I have a 945GCM S2L Motherboard which has 4GB memory (2x2GB) running windows vista x64.

The RAM is getting reported as 3GB in the BIOS, Total Physical Memory also shows 3GB, Task manager also shows 3GB. Am I missing anything ?

EDIT: Is there any way of updating BIOS from a x64 Vista installation, 16bit applications are not supported by x64 & my manufacturer does not have a 64 bit install

8 Answers 8


Your motherboard does support 4gb of memory, with some caveats. Your onboard video will reserve some of that memory you have on there for its use, and you will also have some of the memory being used for system maintenance and such. After all this is said and done, you will only have 3 gb available for the os and any apps to use. This is the bad part about onboard video... shared memory allocation.

  • 2
    I didn't even notice the on-board video on the spec. Now I am a sad panda.
    – TheTXI
    Jul 30, 2009 at 18:40
  • It seems pretty excessive to allocate an entire gig of RAM for video memory on an onboard video card, when the GeForce GTX 275 has 896 megs. 256 Megs would seem more reasonable. Besides, the BIOS should still show that there are 4 gigs of total physical memory, even if some of that RAM is used for video. Jul 30, 2009 at 18:47
  • Some physical address space might also be reserved for memory-mapped IO. This needs to be below 4G to be usable on a 32-bit OS, and if the motherboard doesn't support remapping the physical memory in the affected range above 4G, you'll lose some physical memory this way.
    – bdonlan
    Jul 30, 2009 at 19:03

According to the product specification sheet here the board only supports 4GB of RAM. However the following is also listed

* Due to standard PC architecture, a certain amount of memory is reserved 
for system usage and therefore the actual memory size is less 
than the stated amount.

Maybe try and update the BIOS since sometimes this can help.

Edit: As already mentioned some of the memory can be reserved for the Video Card.

  • How to update BIOS ?
    – Maveňツ
    Oct 21, 2018 at 21:51

Maybe there is a shared memory used by graphics card? Some cards use main memory from computer in order to display contents of the screen.

Look for shared memory (or video memory) and make the value lower.


Does the mainboard of your computer even support such an amount of memory? Many mainboards still only support less than 4GB of memory (because paging is a problem with such a great amount of pages, afair).

  • I posted a link in my answer to the specs on that motherboard. It looks like it should allow for 4GB.
    – TheTXI
    Jul 30, 2009 at 18:37
  • No, paging is not a problem with a great amount of pages of RAM. If you have more RAM you'll page less, not more. Jul 28, 2017 at 14:05

According to this

Your motherboard should be able to support up to 4GB of memory. If your BIOS is not recognizing it (and then it's not getting recognized by Vista) then you may need to look into making sure your BIOS is updated and on the up and up. I wouldn't imagine that a RAM stick would be bad because if it was I would assume it would knock down to 2gb instead of 3gb...

You may need to consult your manufacturer.

Edit Or better yet, consult RascalKing's answer which includes an explanation of the RAM usage by the on-board video.


GigaByte GA-945GCM-S2L

  • 2 x 1.8V DDR2 DIMM sockets supporting up to 4 GB of system memory

Try update BIOS


Some bioses seem to default to have memory mapping (or re-mapping) disabled which means you can't see the full 4Gb. (Some don't even give you an option to enable it). So that would be the first thing to look for. (Of course, every BIOS will probably describe it slightly differently)

Is there any way of updating BIOS from a x64 Vista installation,

If the update utility won't run, you need to find boot into a operating system that will run it. Some manufacturers give a dos or linux install that can run when you boot from a floppy (if you have one) or a cd.

(I keep having to remind myself that not everyone has a Rescue Disk like Bart CD , but that's what I would use).


On a 32 bit XP computer you just cannot use the whole 4GB of physical RAM. It'll always be capped at 3GB.

What happens is that the 32bit processor's use 32 pointers to address the bytes in the RAM. So there are 2 raised to the power 32 (2^32) possible addresses. But, in addition to RAM a good number of these addresses are also used to address other memories like the BIOS, the video memory etc. This uses up one fourth of the 2^32 addresses previously mentioned. So you're left with only 2^30 addresses for the RAM.

These many addresses can address no more than 3GB of RAM (2^30 = 3GB).

This is the reason you're only seeing 3GB of installed physical memory. It has got nothing to do with whether you've a shared video memory or not. This limit will exist in ALL 32bit computers.

In windows XP there is a way to go around this limit. It's called Physical Address Extension (PAE). This can take you up to 4GB (but not beyond). I'm not sure how to enable it. Try googling for it.

  • I'm not sure where you're getting x86 XP from. The OP said Vista x64
    – MDMarra
    Aug 19, 2009 at 3:36
  • 3
    "It'll always be capped at 3GB." - This is simply not true. The limit depends on the hardware configuration and capabilities. I've got 32 bit systems that see 3.25, 3.5, even 3.99 GB of RAM. More technical and in-depth info from a recognized expert: blogs.technet.com/markrussinovich/archive/2008/07/21/… Sep 30, 2009 at 20:01

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .