A disassembler is a computer program that translates machine language into assembly language—the inverse operation to that of an assembler. A disassembler differs from a decompiler, which targets a high-level language rather than an assembly language. Disassembly, the output of a disassembler, is often formatted for human-readability rather than suitability for input to an assembler, making it principally a reverse-engineering tool.
Assembly language source code generally permits the use of constants and programmer comments. These are usually removed from the assembled machine code by the assembler. If so, a disassembler operating on the machine code would produce disassembly lacking these constants and comments; the disassembled output becomes more difficult for a human to interpret than the original annotated source code. Some disassemblers make use of the symbolic debugging information present in object files such as ELF. The Interactive Disassembler allow the human user to make up mnemonic symbols for values or regions of code in an interactive session: human insight applied to the disassembly process often parallels human creativity in the code writing process.
Disassembly is not an exact science: on CISC platforms with variable-width instructions, or in the presence of self-modifying code, it is possible for a single program to have two or more reasonable disassemblies. Determining which instructions would actually be encountered during a run of the program reduces to the proven-unsolvable halting problem.
 Problems of disassembly
Writing a disassembler which produces code which, when assembled, produces exactly the original binary, is non-trivial; there are often differences. However, even when a totally correct disassembly is produced, problems remain if the program is to be modified. For example, the same machine language jump instruction can be generated by assembly code which jumps to a specified location (for example, to execute specific code), or which jumps by a specified number of bytes (for example, to skip over an unwanted branch). A disassembler cannot know what is intended, and may use either syntax, generating a disassembly which reproduces the original binary. However, if a programmer wants to add instructions between the jump instruction and its destination, it is necessary to understand the program's operation to determine whether the jump should be absolute or relative, i.e., whether its destination should remain at a fixed location, or be moved so as to skip both the original and added instructions.
 Examples of disassemblers
A disassembler may be stand-alone or interactive. A stand-alone disassembler, when executed, generates an assembly language file which can be examined; an interactive one shows the effect of any change the user makes immediately. For example, the disassembler may initially not know that a section of the program is actually code, and treat it as data; if the user specifies that it is code, the resulting disassembled code is shown immediately, allowing the user to examine it and take further action during the same run.
Any interactive debugger will include some way of viewing the disassembly of the program being debugged. Often, the same disassembly tool will be packaged as a standalone disassembler distributed along with the debugger. For example, objdump, part of GNU Binutils, is related to the interactive debugger gdb.
- OllyDbg is a 32-bit assembler level analysing debugger
- OLIVER and SIMON include disassemblers for Assembler, COBOL, and PL/1
 See also
||This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (December 2009)|
- L. Vinciguerra, L. Wills, N. Kejriwal, P. Martino, and R. Vinciguerra, "An Experimentation Framework for Evaluating Disassembly and Decompilation Tools for C++ and Java", Proc. of 10th Working Conference on Reverse Engineering (WCRE) 2003.
- B. Schwarz, S. Debray, and G. Andrews, "Disassembly of Executable Code Revisited", Proc. of 9th Working Conference on Reverse Engineering (WCRE), pp. 45–54, 2002.
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