This paper describes the process ofdeveloping an instruction-level simula tion of a digital computer for program checkout, with particular reference to a simulation of the Viatron 2140/2150 minicomputers on the IBM 360. The architecture of these computers is described. Design decisions to be made in simulator development are then covered, including programming language, data representation, approach to timing, and debugging facil ities. A method ofreducing the size of the simulator by "paging" the simu lated memory into and out of the host computer's core memory is discussed. Results of operational use of the simulator are given. Through out, emphasis is placed on those factors which tend to differentiate the simulation of a minicomputer from the simulation of a commercial data- processing computer.