HP Professional sp750 Compaq Professional Workstation SP750 and AP550 Key Tech - Page 14

WHITE PAPER (cont'd) Compaq Professional Workstation - Key Technologies In an AGP architecture, the 2D or 3D graphics accelerator is placed on its own bus and given direct access to system memory. The real features of AGP allow for highly pipelined data transfers between the graphics accelerator and system memory; providing much higher sustained bandwidths in comparison to the PCI bus. The performance advantages of AGP are particularly apparent when 3D applications utilize larger and more detailed textures and command lists, or when vertex data is being communicated. 2D graphics accelerators can also take advantage of the AGP architecture because of the increased bandwidth noted above. Since the graphics accelerator has direct access to system memory, a cost savings is also possible as the graphics accelerator may not require as much on-board memory. Another advantage of AGP is the fact that the graphics accelerator is the only device resident on the AGP bus, therefore, it alone is the bus master. Since this means reduced bus arbitration latency, the graphics accelerator can immediately initiate a transaction when needed. Finally, the removal of the graphics accelerator from the PCI bus increases the bandwidth for other devices on the PCI bus, increasing overall system performance. AGP Modes Currently, there are three data transfer rates supported by AGP (all AGP modes support a 32-bit wide bus with a 66 MHz bus clock). (These data transfer rates are theoretical maximums). • AGP 1X Mode, which yields 264 MB/second throughput • AGP 2X Mode, which yields 532 MB/second throughput • AGP 4X Mode, which yields 1 GB/second throughput To put the above throughput numbers in perspective, the typical PCI bus supports a bus width of 32 bits, clocked at 33 MHz yielding a maximum rate of 132 MB/second throughput. The above throughput numbers make it quite clear that AGP is significantly more efficient than PCI. There are also four AGP data transfer modes that can be utilized to pass data to and from the AGP controller: • CPU-GC (Frame Mode only) • GC-MMF (AGP Frame Mode) • GC-MMS (AGP Sideband Addressing Mode) • GC-MMP (AGP Pipelining Mode) Mode costs, performance and functionality are taken into account by engineering designers when determining the best modes to implement in an AGP controller. Intel continues to encourage AGP controller designers to move from Frame Mode to Pipelining and to Sideband Addressing in their new design to achieve maximum performance. 14