Initialization of general-purpose registers (GPRs)

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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.