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 languagesource 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 provide a built-in code commenting feature where the generated output gets enriched with comments regarding called API functions or parameters of called functions. Some disassemblers make use of the symbolic debugging information present in object files such as ELF. For example, IDA 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 codewriting process. On CISC platforms with variable-width instructions, more than one disassembly may be valid. Disassemblers do not handle code that varies during execution.
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, or to jump a specified number of bytes. 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.
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.
A dynamic disassembler can be incorporated into the output of an emulator or hypervisor to 'trace out', line-by-line, the real time execution of any executed machine instructions. In this case, as well as lines containing the disassembled machine code, the register and/or data change that each individual instruction causes can be shown alongside or beneath the disassembled instruction. This provides extremely powerful debugging information for ultimate problem resolution, although the size of the resultant output can sometimes be quite large, especially if active for an entire program's execution. OLIVER provided these features from the early 1970s as part of its CICS debugging product offering and is now to be found incorporated into the XPEDITER product from Compuware.
Length disassembler
A length disassembler, also known as length disassembler engine, is a tool that, given a sequence of bytes, outputs the number of bytes taken by the parsed instruction. Notable open source projects for the x86 architecture include ldisasm, Tiny x86 Length Disassembler and Extended Length Disassembler Engine for x86-64.
