CICS Runtime offers a library of CICS API reproducing the functionality of the z/OS CICS API and offering equivalent services to the migrated CICS applications, and in addition it offers BMS capabilities with support for 3270 screens.

In a CICS application on a z/OS platform all interactions with resources are done thru the EXEC CICS API (with the exception of DB2).

The CICS Preprocessor (on Z/OS) transforms these EXEC CICS into calls to the CICS library.

On the target platform, the same philosophy is used; an CICS Runtime Preprocessor (an CICS Runtime compile-time tool) transforms these EXEC CICS into calls to the CICS Runtime library.

For the software developer, there is little that changes. The CICS Runtime preprocessor automatically changes the CICS API that is called. There are some limitations in the command that can be used; these are described in CICS Commands and Parameters Coverage.