The "Provably Secure Operating System"
See PSOS Revisited at http://www.csl.sri.com/neumann/psos03.pdf

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This has nothing, as far as I can tell, to do with the the RealTime OperatingSystem pSOS, currently sold by WindRiverSystems (which they bought, picked out some technology, and discarded in favor of WRS's own product VxWorks)


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ObjectCapabilityOperatingSystem, CategoryOperatingSystem