Comment by tyingq

For the most part, it's not the COBOL itself that makes it hard to port. It's the features of the surrounding OS for things like screen layouts, batch process management (not cron like, but deep job dependencies, etc), database management, file transfer, user management, etc. So to port the app, you have to know quite a lot about what things like RACF, TSO, CICS, JCL, Tivoli, Adabase/IDMS/etc, are contributing to the overall app. Porting the COBOL is sometimes not even half the effort.