This report describes the detailed design and code of the control program for the isocentric treatment unit of the Clinical Neutron Therapy System (CNTS). It is primarily intended for software developers. It describes design features which should be preserved when the program is modified, and explains some unobvious implementation details. The control program was developed from a detailed design expressed in the Z notation. It is coded in C. We used only the ANSI Standard C library and a few libraries needed to use the real-time operating system and the X window system (Xlib only, not Motif or any other toolkits). The program text (C data and functions) is partitioned into modules that make it easy for us to produce different program versions that run in different environments. The executing program comprises several concurrent tasks, coordinated by appropriate communication and synchronization methods. The behavior of the user interface is determined by a table. The graphical user interface is separable; the program can also read text commands from a keyboard or script file (for test automation). Most of the code was developed on a general-purpose workstation, simulating tasking in a single process while reading and writing simulated device controller commands and data from files. Many of the program source code files are re-used (without changes) in utility programs that run on a workstation.

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