Abstract: A virtual memory system is disclosed, providing a virtual address space of virtual addresses simultaneously available to a given process. The virtual address space includes a process private space accessible only to a given process, as well as a shared space of virtual addresses potentially accessible to two or more processes. The process private space includes virtual addresses on one side of a private/shared virtual address boundary, while the virtual addresses within the shared space are on the other side of the private/shared virtual address boundary. The private/shared boundary is ensured to be greater than a predetermined highest process private (or shared) virtual address of a different virtual memory system, and less than or equal to a predetermined lowest shared (or process private) virtual address of that different virtual memory system. Compatibility is ensured for software written for the different virtual memory system, allowing such software to run without modification on the present system.