Consider the following scenario:
You have onboard UMA 384 MB + 1GB from the video adapter and you expect the total video memory to be about = 384MB + 1024 MB => 1408 MB
However, in the Control Panel -> Appearance and Personalization ->Display ->Screen Resolution Panel, click [Advanced Settings]. Under the "Adapter" tab, you will see the Dedicated Video memory value displayed as 1536MB.
Basically, the UI is coded to display the Dedicated Video Memory in the form of power of 2. So, it uses this function “NextPowerof2(uNumMegs/8)” to calculate the allowed adjustment. And here are the allowed adjustments for different ranges of memory:
// 129M-256M rounds in multiples of 32M
// 257M-512M rounds in multiples of 64M
// 513M-1G rounds in multiples of 128M
// 1G+-2G rounds in multiples of 256M
// 2G+-4G rounds in multiples of 512M
And we can see that for the case of UMA_384+DSC(1024MB), the adjustment is in multiples of 256MB. It is not possible to display 1408MB. And since 1408MB is greater than 1280MB(256*5), that is why you are seeing 1536MB(256*6) by design.
This power of 2 algorithm was implemented per KB938838.
Windows Vista displays the incorrect dedicated video memory size for certain display adapters
To display a more accurate video memory figures, use the videomemory.exe tool from the DirectX SDK tools. Link:http://msdn.microsoft.com/en-us/directx/aa937788