Abstract: A user can selectively replay portions of a computer program execution, so that the entire program need not be run again to support further test and debug. A run-time instrumented version of the program is created by inserting special instructions into the original program. The run-time instrumented version is executed to create trace files of memory accesses and system calls, as well as identification of interrupts. During execution of the run-time instrumented version, a state of each accessed memory location is monitored and updated to determine which memory accesses should be traced and when. This monitoring and updating is performed in a manner which minimizes interference with the execution. A user then may select a desired portion of the original computer program for replay and, in response, appropriate data is stored in corresponding memory locations so that the desired portion of the program may be replayed accurately.