Windows XP Professional, like all 32-bit versions of Windows, is limited to a total of 4 GB of virtual memory address space. This 4 GB is divided into 2 GB for user processes and 2 GB shared by all processes for the operating system. In special circumstances, the user virtual address space can be expanded to 3 GB by forcing all the system processes to fit within 1 GB using the /3GB boot time switch, but this only affects programs that are specifically compiled to take advantage of the additional address space. And all programs will be constrained by the limited memory available to the operating system.