A method for running application programs supporting multiple contexts on a computer system having an operating system supporting threads, and not supporting multiple context processing instructions. The method is characterized by processing (210) a context processing instruction from said run-time application program, evaluating (220) said instruction in relation to program-flow control yielding context-defining processing parameters, analyzing (230) said context-defining processing parameters (40) from the execution context of said context processing instruction in regard of program flow management by threads, mapping said context-processing instruction to a selected thread managing instruction, or to a selected set of thread managing instructions, having a respective program flow control effect equivalent to that of said context-processing instruction, invoking (240, 250, 260) said selected thread managing instruction or said set thereof, together with selected parameters for guaranteeing that during runtime of said application program only one thread is allowed to execute at a time, and the program state of a thread may be stored at a given point in time and may be restored later from said point in time.