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1) Specification as an Abstract Machine

As in some programming languages, the system was conceived as implementing an abstract machine which dealt with a number of different types of abstract objects. Interaction with the system was to be accomplished through virtual instructions, which were provided in addition to the standard hardware instructions. Each of these instructions was to operate on specific types of objects, and an error was to be returned to the user if the wrong type of object was presented to such an instruction. These instructions were to be understood independently of their implementation, and to be described in terms of (possibly a sequence of) atomic changes in the state of the object.

Paul McJones