3.75 GB of 4 GB @ MPB w/ discrete GPU

If you switch to better performance you must restart the computer to recover the shared memory used by the other GPU.

Do not reset the PRAM. Just switch to performance mode in Energy Saver then restart the computer.

Then take the computer in for service as something isn't working. Or you may try one of the following:

1. Download and reinstall the Mac OS X 10.6.8 Update Combo to see if that fixes the issue.
2. Reinstall Snow Leopard.

Reinstall OS X without erasing the drive

Do the following:

1. Repair the Hard Drive and Permissions

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

2. Reinstall Snow Leopard

If the drive is OK then quit DU and return to the installer. Proceed with reinstalling OS X. Note that the Snow Leopard installer will not erase your drive or disturb your files. After installing a fresh copy of OS X the installer will move your Home folder, third-party applications, support items, and network preferences into the newly installed system.

Download and install the Combo Updater for the version you prefer from support.apple.com/downloads/.

system monitor should show _INSTALLED_ memory, not free for system. Thus doesn't matter which of adapters work. Take a closer look - what it shows in memory about memory banks, try to run hardware test. Looks like your memory is partially broken

About OS X Memory Management and Usage

Reading system memory usage in Activity Monitor

Memory Management in Mac OS X

Performance Guidelines- Memory Management in Mac OS X

A detailed look at memory usage in OS X

Understanding top output in the Terminal

The amount of available RAM for applications is the sum of Free RAM and Inactive RAM. This will change as applications are opened and closed or change from active to inactive status. The Swap figure represents an estimate of the total amount of swap space required for VM if used, but does not necessarily indicate the actual size of the existing swap file. If you are really in need of more RAM that would be indicated by how frequently the system uses VM. If you open the Terminal and run the top command at the prompt you will find information reported on Pageins () and Pageouts (). Pageouts () is the important figure. If the value in the parentheses is 0 (zero) then OS X is not making instantaneous use of VM which means you have adequate physical RAM for the system with the applications you have loaded. If the figure in parentheses is running positive and your hard drive is constantly being used (thrashing) then you need more physical RAM.

Adding RAM only makes it possible to run more programs concurrently. It doesn't speed up the computer nor make games run faster. What it can do is prevent the system from having to use disk-based VM when it runs out of RAM because you are trying to run too many applications concurrently or using applications that are extremely RAM dependent. It will improve the performance of applications that run mostly in RAM or when loading programs.

The 256MB of RAM that is "shared" with the integrated GPU in your MBP model (like mine) is not really shared — it's given, and it is never released. Activity Monitor will always show 3.75GB of RAM if you have 4GB installed, no matter which GPU is in use. This has always been true of the MBPs that have 9400M and 9600M GT GPUs. If you need more than 3.75GB of RAM, add more, but don't ever expect to be able to use the 256MB that are allocated to the integrated GPU.

This is correct, I believe, if you have the model that can switch graphics GPU on the fly. My MBPs do not and if you switch to performance mode and reboot, then the shared RAM is recovered.

My late-2008 MBP (the first unibody one, and the first with the dual GPUs — not switchable on the fly) has never released those 256GB, and there were threads in these forums for a while after the early unibodies came out addressing precisely this issue. The answer, then as now, is that the "shared" RAM is never available for any other use, and Activity Monitor never shows it even though System Profiler does.

I've just confirmed the above on my machine by switching to the discrete GPU (doing the required logout-login routine) and then restarting the computer. Activity monitor still shows 3.75GB.

I participated in several of those threads. I have two 2009 MBPs - 15" and 17" - that have the 94/9600M GPUs manually switched via Energy Saver. I have experimented with both of them back when those discussions were taking place. The shared memory was indeed recovered after switching to the 9600 and rebooting (relogging does not do it, however.)

I tested this at the specific request of another purchaser of the 15" MBP. Now back then these computers came with Leopard. I have since upgraded both to Snow Leopard, but not retested the GPU switch so I do not know if this behavior changed with the introduction of Snow Leopard. Since I don't have the 15" model here I can only retry this with the 17" model. I'll report what I find.

Oh, the switch only works with the AC adaptor. If on battery power the computer sticks with the 9400 integrated GPU.