Abstract: A software development system with improved facilities for debugging optimized code. Certain parts of the operations specified by source statements are categorized as "data change points". A compiler 102 identifies the correspondence between the machine instructions that perform data change point operations and the source statements from which these machine instructions were derived and stores this mapping data 110 for use by a debugger program 104. This source code/machine instruction mapping data is used by the debugger to permit users to specify machine instruction locations (such as for setting breakpoints) by identifying a source statement and specifying that the debugger use the data change point of that statement; as another example, the step command includes an option that permits a user to continue execution until the next data change point instruction is reached. The debugger's source code display (FIGS.