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We develop a constant time interval production planning and control methodology, called CONSTIN, and its associated simulation system for a large-scale and unstable semiconductor manufacturing process. CONSTIN moves work-in-process inventories (WIP) between processes only at a constant time interval, and consequently maintains a desirable level of WIP. Our theoretical and experimental analysis shows a clear relationship between WIP levels and the time interval in CONSTIN. Computational experiments with realistic wafer fabrication process data demonstrate that CONSTIN is comparable in simulation accuracy to a popular event-driven simulator and can run much faster. Additional experiments also manifest that, with appropriate control rules, CONSTIN can restore the desired levels of WIP from extreme deviations and maintain them. Therefore, we conclude that CONSTIN is a promising methodology of production planning and WIP control for the semiconductor manufacturing process.