z/OS Unicode Services interfaces follow the MVS linkage conventions described in "Linkage Conventions" of z/OS MVS Programming: Assembler Services Guide. The topic for each z/OS Unicode interface gives specific details about the conventions that are followed. In general,

• GPR 1 - Caller must set to the address of the parameter area.
• GPR 13 - Caller must set to the address of a save area.
• GPR 14 - Caller must set to the return address.
• GPR 15 - Caller must set to the entry address. The stub routines do this automatically.