will all draw their RAM out of the Windows pool, rather than the larger pool available to the Mac. Depending on whether the Windows installation is 32-bit or 64-bit, you may be limited in the amount of RAM you can allocate to Windows. The first thing you should do is check the Parallels preferences for your Windows environment, and see how much RAM you've allocated to it. If you've authorized Windows Whatever to use 4GB RAM (for example), that 4GB is not available to the OS X environment until you shut down the Windows environment.
The memory space for each virtualized OS is set in the Parallels preferences on an OS-by-OS basis. This virtualized environment also commands its own memory space, because the virtualized OS doesn't know it's not the boot OS. Parallels, like other virtualization middleware applications, runs an entirely different operating system with its own native applications.
