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Question about maximum memory restrictions on motherboard and accessible memory amounts

I have a system with an Intel motherboard - D865GBF.  It is a Pentium 4 based board that I've been using for awhile (at least over a year, maybe longer).  I have 1GB of RAM in it currently (2 x 512).  I would like to add more memory to it.  I am going to remove the memory that is in the system now and use it in another system of mine.  I would like to add, at minimum, 2GB (2 x 1GB) (DDR400).  I was considering, though, adding the maximum for this board which is 4GB total (there are 4 slots, so 1GB per slot).  What has confused me, is in the manual for my board is the following paragraph:

"NOTE
System resources (such as PCI and AGP) require physical memory address locations that reduce
available memory addresses above 3 GB. This may result in less than 4 GB of memory being
available to the operating system and applications."

I just don't get it.  Does this mean that if I added 4GB of ram, that I could never really use more than 3GB?  I'm confused.  Why would they say the board supports up to 4GB, if you really can only use 3GB?  It doesn't make sense, so I think I'm not understanding this quite.  I'm hoping someone here understands this better than I and can explain what exactly this means.

Thank you so much for taking the time to help.
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sah18
Asked:
sah18
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1 Solution
 
CallandorCommented:
This is a well-known problem with 32-bit OSes: you cannot address more than 4GB with 32-bit registers.  Now part of that 4GB includes ports and memory addresses of devices like IDE controllers and video controllers so their addresses are not available for you to use.  These devices usually reside in high memory in the addresses above 3GB, so this is why installing 4GB only gives you 3GB usable.  You can get around it by running a 64-bit OS, and there are server motherboards that let you install more than 4GB RAM.  Some desktop motherboards let you see 3GB; others limit you to 2GB unless you use the /3GB and /PAE switches in the boot.ini file: http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx
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sah18Author Commented:
Thank you for your explanation.  This is starting to make more sense to me now.  I just have a couple more questions/clarifications on this topic.

1. So, given my current os (Windows XP Professional) and motherboard, really the most (usable) memory I could put in this system would be 3GB total (2 x 1GB and 2 x 512MB), correct?  Adding an extra 1GB would be totally a waste of money, as it would not be accessible anyway, is this right?

2. If I kept all my current hardware, and switched to using a 64-bit os, would I then be able to install the full 4GB of ram, or is this still limited by my motherboard in some way (motherboard max says it will accept 4gb max)?  

3. What 64-bit oses are there?

4. To use a 64-bit os, do I really need to also have a different motherboard, processory, etc?  Or does it not matter?

Thank you again!!

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CallandorCommented:
1. Correct.  If you plan to run WinXP64 or Vista in the future, though, you may want the extra memory.

2. Yes, you can access the full 4GB with a 64-bit OS.  The device addresses will be relocated to a different space.

3. WinXP64, Linux, Vista (in the future), server-class OS'es.

4. As long as there are 64-bit drivers provided for your motherboard, you can use them.
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sah18Author Commented:
Outstanding!  You've been extremely helpful!!
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