A coverage-testing tool ATACOBOL (Automatic Test Analysis for COBOL) that applies data flow coverage technique is developed for software development on IBM System/390 mainframe. We show that the data flow coverage criteria can identify possible problematic paths that maps to the actual testing semantic required by Y2K compliance software testing. However, the mainframe environment lacks testing tools that equip the data flow coverage measure. Up to the current implementation, ATACOBOL is able to perform block coverage, decision coverage and all-uses measures. We extend the rules of data flow coverage criteria to adapt data structures that modern high-level languages usually employ.