I increased the amount of RAM on a 1.8 Ghz Powermac (the 1st generation one with a 900 MHz frontside bus and 8 Gb of maximum RAM) from 3 Gb to 5 Gb, and strangely enough, although About This Mac shows 5 Gb, in the Activity Monitor under System Memory, it only shows 1 Gb of System Memory. The strange thing is that before when there was 3 Gb in the system the Mac showed 3 Gb of system memory.
This tech article from Adobe sheds some light on what I’m seeing:
Photoshop CS2 is a 32-bit application, and when you run it on a 32-bit operating system, such as Mac OS 10.2.8, it still can only access 2 GB of RAM. The operating system uses some of this RAM, so the Photoshop Memory Usage preference displays only a maximum of 1.6 or 1.7 GB or total available RAM.
When you run Photoshop CS2 on a 64-bit operating system, such as Mac OS 10.3 and higher, it can access up to 8 GB of RAM. You can see the actual amount of RAM Photoshop can use in the Maxiumum Used By Photoshop number when you set the Maximum Used by Photoshop slider in the Memory & Image Cache preference to 100%. The RAM above the 100% used by Photoshop, which is from approximately 3 GB to 3.7 GB, can be used directly by Photoshop plug-ins (some plug-ins need large chunks of contiguous RAM), filters, actions, etc. If you have more than 4 GB (to 8 GB), the RAM above 4 GB is used by the operating system as a cache for the Photoshop scratch disk data. Data that previously was written directly to the hard disk by Photoshop, is now cached in this high RAM before being written to the hard disk by the operating system. If you are working with files large enough to take advantage of these extra 2 GB of RAM, the RAM cache can speed performance of Photoshop.
Apparently, OS X is now allocating all that extra RAM as a substitute for the hard drive for virtual memory.
Addenda 2/17/05 – After upgrading to OS X 10.4, all 5 Gb of RAM shows up.