1) Scope system simulator

A large amount of necessary software, such as assemblers, compilers and loaders already existed for SCOPE, the CDC distributed batch operating system. We needed a method for running that software, and we did not want to build it directly into our system. Therefore, we decided that it must be possible to write, as a user program on our system, a simulator for the SCOPE operating system. In order to be efficient, the simulator code should only be invoked when the user program made a call intended for the real SCOPE system. The cost of making the call on the simulator should be comparable to the cost of calling a PPU in the SCOPE system. Finally, once the simulator has been called, it must have full access to the data of the users program, similar to the access available to a PPU under the real SCOPE system.