Built-in Function Support › Internal Structure Of Built-In Functions › Runtime Processing of Built-In Functions

Runtime Processing of Built-In Functions

At runtime, the following processing sequence occurs for each function:

1. The runtime system begins processing the function, as follows:
   • Moves each parameter in the function to an intermediate result area (IRA). If a parameter is coded as a multi-operand expression, the expression is evaluated and only the result is moved to the IRA. Data conversions are performed as necessary.

     The runtime system maintains an operand XDE/VXDE for each function parameter. The XDE/VXDE pair describes the parameter as it is stored in the IRA and contains the parameter's IRA address.

   • Passes control to the processing program module named in the function XDE. The runtime system places in register 1 the address of the function VXDE. (A function VXDE contains addresses that enable access to the work area, the result field in the IRA, and the function's operand XDEs/VXDEs.)
2. The processing program continues processing the function, as follows:
   • Processes initialization statements, as illustrated earlier in this appendix. Register information is saved by using standard z/OS conventions. Register 1 is used to access the function VXDE, which in turn enables access to all the information required for processing the function, as illustrated below.
   • Branches to the appropriate program by using a branching routine in conjunction with the XDEUFUNC field of the function XDE.
   • Processes the program statements.
   • Processes final statements. If the program did not encounter an error condition, register 15 is set to 0. If an error was encountered, register 15 is set to a nonzero value and an optional error message is moved to the first 80 bytes of the work area. Registers are restored and the module passes control back to the runtime system.
3. The runtime system finishes processing the function by checking the value returned in register 15. If register 15 equals 0, the runtime system resumes the process. The result of the function is used in the statement in which the function is coded. If register 15 is greater than 0, the runtime system aborts the dialog and displays the Dialog Abort Information screen along with the optional error message.

Internal Representation of a Function at Run Time

The internal representation of a substring function is illustrated below.

An arrow indicates that the source structure contains the address of the object structure, illustrating that the processing program module can use register 1 to gain access to all required information.

Field names containing the addresses are listed next to the arrows.

 SUBSTRING(EMP-NAME, START-POS-1)

 XDEs                       VXDEs                      INTERMEDIATE RESULT AREA
┌───────────────┐          ┌───────────────┐            ┌─────────────────────┐
│Operand XDE    │ VXDEXDEA │Operand VXDE   │ VXDEDADR   │                     │
│for first      ◄──────────┤for first      ├────────────►  │First parameter │ │
│parameter      │          │parameter      │            │  └────────────────┘ │
│(EMP-NAME)     │          │(EMP-NAME)     │            │                     │
└───────────────┘          └───────────────┘            │                     │
                                  ▲                  ┌──►  │Second parameter│ │
                                  │ VXDESNXT         │  │  └────────────────┘ │
┌───────────────┐          ┌──────┴────────┐         │  │                     │
│Operand XDE    │ VXDEXDEA │Operand VXDE   │ VXDEDADR│  │                     │
│for second     ◄──────────┤for second     ├─────────┘  │                     │
│parameter      │          │parameter      │         ┌──►  │Result field    │ │
│(START-POS-1)  │          │(START-POS-1)  │         │  │  └────────────────┘ │
└───────────────┘          └───────────────┘         │  └─────────────────────┘
                                  ▲                  │
                                  │ VXDESNXT         │
┌───────────────┐          ┌──────┴────────┐         │
│Dummy operand  │ VXDEXDEA │Dummy operand  │         │
│XDE for omitted◄──────────┤VXDE for omit─ │         │
│third parameter│          │ted third      │         │
└───────────────┘          │parameter      │         │
                           └───────────────┘         │
                                  ▲                  │
                                  │ VXDESNXT         │
┌───────────────┐          ┌──────┴────────┐ VXDEDADR│  ┌───────────────────┐
│Function XDE   │ VXDEXDEA │Function VXDE  ├─────────┘  │                   │
│for substring  ◄──────────┤for substring  ├────────────►     WORK AREA     │
│function       │          │function       │ VXDEUWKA   │                   │
└───────────────┘          └───────────────┘            └───────────────────┘
                                  ▲
                                  │
                           ┌──────┴────────┐
                           │  Register 1   │
                           │               │
                           └───────────────┘

More information:

Steps for Generating a User-Defined Built-In Function