AGP

9/12/97

AGP - Accelerated Graphics Port. A technology that allows the graphics processor more or less direct access to a PC's memory in order to facilitate rapid transfer of data from memory to the graphics processor. The operating system, computer chipset and graphics processor must all be AGP aware in order for most AGP modes to work. Devices other than memory can communicate with the graphics processor over the PCI bus rather than via AGP. AGP applies to data transfers from main memory to display memory. The speed of transfer from display memory to the screen is unaffected by the choice of data bus. The ability to access main memory quickly is of great importance to display intensive tasks that can not fit all display information into display adapter memory.

AGP can operate as a 66MHz PCI device or as a 133MHz (AGPx2) device transferring data on the trailing edge of the PCI clock as well as the rising edge. AGP also permits pipelined data transfers. Depending on the situation AGP may be able to achieve effective data rates of 200 to 400 MegaBits/Sec (The spec sheet will claim 532Megabits/Sec). An unaugmented 33Mhz PCI bus is limited to 100 MegaBits/Sec. AGP may use an interrupt. It is not clear whether the AGP interrupt can be shared with PC or how great the performance impact would be of sharing the IRQs. Early testing indicated that AGP may allow displays to be updated as much as eight times faster than PCI -- presumably with some negative affect on the CPU throughput as the CPU may be locked out from memory access. Later testing indicated that AGP was faster than PC, but no where near eight times faster.

Last Update 990630.

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