Restrictions for debugging self-modifying code

z/OS® Debugger cannot debug self-modifying code. If your program contains self-modifying code that modifies an instruction while the containing compilation unit is being debugged, the result can be unpredictable, including an abnormal termination (ABEND) of z/OS Debugger. If your program contains self-modifying code that completely replaces an instruction while the containing compilation unit is being debugged, the result might not be an ABEND. However, z/OS Debugger might miss a breakpoint on that instruction or display a message indicating an invalid hook address at delete.

The following coding techniques can be used to minimize problems debugging self-modifying code:

  1. Do not modify part of an instruction. Instead, replace an instruction. The following table compares coding techniques:
    Coding that modifies an instructions Coding that replaces an instruction 
    ModInst  BC 0,Target
   ...
   MVI ModInst+1,X'F0'
    		 
    ModInst  BC 0,Target
   ...
   MVC ModInst(4),NewInst
   ...
NewInst  BC 15,Target
  2. Define instructions to be modified by using DC instructions instead of executable instructions. For example, use the instruction ModInst DC X'4700',S(Target) instead of the instruction MVC ModInst(4),NewInst.