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Early Desire (Direct Executing Simulation in Real Time) is a floating point equation-language system for interactive dynamic system simulation. It runs 1.3 to five times faster than threaded Fortran and executes immediately on a RUN command without any external compiler or linker. An experimenter thus obtains results of model changes at once. An interpreted job-control language serves for interactive program entry, editing, and file manipulation and for programming multirun simulation studies. The dynamic program segment containing differential equations in first-order form is entered just like the jobcontrol statements and accesses variables with the same name. An efficient, extra-fast minicompiler translates the dynamic segment practically instantaneously. Different precompiled integration routines can be overlaid from mass storage while the program runs. Early Desire runs on any 28K-word PDP-II or LSI-II processor with FIS or FPU instructions under the RT-II operating system and is configured for up to 40 state variables. The system provides a true CRT screen editor with cursor-control and number-pad keys and produces runtime graphics even on VT-52 alphanumeric terminals and on VT-II displays. Future Desire systems will run on 32-bit superminicomputers and, for extra speed, on minicomputers with attached array processors.