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Stephen Johnson Level 1 Level 1

I just upgraded to 10.6.3 and then 10.6.8 on my Mac Mini. With 10.6.3 Activity Monitor started showing total of 3 GB of RAM instead of 4 GB as System Profile does show. I ran Rember and all tests passed. It showed 4 GB total memory and 1690 MB available memory which seems consistent with AM's Free memory.

Any ideas why the system seems to "see" (System Profiler) all the memory but not be using it (AM)?

 

Stephen


Mac mini, Mac OS X (10.6.8), 1.83Ghz, 4 GB RAM
Solved by BobHarris on Dec 2, 2012 6:50 AM Solved

Most computers today use the address space to talk to the outside world (RAM, Disks, Networking, Displays, USB devices, Firewire devices, etc...).

 

The computer system designers generally have an chip on the motherboard that takes the address lines from the CPU chip and interfaces them with all the things the computer must talk to.  On the Mac mini this is called the "North Bridge" and "South Bridge" chip sets.  There is even a Wikipedia pages describing the North Bridge and South Bridge chip sets

<http://en.wikipedia.org/wiki/Northbridge_(computing)>

<http://en.wikipedia.org/wiki/Southbridge_(computing)>

NOTE:  The Wikipedia articles are Windows PC specific, but the generic concepts are the same for a Mac mini.

 

The North and South Bridge chips used on the your's and my Mac minis can only handle 32 address lines, and 32-address lines has a maximum address range of 0 to 4,294,967,295 (0x0 to 0xFFFFFFFF or 4 billion in computer speak).

 

The Mac mini North Bridge Chip sends most of those addresses to the RAM chips you have installed (3,221,225,472 or 3 billion aka 3 gigabytes in computer speak).

 

The remaining 1 billion (1,073,741,824) addresses are used by the North and South Bridge chip sets to talk with I/O devices.

 

These remaining 1 billion addresses are mapped to control registers and device buffers in the various Graphic chips, USB controllers, Firewire controllers, and misc support chips used by the Mac mini (PRAM, SMC power controller, Time-of-Day clock (the wrist watch that keeps time when the computer is powered off, etc...)).

 

NOTE:  The RAM chips NEVER see any signals from these last billion addresses.  For example, you decide you need more seating  in your home, so you buy 2 double length living room sofas (two 2GB DIMMs), however, your living room will only hold 1 and a half of your sofas (3GB), and the other half of a sofa is sticking out the  door into your garage, and you cannot sit on that part because it is not properly supported underneath, and besides you use your garage for other things, such as cars, lawnmowers, snowblowers, garden tools, etc... (poor analogy, but hopefully you know what I mean).  That last 1GB of RAM is just hanging around doing nothing, and cannot be used.

My point/question is that, if that "last 1GB of address space" were not there then the system would use memory out of the 3GB that is available for general use?

The CPU needs to steal address space to talk with its I/O devices and support chips.  It does not need your RAM, just the addresses.  If it only had 3 billion addresses, the computer system designers would use some of those addresses to talk to its I/O devices and you would not be able to address all of that 3GB of RAM.

 

Way back when, the early 8080 chips could only address 1 million addresses.  The computer system designers gave 640 kilobytes to RAM and the remaining addresses to I/O devices.  And when they first came out, we couldn't afford to put that much RAM into those systems :-)

Reply by Jeffrey Jones2 on Sep 14, 2012 9:00 AM Helpful

Check out the VM size, too. 144 GB. My harddrive is not big enough for that much VM!

VM size has nothing to do with your disk size. Don't confuse it with swap space -- it's only using 57 MB of swap space. VM size is pretty much an abstract concept that you should ignore. 144 GB is unremarkable.

Reply by John Galt on Dec 1, 2012 8:01 PM Helpful

As far as your Mac is concerned, it only has 3 GB RAM. The remainder cannot be addressed by the hardware and may as well not exist, since the addresses your Mini would use to address it are reserved for other hardware.

 

2007 was not that long ago but in those days memory was simply not available in configurations that would result in 4 GB of addressible RAM. When your Mini was engineered, only 1 GB modules were available, hence its specified maximum of 2 GB.

All replies

  • Stephen Johnson Level 1 Level 1

    Here is a screen shot of AM and System Profiler:

    memory.jpg

    Check out the VM size, too. 144 GB. My harddrive is not big enough for that much VM!

  • BobHarris Level 6 Level 6

    Chances are your Mac mini can ONLY use 3 of the 4GB you have installed.  My 1.83GHz Core 2 Duo Mac mini has two 2GB DIMMs installed, HOWEVER, I can ONLY use 3GB of that RAM.

     

    My guess is System Profiler is telling you the physical RAM installed, but Activity Monitor is telling you want the operating system is able to see.

     

    This is ONLY a guess, as maybe your Mac mini is different from mine.

  • WZZZ Level 6 Level 6
    expertise.macosx
    Mac OS X

    I have a related question about the pie chart.

     

    Here's mine:

     

    Screen shot 2012-09-14 at 11.10.07 AM.png

     

    I have 12GB installed. Free at 9.72 + Used at  2.28 adds up to 12GB.

     

    But adding up Wired at 957.4MB, Active at 754.1MB, and Inactive at 632.4MB comes to 2.3GB (rounded). Adding that to the pie chart figure of 11.75 gives a total of 14.05GB, which isn't possible. What does the pie chart figure at 11.75GB represent? It can't be the 12GB minus the Wired, Active and Inactive either, since that would be much less than 11.75. And if it's supposed to be the total of installed Memory, why the missing .25GB?

  • Jeffrey Jones2 Level 6 Level 6

    Check out the VM size, too. 144 GB. My harddrive is not big enough for that much VM!

    VM size has nothing to do with your disk size. Don't confuse it with swap space -- it's only using 57 MB of swap space. VM size is pretty much an abstract concept that you should ignore. 144 GB is unremarkable.

  • Stephen Johnson Level 1 Level 1

    BobHarris wrote:

     

    Chances are your Mac mini can ONLY use 3 of the 4GB you have installed.  My 1.83GHz Core 2 Duo Mac mini has two 2GB DIMMs installed, HOWEVER, I can ONLY use 3GB of that RAM.

     

    My guess is System Profiler is telling you the physical RAM installed, but Activity Monitor is telling you want the operating system is able to see.

     

    This is ONLY a guess, as maybe your Mac mini is different from mine.

     

    But when I boot back into 10.5.8, all 4GB are there:

     

    AM10.5.8.jpg

     

    Something changed in 10.6. Maybe I need to do a fresh install or go on and upgrade to 10.7.

  • baltwo Level 9 Level 9

    WZZZ wrote:

    I have a related question about the pie chart.

     

    I have 12GB installed. Free at 9.72 + Used at  2.28 adds up to 12GB.

     

    But adding up Wired at 957.4MB, Active at 754.1MB, and Inactive at 632.4MB comes to 2.3GB (rounded). Adding that to the pie chart figure of 11.75 gives a total of 14.05GB, which isn't possible. What does the pie chart figure at 11.75GB represent? It can't be the 12GB minus the Wired, Active and Inactive either, since that would be much less than 11.75. And if it's supposed to be the total of installed Memory, why the missing .25GB?

    Wired+Active+Inactive = Used. Thus, 9.72+2.28 ≃ 9.7.2+2.3 ≃ 12. The 11.75 is a mucked up AM reading. Check the memory via About this Mac->More info. Mine shows exactly 12 GB.

     

    Screen shot 2012-09-15 at 2.09.29 PM.png

  • WZZZ Level 6 Level 6
    expertise.macosx
    Mac OS X

    System Profiler sees all 12GB. I was concerned I might be in the same boat as the OP, losing some off the top. Either that or AM is messed up. It's always showing 11.75, even on a restart. But, interestingly, Free and Used always adds up to 12.

     

    I can see what the breakdown is later using top, and if it agrees with AM. I might trust top to be more accurate.

  • baltwo Level 9 Level 9

    top shows:

     

    PhysMem: 696M wired, 1871M active, 1309M inactive, 3876M used, 8405M free.

     

    I didn't add them up, being too lazy and since AM's always been accurate.

  • WZZZ Level 6 Level 6
    expertise.macosx
    Mac OS X

    I'm supposing if Free and Used always adds up to 12, I can disregard the pie chart.

  • baltwo Level 9 Level 9

    That's what I'd do. However, you could nuke the activity monitor plist file and relaunch it.

  • WZZZ Level 6 Level 6
    expertise.macosx
    Mac OS X

    AM plist nuked, no difference. I have now filed this under useless dead-end investigations of glitches and anomalies.

  • baltwo Level 9 Level 9

    thumbsup.gif

  • Stephen Johnson Level 1 Level 1

    fresh install of 10.6.3 has the same behavior, upgrade to 10.7.4 still inaccurate; different than WZZZ's case my 1GB is missing in both the pie chart and the Free + Used and does not seem to be available for use by the system, though System Profiler continues to show all 4 GB.

    I use Virtual PC and it has a slider bar to set the RAM for the PC. It shows a little more than 3.5 GB available though above 3 GB it is highlighted in yellow, as if caution about trying to use it. So I set it to a little more than 3GB, started V PC and watched its usage climb in Activity Monitor. When it reached about 3GB the whole Mac OSX locked.

    Here is a related issue and the poster's solution was to switch the riser cards around:

    https://discussions.apple.com/message/11022205?tstart=0#11022205

    Maybe I need to swap my memory cards around.

  • Stephen Johnson Level 1 Level 1

    Well, I finally decided to go on and search for the specs on my Mini, even though all 4 GB work under 10.5.8. Here it is:

    http://support.apple.com/kb/SP7

     

    "1GB of 667MHz DDR2 SDRAM (PC2-5300) on two SO-DIMMs; supports up to 2GB"

     

    I bought it in 10/2007. Even for that long ago, it seems like 2GB would have been quite limiting. I upgraded to 4GB soon after I got it. So, I suppose that I get anything above 2GB is a bonus.

    Seems strange and sad to me but I suppose it is true.

     

    Still will try swapping the cards around... maybe I can get the newer OS and all 4GB back.

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