Single Address Space Operating Systems (SASOSs) are OSs in which all code and data exists within one, big, shared address space, often threaded. Using one address space facilitates and enhances sharing and cooperation, because it lets addresses have a unique (for all time) representation or interpretation. Thus, pointer-based data structures can be directly communicated and shared between programs at any time, and can be stored directly on secondary storage with no need to translate. Such structures are simplified by using larger address spaces.
All earlier OSs were SASOSs, because computers only used single address spaces; that was all that existed, there was nothing else. Segmented/paged address spaces were a later development. Most present, common personal computers use the x86 architecture, which began as the segmented 8088/8086; though i386 and later IA32 processors have a single address space (flat) memory mode. Future, larger processors may return more to single address spaces as the high-end of the computer industry moves to 64-bit CPUs. Or, OSs may keep using processes, which use a different address space for each process.
Where SASOSs run as distributed OSs, SASOSs treat a network of nodes as one shared memory machine, using distributed virtual shared memory: DVSM.
Some SASOS benefits:
1) Can be made as secure as traditional systems.
2) Are not inherently less efficient than traditional systems.
3) Improve performance over traditional systems on some types of important applications.
4) Give a dual cost advantage: lower initial cost, lower incremental cost.