Photoshop and memory allocation??

[Macinposh]

So, just realized such a thing,that when using photoshop (cs2) with my Macpro, the maximum amount of memory that it "uses" is around 1400MB.


I have 2GB in total, but the ratio 1400MB used, 600MB free is almost constant. Even if i have 1 or 15 72MB pictures open, if i run lightroom in the background etc...
It never seem to "max out", or move beyond that,even if my "memory usage" percentige is set to 90%.




Hmm, so,whats this all about?
Does the system keep that 600MB in back up or what?
What is the reason for this behiavour?

Just curious, not panicing...