Ask Dan: What's with the 3Gb memory barrier?

[GomJabbar]

I saw the following link posted on another forum yesterday. It is an interesting read that is not overly technical.
[I apologize in advance to those that find this "old news"]

Basically it explains why 32-bit operating systems (and 32-bit CPU's) cannot access much more than 3Gb of RAM (and sometimes even less). Additionally, it might make you reconsider that expensive pair of video cards you were about to buy (or make you regret that you bought them).

Ask Dan: What's with the 3Gb memory barrier?

The following excerpt gave me a laugh.

(This fact has apparently not stopped certain unscrupulous companies, coughDellcough, from allowing people to buy a computer with WinXP, 4Gb of RAM, and a pair of Nvidia's oddball 1Gb GeForce 7950 GX2 cards. Result: 56.25% of the installed memory absent without leave. You might as well have only bought 2Gb.)

[Turbo Audi]

How about Physical (Virtual) Address Extension?

[jamerslong]

good read, but one question ( i must be crazy cause i dont get it)

Lets say I install 4GB of RAM and have a 512MB Video card. would the memory for the video card be reserved in the upper memory my system cannot see? or.. would it kill a chunk of my visible memory?

[GomJabbar]

I am no expert at this, but the way I understand it, if you are using a 32-bit OS like Windows XP, that 512GB video card will use the memory area (addresses) between 3 and 4 GB. XP will have available about 3GB of system RAM. That extra GB will be useless. Only if you use video card(s) that total over 1GB of memory will you begin reducing the available system RAM to below 3GB.

If you have a 64 bit processor and 64 bit OS, then you can install and use over 4GB and use it, but you will still probably have a memory hole between the 3 and 4 GB address space. So with only 4GB of RAM installed, you still will not see only about around 3GB of system RAM available. If you install 8GB, you would likely see about 7GB of RAM available. It is noted in the article that some modern motherboards can remap that memory hole to make more of your installed RAM available to the OS.

[jamerslong]

ok, heres what is going through my head....

if I only have 3 gigs of ram installed then it still takes that 512 MB ram anyway right? the only difference is my operating system sees it. so couldnt I free up 512MB ram for system use by adding the fourth gig?

or is it saying that the system assigns 512MB of my "4th" GB to my video card because it doesent know what else to do with it and windows cant have it.

[GomJabbar]

I believe it is the memory address between 3 and 4 GB that the video card is assigned to, so most (if not all) of your 3GB of system RAM should be available. The video card in this example does have it's own 512MB of RAM, but the system has to assign it an address below the 4GB limit and hopefully above the addresses used by the system RAM.