A method is provided for obtaining data to be used in evaluating performance of a computer processor. More specifically, the method provides for efficiently obtaining traces from an application program for use in a simulation of a computer processor. The method uses both an original code defining the application program and an instrumented version of the original code ("instrumented code"). The method includes apportioning a total time of execution of the application program between the original code and the instrumented code. Transition of execution between the original and instrumented codes is conducted through either modification of function calls or through consultation with a mapping of instruction address correspondences between the original and instrumented codes.