texture maps in system memory: pipelining and sideband addressing. In pipelining, AGP overlaps the memory or bus access times for a request ("n") with the issuing of following requests ("n+1"..."n+2"... etc.). In the PCI bus, request "n+1" does not begin until the data transfer of request "n" finishes. While both AGP and PCI can "burst" (transfer multiple data items continuously in response to a single request), such bursting only partly alleviates the non-pipelined nature of PCI. The depth of AGP pipelining depends on the implementation, and remains transparent to application software. With sideband addressing, AGP utilizes 8 extra "sideband" address lines which allow the graphics controller to issue new addresses and requests simultaneously while data continues to move from previous requests on the main 32 data/address wires. Chapter 4 Table of Contents