GPU not using all its RAM

[The77s]

Hey.

My GPU (HD Radeon 5870 1GB GDDR5 SDRAM) is, for some reason, only using 753MB of the 1GB of RAM it has, I figured this out when I went to CanYouRunIt and checked if I could run Skyrim, I passed everything apart from the GPU RAM, which I found strange, because I know it has 1GB of RAM, so I checked dxdiag and it confirmed it's only using 753MB of the RAM.

I was wondering, if there is a reason for this, is this suppose to happen? It says 1GB but in reality it's a little bit less than that?

And if this isn't suppose to happen, am I able to fix it somehow?

Also, I have 8GB or DDR3 RAM, will that make up for the 250MB odd of missing Video RAM?

And, if there is a problem with it, should I put effort into fixing it, or is there really no need and it won't affect performance?

Sorry for all the questions, and thank you for any help, I'll post the rest of my Specs, I have no idea if they will affect the GPU, so I'm posting them just in case.

Processor - AMD Phenom II X4 970 ~3.5GHz
Hard Drive - 500GB Western Digital, 2TB Hitachi
Video Card - ATI Radeon HD5870
Sound Card - Sound Blaster X-Fi
Operating System - Windows 7 Ultimate 64-bit
Motherboard - Asus Crosshair III Formula AMD 790FX

Again, thank you very much for reading.

 

[Kari]

have you checked it with GPU-Z
http://www.techpowerup.com/gpuz/

 

[The77s]

Memory Size - 1024 MB

So it's showing me that I do have 1GB of RAM in the GPU, but for some reason it only uses 753MB of it.

 

[Kari]

most likely canyourunit has the info messed up somehow.
did you notice there is a sensors tab on gpu-z? there should be a reading for vram usage as well. tick the box for 'continue refreshing when in background' and start the most demanding game you've got with all the settings maxed, see how much vram the game uses. Test all your games if you want.
for ex. gta iv is a real memory hog when maxed, it should use everything the card has to offer, a lot of games won't use 1 gig anyways under any circumstances anyhow...

 

[The77s]

Okay, I put GTAIV on Max, everything VHigh and max View Distance and stuff (And, something I never knew I was capable of happened, my PC ran it at 60FPS with all the graphics full, Night Shadows on and everything, it lagged on certain bits, but it seemed fine, I always just used the Auto Configure) and I played it for about 10 minutes then it crashed, so I thought I'd check how much it was, and Memory Usage (Dedicated) was around 833MB max and slowly falling cause I had quit, so I think that my GPU does use all the Memory, because it was using 833 there, not 753.

Would I be right in thinking that?

 

[Kari]

I'd think so, yep

 

[The77s]

Okay, thanks for all the help Kari, it is just dxdiag and CanYouRunIt acting weird.