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Some Issues for Single Address Space Systems Jeff Chase, Mike Feeley, and Hank Levy

Summary: Some Issues for Single Address Space Systems
Jeff Chase, Mike Feeley, and Hank Levy
Department of Computer Science and Engineering, FR­35
University of Washington
Seattle, WA 98195
At the last WWOS we described Opal, an OS environment that has a single virtual address space
common to all protection domains, rather than the usual private virtual address space per protection
domain (e.g., a Unix process). All threads on an Opal node see the same mapping of virtual
to physical addresses; any thread can reference any virtual address, but access to the data is
determined by the thread's protection domain. The single address space approach is made feasible
by the large virtual address spaces of the newest RISC processors. It allows protection domains to
pass and share memory segments by mutual consent, while preserving the meaning of pointers.
At the time Opal was a paper design, with only a few pieces of implementation. Since that time the
key features of Opal have been prototyped [Chase et al. 93]. While the current prototype runs on
32­bit MIPS systems, it demonstrates that the model is both workable and useful. Yet the single
address space idea remains a difficult adjustment, both conceptually and practically, for those of
us long accustomed to per­process address spaces. Having spent much of the past year arguing the
benefits of the model, our purpose now is to outline some of its limitations and complications. The
goal is to separate the real problems from the false ones, and to focus the debate on the areas that


Source: Anderson, Richard - Department of Computer Science and Engineering, University of Washington at Seattle


Collections: Computer Technologies and Information Sciences