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I was using MacOSX 10.5 on my MacBook until today and I had 4GB of memory there.

Now I have updated to MacOSX 10.6 and it only displays 3GB.

Why is that? How can I fix it?

Also, I am a bit wondering why most people (well, most of the Google hits explained the 3GB issue that way -- leaving out the fact that it has worked earlier) are saying that a 32bit system can under no circumstances access more than 3.2GB. Don't we have PAE nowadays in most systems?

Thanks, Albert

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What does system profiler say is installed in each memory slot? –  Troggy May 12 '10 at 22:12
    
Each 2 GB. Also "About this Mac" says there is 4GB. Just the Taskmanager (and also console tools like top) say there is only 3GB available. This was different in MacOSX 10.5 where they all showed up 4GB. –  Albert May 12 '10 at 22:26
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3 Answers

I don't pretend to be an expert in this, but I think it's a more accurate relection of the usable RAM in 10.6, possibly as a result of RAM set aside for the video card?

This forum seems to address some users with your exact situation:

http://forums.macrumors.com/showthread.php?t=770235

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Hm I don't understand a few things. What is wired memory? What is the difference between wired mem and reserved mem? And what exactly is different in 10.6 than in 10.5? Where exactly did that 1GB go? –  Albert May 13 '10 at 15:18
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up vote 1 down vote accepted

We just tested it again in MacOSX 10.5. The Activity Monitor shows up 4GB. However, free + reserved + active + inactive mem sums up to 3GB.

We also tried to allocate as much memory as possible. The maximum amount of used memory was 3GB. Interestingly, in the graphic stats diagram (saying "4GB") -- the green part saying how much free memory there is -- disappeared once we had used 3GB.

So it kind of seems that it is a bug in the diagram and it should have said "3GB" instead of "4GB". It probably doesn't calculate how much memory there really is in sum but just takes the value out of the system profile.

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I don't pretend to be an expert in this, but I think it's a more accurate relection of the usable RAM in 10.6, possibly as a result of RAM set aside for the video card?

For one thing, the 945 and older chipset families cannot map anything over the 4GB line, as it only have 32 address lines.

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