Considerations for Assembler Programs in a DC/UCF Online System › Making Your Assembler Program Reentrant

Making Your Assembler Program Reentrant

All programs that are designed to run in an online DC environment should be written using fully reentrant coding techniques. This means that the program should never update its own storage. Any variable storage that your program needs to update should be in an area reserved for the exclusive use of a single task. Typically, you would define a DSECT to map this area. Several techniques can be used to achieve this goal. Two or more of these techniques can be combined in a single program.

• Specify ISASIZE on the PROGRAM statement in the DC Sysgen. On entry to your program, register 11 will be set to point to an area of this size reserved for the use of your program.
• On entry to your program, code a #GETSTG ...PLIST=*,LENGTH=constant.... This form of the #GETSTG macro does not update any program variable storage. The sample program in Appendix C of this manual uses this technique.
• An assembler subprogram can use storage passed to it from its caller provided that storage is itself reentrant.
• If one of the above techniques is used to obtain enough storage for a PLIST, then a more generic form of the #GETSTG macro can be used to obtain further variable storage.
• Specify SAVEAREA on the PROGRAM statement in the DC Sysgen. See section 7.4 below for more information.

A non-reentrant assembler routine can be used in an online DC environment under certain limited circumstances, but this is not recommended for reasons explained below.

• A non-reentrant stand-alone assembler program, i.e., an assembler program which is linked as its own load module or phase, can be invoked directly from a TASK CODE or can be called via a high level language TRANSFER CONTROL, a #LINK from another assembler program or a LINK PROGRAM from an ADS dialog. Such a program must be defined in the DC/UCF Sysgen as non-reentrant. Note that this will cause separate copies of the load module to be loaded for every concurrent task using the program. This is generally highly inefficient.
• A non-reentrant assembler subprogram can be called via a dynamic or static COBOL CALL verb, or a PL/I CALL verb if all of the following conditions are met:
  • The subprogram does not issue any SVC or PC calls. Note that many IBM macros generate such calls.
  • The subprogram does not issue any DML calls.
  • Multitasking is not in effect on the DC/UCF system.

  This technique is not recommended because a small change in the program or online environment may cause the program to stop functioning correctly and potentially allow it to cause storage and data corruption.