MSI 645 Ultra Manual page 53

Chapter 3
Graphic Win Size
The field selects the size of the Accelerated Graphics Port (AGP) aperture.
Aperture is a portion of the PCI memory address range dedicated for graphics
memory address space. Host cycles that hit the aperture range are forwarded
to the AGP without any translation. Settings: 4MB, 8MB, 16MB, 32MB, 64MB,
128MB, 256MB.
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