Calculating Graphics Memory

The video memory manager (VidMm) must calculate the total amount of graphics memory before it can report an accurate account. It uses the following to calculate graphics memory numbers:

• Total system memory

  This value is the total amount of system memory accessible to the operating system. Memory that the BIOS allocates doesn't appear in this number. For example, a computer with a 1 GB DIMM (that is, 1,024 MB) that has a BIOS that reserves 1 MB of memory appears to have 1,023 MB of system memory.

• Total system memory that's available for graphics use

  This value is the total amount of system memory that is dedicated or shared to the GPU. This number is calculated as follows:

  TotalSystemMemoryAvailableForGraphics = MAX((TotalSystemMemory / 2), 64MB)
  

• Commit limit on aperture segment

  This value is the amount of system memory that VidMm allows kernel-mode display miniport drivers (KMDs) to pin down for GPU use at any given instant. That is, it's the amount of system memory that KMDs can memory map through an aperture segment. The total amount of system memory that is allocated for the GPU might exceed the commit limit greatly; however, VidMm ensures that only up to a commit limit amount is actually resident in an aperture segment at any one time.

  By default, the commit limit on a particular aperture segment is the size of that segment. The KMD can specify a different commit limit in the CommitLimit member of the DXGK_SEGMENTDESCRIPTOR structure when the driver describes the segment. A commit limit that is specified in such a way applies only to the particular segment that the driver describes.

  In addition to per-segment commit limit, there's a global commit limit on all aperture segments. This global commit limit is also referred to as shared system memory. VidMm computes this value. A KMD can reduce this value to a lower value in the ApertureSegmentCommitLimit member of the DXGK_DRIVERCAPS structure; however, we don't recommend this practice.

  VidMm doesn't allow a KMD to violate the per-segment commit limit nor the global commit limit. If a particular segment has a commit limit of 1 GB but the global commit limit is 256 MB, VidMm doesn't allow a KMD to map more than 256 MB of system memory into that segment.

• Dedicated video memory

  This value is the sum of the size of all memory segments for which the KMD didn't specify the PopulatedFromSystemMemory member in the DXGK_SEGMENTFLAGS structure for each segment.

• Dedicated system memory

  This value is the sum of the size of all memory segments for which the KMD specifies the PopulatedFromSystemMemory member in the DXGK_SEGMENTFLAGS structure for each segment. This number can't be greater than the total system memory that is available for graphics use (TotalSystemMemoryAvailableForGraphics).

• Maximum shared system memory

  This value is the maximum amount of system memory that is shared to the GPU. It's calculated using the following formula:

  MaxSharedSystemMemory = TotalSystemMemoryAvailableForGraphics - DedicatedSystemMemory
  

• Shared system memory

  This value is the amount of system memory that is shared to the GPU. It's calculated using the following formula:

  SharedSystemMemory = MIN(MIN(SumOfCommitLimitOnAllApertureSegment, DXGK_DRIVERCAPS.ApertureSegmentCommitLimit), MaxSharedSystemMemory)
  

• Total video memory

  This value is the total amount of video memory. It's calculated using the following formula:

  TotalVideoMemory = DedicatedVideoMemory + DedicatedSystemMemory + SharedSystemMemory