Stepping (debugging)
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Techniques for program animation
Program animation is a particular feature of some Testing tools allowing programs to be executed step-by-step at either source code level or machine code level. Some tools permit animation at both levels depending upon the availability of data collected at compile time.
There are at least three distinct software techniques for creating 'animation' during programs execution.
- instrumentation involves adding additional source code to the program at compile time to call the animator before or after each statement to halt normal execution.
- Induced interrupt This technique involves forcing a breakpoint at certain points in a program at execution time, usually by altering the machine code instruction at that point (this might be an inserted system call or deliberate invalid operation) and waiting for an interrupt. When the interrupt occurs, it is handled by the testing tool to report the status back to the programmer. This method allows program execution at full speed (until the interrupt occurs) but suffers from the disadvantage that most of the instructions leading up to the interrupt are not monitored by the tool.
- Instruction Set Simulator This technique treats the compiled programs machine code as its input 'data' and fully simulates the host machine instructions, monitors the code for conditional or unconditional breakpoints or programmer requested "single cycle" animation requests between every step.
Comparison of methods
The advantage of the last method is that no changes are made to the compiled program to provide the diagnostic and there is almost unlimited scope for extensive diagnostics since the tool can augment the host system diagnostics with additional software tracing features. The second method only alters the instruction that will halt before it is executed and may also then restore it before optional resumption by the programmer. Some animators optionally allow the use of more than one method depending on requirements. For example, using method 2 to execute to a particular point at full speed and then using instruction set simulation thereafer.
Additional features
The animator may, or may not, combine other test/debugging features within it such as program trace, dump, conditional breakpoint and memory alteration, program flow alteration, code coverage analysis, "hot spot" detection or similar.