You can write new programs that will run on systems that use 31-bit addressing. If these programs do not need to use any new MVS™ functions, the best way to avoid errors is to assemble the programs on MVS/370 with macro libraries from a 31-bit addressing system. You can also assemble these programs on 31-bit addressing systems with macro libraries from MVS/370, but you must generate MVS/370-compatible macro expansions by specifying the SPLEVEL macro at the beginning of the programs.
Programs designed to execute on either 24 or 31-bit addressing systems must use fullword addresses where possible and use no new functions on macros except the LOC parameter on GETMAIN. These programs must also be aware of downward incompatible macros and use SPLEVEL as needed.