Protection domain extensions in Mungi

The Mungi single address space operating system provides a protected procedure call mechanism named protection domain extension (PDX). The PDX call executes in a protection domain which is the union of (a subset of) the caller's domain, and a fixed domain associated with the procedure. On return, the caller's original protection domain is re-established. Extensive caching of validation data allows amortisation of setup costs over a possibly large number of invocations. The PDX mechanism forms the basis for object support in Mungi, particularly encapsulation. It is also used for accessing devices, and to implement user-level page fault handlers and other services.