Short for Operation Code, which is the part of an instruction in machine language to specify the operation to be performed. A complete machine language instruction consists of an opcode and zero or more operands with which the specified operation is performed. Examples are “add memory location A to memory location B,” or “store the number five in memory location C.” “Add” and “Store” are the opcodes in these examples. Because virus scanners try to detect and remove malicious patterns of machine instructions, virus writers have now turned to metamorphic viruses that rewrite themselves using equivalent opcodes, or that re-order the machine instructions to achieve the same computational result while at the same time avoiding detection.

See Also: Virus.