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A debugger or debugging tool is a computer program that is used to test and debug other programs (the "target" program). The code to be examined might alternatively be running on an instruction set simulator (ISS), a technique that allows great power in its ability to halt when specific conditions are encountered but which will typically be somewhat slower than executing the code directly on the appropriate (or the same) processor. Some debuggers offer two modes of operation - full or partial simulation, to limit this impact.

When the program "crashes" or reaches a preset condition, the debugger typically shows the position in the original code if it is a source-level debugger or symbolic debugger, commonly now seen in integrated development environments. If it is a low-level debugger or a machine-language debugger it shows the line in the disassembly (unless it also has online access to the original source code and can display the appropriate section of code from the assembly or compilation).(A "crash" happens when the program cannot normally continue because of a programming bug. For example, the program might have tried to use an instruction not available on the current version of the CPU or attempted to access unavailable or protected memory.)

Typically, debuggers also offer more sophisticated functions such as running a program step by step (single-stepping or program animation), stopping (breaking) (pausing the program to examine the current state) at some event or specified instruction by means of a breakpoint, and tracking the values of some variables. Some debuggers have the ability to modify the state of the program while it is running, rather than merely to observe it. It may also possible to continue execution at a different location in the program to bypass a crash or logical error.

The importance of a good debugger cannot be overstated. Indeed, the existence and quality of such a tool for a given language and platform can often be the deciding factor in its use, even if another language/platform is better-suited to the task.[citation needed]. The absence of a debugger, having once been accustomed to using one, has been said to "make you feel like a blind man in a dark room looking for a black cat that isn’t there"[1]. However, software can (and often does) behave differently running under a debugger than normally, due to the inevitable changes the presence of a debugger will make to a software program's internal timing. As a result, even with a good debugging tool, it is often very difficult to track down runtime problems in complex multi-threaded or distributed systems.

The same functionality which makes a debugger useful for eliminating bugs allows it to be used as a software cracking tool to evade copy protection, digital rights management, and other software protection features. It often also makes it useful as a general testing verification tool test coverage and performance analyzer, especially if instruction path lengths are shown.

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