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Unibody MacBook: real available ram

B

beneventi

macrumors member
Original poster
Jun 6, 2006
42
0
Italy
Dear forum users, I've just purchased a new unibody MacBook with 2GB of ram to replace my black MacBook (with 2GB of ram too).

I have Bootcamp installed on both of them and I've noticed in Windows XP that the real ram in the new unibody MacBook is less than the old one. To be precise: the old one gives me 1,96 GB of ram, the new unibody gives me 1,72. Is this a problem?

Where are those 256 MB of Ram? Are they for the NVIDIA Chipset? I thought that the new graphics card had its own ram.

Best regards

Alessandro
 
beneventi said:
Dear forum users, I've just purchased a new unibody MacBook with 2GB of ram to replace my black MacBook (with 2GB of ram too).

I have Bootcamp installed on both of them and I've noticed in Windows XP that the real ram in the new unibody MacBook is less than the old one. To be precise: the old one gives me 1,96 GB of ram, the new unibody gives me 1,72. Is this a problem?

Where are those 256 MB of Ram? Are they for the NVIDIA Chipset? I thought that the new graphics card had its own ram.

Best regards

Alessandro
Click to expand...

The 256MB is shared, not dedicated. The 9400M is still an integrated card, a better one, but still integrated.
 
I believe XP assigns 512mb to the NVidia chip. Windows 32-bit can only address 4GB of total memory space, so its probably seeing around 3.25gb of your system RAM and then taking 512mb of that for the video card. Vista x64 will see all 4GB and dedicate 256mb of that to the video chip, like OSX.
 
Eric S. said:
I have 4 GB in my aluminum MB and Windows XP says 2.72 GB. Will XP not use anything above 3 GB?
Click to expand...
32 bit is technically supports 4 but i have never seen it register more than 3 gig. vista sp1 will show 4 but it wont use more than 3.
 
32Bit can use 4GB, however, remember the overhead (All the controllers that also require their own memory space). The overhead takes up ~1GB of space. Since 32bit cannot see above 4GB. 4GB - 1GB = 3GB of available memory space for the actual "Ram". Then minus 256MB for the 9400M video card and you get ~2.75GB of usable ram in Windows XP or any 32-bit OS.
 
ayeying said:
32Bit can use 4GB, however, remember the overhead (All the controllers that also require their own memory space). The overhead takes up ~1GB of space. Since 32bit cannot see above 4GB. 4GB - 1GB = 3GB of available memory space for the actual "Ram". Then minus 256MB for the 9400M video card and you get ~2.75GB of usable ram in Windows XP or any 32-bit OS.
Click to expand...

But then I wonder why the OP said he has 2 GB and the system reports 1.72, but I have 4 and the system reports 2.72. It almost seems as though it's starting from 3, not 4.
 
Eric S. said:
But then I wonder why the OP said he has 2 GB and the system reports 1.72, but I have 4 and the system reports 2.72. It almost seems as though it's starting from 3, not 4.
Click to expand...

He has 2GB. 2GB - 256MB (video card) = ~1.75GB.

You have 4GB. 4GB - 1GB (overhead) = 3GB - 256MB (video card) = ~2.75GB
 
Eric S. said:
I don't get it. Why would there be overhead on 4GB and not on 2GB?
Click to expand...

One possible explanation is that Window XP only recognize as much as 3GB.

Try taking off one of the 2GB modules and put the original 1GB stock RAM back, run XP and see if it still only recognize 3GB. If it does, then the max RAM recognized for your XP is only 3GB.
 
Eric S. said:
I don't get it. Why would there be overhead on 4GB and not on 2GB?
Click to expand...

Okay. For 32bit, the max it can see is 4GB. That includes both overhead/available ram, everything.

Now, the overhead takes up ~1GB. Since 32bit is max of 4GB, the overhead moves into that space and steals 1GB. Therefore, you're left with 3GB possible of being accessed for the physical ram.

The entire 32bit coding only allows 4GB.

This is the maximum 32-bit can occupy:

| ---- | ---- | ---- | ---- | = 4GB

Now, if we include in the overhead which is required for the stuff such as USB controllers, input/output and such, etc.

| OVHD | ---- | ---- | ---- | = 4GB

Here, 1GB is taken for overhead. 3GB is left for everything else.

When you have 2GB of physical ram installed, it is (Physical ram = PHRA):

| OVHD | PHRA | PHRA | ---- | = 4GB

Here, you see that we haven't reached the 4GB limit. Thats why all of the 2GB of physical ram can be used.

Now, if we have 4GB installed:

| OVHD | PHRA | PHRA | PHRA | = 4GB

Uh oh, we only got 3GB of physical ram to fit in the room. What happened to the extra 1GB? It is wasted and not used at all. There's not enough room for the extra "1GB" to fill anywhere since overhead is required and cannot be avoided.

Note: this is a crude explanation but simple explanation.
 
xkRoWx said:
One possible explanation is that Window XP only recognize as much as 3GB.
Click to expand...

That's what I've been saying. :)

Try taking off one of the 2GB modules and put the original 1GB stock RAM back, run XP and see if it still only recognize 3GB. If it does, then the max RAM recognized for your XP is only 3GB.
Click to expand...

Well I'm curious about it but not enough to perform surgery on the MB. Anyone it doesn't have an "original" 1GB RAM; it came this way.

ayeying said:
this is a crude explanation but simple explanation.
Click to expand...

OK, I see what you're saying now. The bottom line is, as I was asking, XP can only use 3GB.
 
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