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Compaq Professional Workstation - Key Technologies
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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).
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AGP 1X Mode, which yields 264 MB/second throughput
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AGP 2X Mode, which yields 532 MB/second throughput
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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:
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CPU-GC (Frame Mode only)
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GC-MMF (AGP Frame Mode)
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GC-MMS (AGP Sideband Addressing Mode)
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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.
