1. A computer, comprising: an entry handler block configured to: process an entry exception raised on entry to a pre-existing operating system at a specified entry point or on a specified condition without modifying the pre-existing operating system of the computer; save a context of an interrupted thread to a memory; modify the thread context; and deliver the modified thread context to the operating system; an exit handler block configured to: process a resumption exception raised on resumption from the operating system when such resumption is complementary to one of the specified entry exceptions without modifying the operating system; and restore the context saved by a corresponding execution of the entry handler from the memory; a thread scheduler block configured to: schedule concurrent threads of control, each thread having an associated context; and maintain an association between a thread and a set of computer resources of the associated context; wherein an association between one of the threads and an extended context of the thread is maintained through a context change induced by the thread scheduler block or by the operating system, without modifying the thread scheduler block or the operating system, the extended context including resources of the computer beyond those resources associated with the thread; a first detector block configured to: detect a specified entry to the operating system from an interrupted thread of the computer; and raise the entry exception; and a second detector block configured to: detect a complementary resumption exception; raise the resumption exception; and return control to the interrupted thread.