Stepping (debugging)

Program animation or Stepping refers to the now very common debugging method of executing code one "line" at a time. The programmer may examine the state of the program, machine, and related data before and after execution of a particular line of code. This allows evaluation of the effects of that statement or instruction in isolation and thereby gain insight into the behavior (or misbehavior) of the executing program. Nearly all modern IDEs and debuggers support this mode of execution. Some testing tools allow programs to be executed step-by-step optionally at either source code level or machine code level depending upon the availability of data collected at compile time.

Instruction stepping or single cycle also referred to the related, more microscopic, but now obsolete method of debugging code by stopping the processor clock and manually advancing it one cycle at a time. For this to be possible, three things are required:

On the IBM System 360 processor range announced in 1964, these facilities were provided by front panel switches, buttons and banks of neon lights.

Other systems such as the PDP-11 provided similar facilities, again on some models. The precise configuration was also model-dependent. It would not be easy to provide such facilities on LSI processors such as the Intel x86 and Pentium lines, owing to cooling considerations.

As multiprocessing became more commonplace, such techniques would have limited practicality, since many independent processes would be stopped simultaneously. This led to the development of proprietary software from several independent vendors that provided similar features but deliberately restricted breakpoints and instruction stepping to particular application programs in particular address spaces and threads. The program state (as applicable to the chosen application/thread) was saved for examination at each step and restored before resumption, giving the impression of a single user environment. This is normally sufficient for diagnosing problems at the application layer.

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