Program that transforms machine language into assembler language (human-readable).
The disassembler software reads the raw byte stream output from the processor and parses it into groups of instructions. These instructions are then translated into assembly language instructions.
The output code is entirely dependent on the various algorithms internal to the disassembler.
A good disassembler needs to be able to fairly accurately distinguish between data and code.
Disassemblers are mainly used as tools to modify existing binary code for which there is no source available and as one of the way of determining the existence of virus or other malicious code in a binary file.