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RAM not being detected correctly?

M

MoBsTa

Limp Gawd
Joined
Jan 22, 2005
Messages
241
SO I bought two sticks of 2GB RAM (4GB total) to place in my computer. I read the motherboard manual correctly and everything and the system starts up and runs, but it tells me I only have 3GB of RAM in the computer. How can this be?

Is there any good way to check how much RAM the system detects? I'm using crappy methods
 
Yea it's definitely only detecting 3GB of RAM when I know I have 4 gigs in there.
 
What operating system are you running? A 32-bit Windows client is going to be constrained when it comes to addressing 4GB of memory.
 
Oh wow I never knew that about XP. So will the video games still recognize 4 gigs or will it stay at 3 gigs until I get WIndows 7
 
Nope, if the operating system cannot utilize the memory, the games certainly won't be able to. The only way to be able to address the entire 4GB or greater is to use a 64-bit OS. The client Windows versions supporting 64-bit operation are XP, Vista, and 7.

With a 32-bit addressing mechanism in place, the maximum number of unique addresses that exist is 2^32. Since the machine is byte-addressable, this translates to a maximum addressable memory of 2^32 bytes, or 4 gigabytes. Of the 4 gigabytes, there's a number of devices (video cards, especially!) which are best used with forms of I/O known as memory-mapped I/O or direct memory access (DMA). In this situation, the range of memory on a device is mapped into main memory and changes to the device are made by writing to this memory. Programmers don't have direct access, but the OS does, but that's another issue.

In a very simple case, let's say that a system has 4GB of memory and is running a 32-bit OS, which, as before, has a 4GB limit. If you have a video card with 256MB of memory, after memory mapping, the most RAM your OS can use is limited to ~3.75GB. If your video card has a larger buffer, such as 512MB or 1024MB, the maximum addressable memory is reduced to 3.5GB or 3.0GB, respectively.

In a 64-bit environment, the game changes and the maximum theoretical memory that can be addressed is 2^64 bytes. This won't really be the case anytime soon, as the most recent memory controllers are 40-bit, physically limiting the maximum to 2^40 bytes.

Understanding Address Spaces and the 4GB Limit is a good read, I definitely recommend it.
 
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