MSI MS-6580 Manual page 62

BIOS Setup
AGP Aperture Size
This setting controls just how much system RAM can be allocated to AGP for
video purposes. The aperture is a portion of the PCI memory address range
dedicated to graphics memory address space. Host cycles that hit the aperture
range are forwarded to the AGP without any translation. The option allows
the selection of an aperture size of 4MB, 8MB, 16MB, 32MB, 64MB, 128MB,
and 256 MB.
3-15