NOP (code)

In computer science, a NOP, no-op, or NOOP (pronounced "no op"; short for no operation) is a machine language instruction and its assembly language mnemonic, programming language statement, or computer protocol command that does nothing. Some computer instruction sets include an instruction whose explicit purpose is to not change the state of any of the programmer-accessible registers, status flags, or memory. It often takes a well-defined number of clock cycles to execute. In other instruction sets, there is no explicit NOP instruction, but the assembly language mnemonic NOP represents an instruction which acts as a NOP; e.g., on the SPARC, sethi 0, %g0. A NOP must not access memory, as that could cause a memory fault or page fault. A NOP is most commonly used for timing purposes, to force memory alignment, to prevent hazards, to occupy a branch delay slot, to render void an existing instruction such as a jump, as a target of an execute instruction, or as a place-holder to be replaced by active instructions later on in program development (or to replace removed instructions when reorganizing would be problematic or time-consuming). In some cases, a NOP can have minor side effects; for example, on the Motorola 68000 series of processors, the NOP opcode causes a synchronization of the pipeline. Listed below are the NOP instruction for some CPU architectures: From a hardware design point of view, unmapped areas of a bus are often designed to return zeroes; since the NOP slide behavior is often desirable, it gives a bias to coding it with the all-zeroes opcode. A function or a sequence of programming language statements is a NOP or null statement if it has no effect. Null statements may be required by the syntax of some languages in certain contexts. In Ada, the null statement serves as a NOP. As the syntax forbids that control statements or functions be empty, the null statement must be used to specify that no action is required. (Thus, if the programmer forgets to write a sequence of statements, the program will fail to compile.