1. A computer, comprising: an entry exception raised on entry to a pre-existing operating system at a specified entry point or on a specified condition without modifying said pre-existing operating system of the computer, the entry exception having an associated entry handler, the entry handler programmed to save a context of an interrupted thread and modify the thread context before delivering the modified thread context to the operating system; a resumption exception raised on resumption from the operating system when such resumption is complementary to one of the specified entries without modifying the operating system, the resumption exception having an associated exit handler, the exit handler programmed to restore the context saved by a corresponding execution of the entry handler; concurrent threads of control scheduled by the operating system, each thread having an associated context, the association between a thread and a set of computer resources of the associated context maintained by the operating system; a detector for detecting a specified entry to the operating system from an interrupted thread of the computer, an operator responsive to said detector for raising and servicing the entry exception; and a further detector for detecting a complementary resumption, a further operator responsive to said further detector for raising and servicing the resumption exception, and returning control to the interrupted thread; the entry exception, resumption exception, entry handler, and exit handler cooperatively designed to maintain an association between one of the threads and an extended context of the thread through a context change induced by the operating system, the extended context including resources of the computer associated with the thread that are beyond those resources whose association with the thread is maintained by the operating system.