The GPRs are initialized by IDF to the value X'FEFEnn0F' (where nn is the register number, 00 through 0F), with the following exceptions:

• R12 and R15 contain the target program's entrypoint address.
• R13 points to a 24-word save area (doubleword aligned).

z/OS

• R1 contains the parameter pointer.

  If the COMMAND option is not specified, R1 points to a standard z/OS CALL format parameter list.

  If the COMMAND option is specified, R1 points to a standard TSO Command Processor Parameter List (CPPL). For more details, see Loading programs (TSO).

• If SVC 97 is being used for breakpoints, R14 points to an SVC 97; otherwise, it points to an X'02FF'. IDF uses either of these to determine if control was returned by the target program.
• If option AMODE64 is in effect, the first word of each register is initialized by IDF to X'00000000'.

z/VM

• R0 and R1 contain parameter pointers in the usual CMS fashion (for more information, see How to specify parameters for your program).
• If the program is a nucleus extension, R2 contains the address of its SCBLOCK.
• R14 points to an X'02FF' used by IDF to determine that the target program has returned control.

z/VSE

• R1 contains the parameter pointer.
• R14 points to an X'01FF' used by IDF to determine if control was returned by the target program.