The BACKREG option controls the backchain register, which is the register used to pass the address of the automatic storage of a parent routine when a nested routine is invoked.
Note: Under the LP(64) option, the BACKREG option
is ignored.
.-5--. >>-BACKREG--(--+-11-+--)---------------------------------------><
For best compatibility with PL/I for MVS™ & VM, OS PL/I V2R3, and earlier compilers, use BACKREG(5).
All routines that share an ENTRY VARIABLE must be compiled with the same BACKREG option, and it is strongly recommended that all code in application be compiled with the same BACKREG option.
Note that code compiled with VisualAge® PL/I for OS/390® effectively uses the BACKREG(11) option. Code compiled with Enterprise PL/I V3R1 or V3R2 also uses the BACKREG(11) option by default.