In one embodiment of the invention, a method is provided to allow an operating system to support both address space layouts of a SAS OS and a MAS OS at the same time, with the choice of which layout type to be used to be made by the application developer. In one embodiment the method includes: selecting one of a mostly private address space (MPAS) model and a mostly global address space (MGAS) model, where if the MPAS model is selected, then a process is permitted to map a shared object in a mostly private address space (MPAS) layout so that the process perceives a behavior as if the process is running on a multiple address space operating system, and where if the MGAS model is selected, then the process is permitted to map a shared object in a mostly global address space (MGAS) layout so that the process perceives a behavior as if the process is running on a single address space operating system.