ULTIM programs can be executed in three different modes:

• In interpretive mode, where the source program is first validated and translated into control blocks which are then executed.
  
  
• In compile-and-go mode, where the source program is validated, compiled in memory and executed immediately. The results are the same as with the interpretive mode, but the execution time is greatly reduced. This is the default mode.
  
  
• In true compiler mode, where an executable module is produced. The source program is validated, compiled and an object module is created (see Compiler). On the PC, this module is immediately executable; under Z/os, the usual linkage editor must be used to make it executable.