AGP APERTURE

3/21/2003

AGP Aperture: An area of memory space that appears to an Accelerated Graphics Processor video system to be a contiguous block of memory. It is a (small) chunk of the 4GB memory space in the 80x8x architecture that is being appropriated for a purpose other than directly accessing memory. It is used in order to avoid burdening PC Operating Systems with the need to work around the addresses used by a fixed video buffer -- an arrangement that has been troublesome in the past.

The aperture address range is mapped to actual physical memory addresses by motherboard hardware containing a set of Graphics Attribute Remapping Tables (GART). These allow potentially discontinuous 4Kbyte pages of PC memory to be mapped into a contiguous memory space seen by the AGP card. The AGP Aperture addresses are always numerically above the highest value of physical memory installed so that there can not be a conflict between AGP and physical addresses.

The AGP Aperture is also used by the PCI bus when accessing video memory in a system with an AGP system available. Access via PCI is allowed, but it is not desirable for high volume transfers. It is slower than accessing the memory via the AGP card or the PC memory subsystem. The PCI bus has a lower bandwidth than either AGP or the PC memory subsystem.

Many BIOSes have a setting for PCI Aperture size. This setting seems to have little or no affect on PC performance. It is probably unwise to set the AGP aperture value in the BIOS to less than the default unless there is a specific recommendation from the video card manufacturer to use a smaller value. Larger values will probably do no harm (nor good).

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