Magical Moving RAM!

[BigJonno]

My PC has 4GB of RAM and a 1GB 8600GT, however dxdiag is showing 3GB of RAM and 2GB on the card. Anyone seen this before, know what's causing it or how to fix it?

[Aggort]

I think it's shared memory (http://en.wikipedia.org/wiki/Shared_Memory_Architecture). Gimme a second I'll help fix it.

EDIT: Check your BIOS. Make sure the memory allocation is correct there and check to see what memory is shared.

[GrenMag]

I'm a bit fuzzy on it (I should know it but I don't dink around with kernel mode display driver code too often) but I believe ATI's reporting 2GB because it's got a 1GB address range for system memory set up to be available in the aperture. It's not actually using it all time, only allocates it as needed. Can't remember how or if that changes the reporting of system memory.

I'm sure somebody will correct me if I'm off my gourd on that.

EDIT: pre-empted by Aggort.

[burger]

First off 32 bit Windows can only address 4 gigs total so because you have 4 gigs worth of physical system memory and 1 gig of physical video memory Windows ignores 1 gigs worth of the physical system memory so it can address the video memory.

Why it says 2 gigs of video memory is most likely for the reasons stated by Aggort and GrenMag.

[Aggort]

Burger does raise a good point, if you're running a 32-bit version of Windows then it'd be forced to allocate the memory elsewhere.

[boratika]

Here's an ancient post by Serapth that should explain the first bit:

Its simple math in a sense, 32 bit addressing is about to address 2^32 ( 2 to the power of 32 ) unique addresses, or 4, 294, 967, 296 bytes. Also known as 4 gigs.

Thing is, with win32, they virtualized memory addressing and limit each process to at most 2 gigs of memory. Another major gotcha is video memory. See, even if you have 4 gigs of RAM on your 32 bit OS, say you have another 512MB of video ram... how is that memory going to be accessed if all 4 294 967 296 different addresses are already taken? Answer is, they cant and ironically enough, tons of video ram can actually kill your performance on a 32bit OS.