—Host-compiled simulation has been widely adopted as a practical approach for fast and high-level evaluation of complex software-intensive systems at early stages of the design process. In such approaches, higher speed is achieved by coarsegrained simulation of the system, which also leads to a loss in timing accuracy. To eliminate the inherent speed and accuracy tradeoff, we present an adjustive software simulator, which automatically controls the timing model of the simulation platform to provide both fast and accurate results. At its core, we propose a novel RTOS model that permanently monitors the state of the system and optimally and automatically adjusts back-annotated timing granularities to provide an error-free task scheduling. We evaluated our approach on an industrial-strength example, and results show that the accuracy of a fine-grain simulation can be achieved while maintaining a speed of close to 900MIPS.