On Wednesday, September 22, 1999, 6:27 AM, Elizabeth Boston
<[log in to unmask]>wrote:

> In the process of trying to figure this problem out, I have come
> across a setting in the BIOS called "AGP Aperture size" it is set
> to 64mb.
>         Can someone explain this setting to me... what it means and what
> changing the value will do?
>

 The default AGP aperture size is 64MB, a setting that  accommodates both
Windows* 9x and NT* 5 environments. Although Windows 9x provides a larger
memory space (2GB virtual memory addressing), NT 5 has more limited system
memory resources.  (Developers are planning to map the entire aperture to
provide error protection.) Because of these constraints, 64MB was chosen as
compromise that worked well in Windows* NT* 5  environments.

Choosing an AGP aperture size is like setting screen
 resolution--it depends on many factors. The default size of
 64MB is only a recommendation. You can increase or decrease
 the default setting to fit your application requirements. If
 the target system has less memory than that specified for the
 aperture, AGP applications will run in available memory.
 To get an idea of the memory overhead associated with AGP, you
 can use this formula: Max DirectX5 Texture Area =
 64MB-(64MB/2). In Win95/OSR2.x Max DX5 Texture Area =
64MB-(64MB/2)-12MB with only 1/2 of the aperture available for
 write combine memory. OSR2.1 VgartD VxD reserves 12MB for the
operating system.

Note that a portion of the aperture is mapped to WC
 (write-combining) and a portion to UC (uncached). On OSR2.x,
 half is assigned to WC and half to UC. Windows* 98 currently
also assigns half to WC and half to UC but that may change.
 The Windows* NT* support has not yet been completed.
Since most DirectDraw* heaps end up being allocated from WC
memory, the amount of virtual memory that is available for
 these heaps is one half the aperture size. A 64MB aperture
 allows 32MB for WC heaps. HTH. :-)

Walter R. Worth
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