Single address space operating system

In computer science, a single address space operating system (or SASOS) is an operating system that provides only one globally shared address space for all processes. In a single address space operating system, numerically identical (virtual memory) logical addresses in different processes all refer to exactly the same byte of data.¹

In a traditional OS with private per-process address space, memory protection is based on address space boundaries ("address space isolation"). Single address-space operating systems make translation and protection orthogonal, which in no way weakens protection.²³ The core advantage is that pointers (i.e. memory references) have global validity, meaning their meaning is independent of the process using it. This allows sharing pointer-connected data structures across processes, and making them persistent, i.e. storing them on backup store.

Some processor architectures have direct support for protection independent of translation. On such architectures, a SASOS may be able to perform context switches faster than a traditional OS. Such architectures include Itanium, and Version 5 of the Arm architecture, as well as capability architectures such as CHERI.⁴

A SASOS should not be confused with a flat memory model, which provides no address translation and generally no memory protection. In contrast, a SASOS makes protection orthogonal to translation: it may be possible to name a data item (i.e. know its virtual address) while not being able to access it.

SASOS projects using hardware-based protection include the following:

• Angel
• IBM i (formerly called OS/400)
• Iguana at NICTA, Australia
• Mungi at NICTA, Australia
• Nemesis
• Opal
• Scout
• Sombrero

Related are OSes that provide protection through language-level type safety:

• Br1X
• Genera
• JX a research Java OS⁵
• Phantom OS
• Singularity
• Theseus OS⁶
• Torsion⁷