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Disassembler
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 allows 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 possible; however, there are often differences. This poses demands on the expressivity of the assembler. For example, an x86 assembler takes an arbitrary choice between two binary codes for something as simple as "MOV AX,BX". If the original code uses the other choice, the original code simply cannot be reproduced at any given point in time. However, even when a fully correct disassembly is produced, problems remain if the program requires modification. For example, the same machine language jump instruction can be generated by assembly code to jump to a specified location (for example, to execute specific code), or to jump to 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 to generate 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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