A calling program must provide its target program with a 72-byte register save area unless the target program's interface requirements have specified otherwise. It is the caller's responsibility to provide a save area that meets the specifications provided by the target program. The calling program obtains storage for the save area from its primary address space. The save area must begin on a word boundary. Before invoking the target program, the calling program loads the address of the save area into general purpose register 13. An AR mode program that is obtaining a save area would put 0 into access register 13. When the target program has required a save area of at least 144 bytes, the save area should begin on a doubleword boundary. An AMODE 64 target program is an example of a program that could require the calling program to provide a save area of at least 144 bytes.