NOP (code)

Short description: Machine instruction that indicates to a computer to do nothing

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.

Machine language instructions

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:

CPU architecture	Instruction mnemonic	Bytes	Opcode	Notes
Intel x86 CPU family	NOP	1; 1–9 for i686 and x86-64	0x90	0x90 decodes to xchg eax, eax in all modes except long mode, where the opcode 0x90 still has no effect. The longer encodings are described in Intel's manual.
Intel 8051 / MCS-51 family	NOP	1	0x00
DEC Alpha	NOP	4	0x47FF041F	Opcode for BIS r31,r31,r31, an instruction that bitwise-ORs the always-0 register with itself.
AMD 29k	NOP	4	0x70400101	Opcode for aseq 0x40,gr1,gr1, an instruction that asserts that the stack register is equal to itself.
ARM A32	NOP	4	0x00000000	This stands for andeq r0, r0, r0. The assembly instruction nop will most likely expand to mov r0, r0 which is encoded 0xE1A00000 (little-endian architecture).
ARM T32 (16 bit)	NOP	2	0xb000	Opcode for ADD SP, #0 - Add zero to the stack pointer (No operation). The assembly instruction nop will most likely expand to mov r8, r8 which is encoded 0x46C0.
ARM T32 (32 bit)	NOP	4	0xF3AF 8000
ARM A64 (64 bit)	NOP	4	0xD503201F
AVR	NOP	2	0x0000	one clock cycle
IBM System/360, IBM System/370, IBM System/390, z/Architecture, UNIVAC Series 90	NOP	4	0x47000000 or 0x470nnnnn or 0x47n0nnnn where "n" is any 4-bit value.	The NOP ("No-Op") and NOPR ("No-Op Register") are a subset of the "Branch on Condition" or "Branch on Condition Register" instructions, respectively; both versions have two options for generating a NO-OP. In the case of both the NOP and NOPR instructions, the first 0 in the second byte is the "mask" value, the condition to test such as equal, not equal, high, low, etc. If the mask is 0, no branch occurs. In the case of the NOPR instruction, the second value in the second byte is the register to branch on. If register 0 is chosen, no branch occurs regardless of the mask value. Thus, if either of the two values in the second byte is 0, the branch will not happen. In the case of the NOP instruction, the second value in the second byte is the "base" register of a combined base register, displacement register and offset address. If the base register is also 0, the branch is not taken regardless of the value of the displacement register or displacement address.
	NOPR	2	0x0700 or 0x070n or 0x07n0 where "n" is any 4-bit value.
SuperH	NOP	2	0x0009
MIPS	NOP	4	0x00000000	Stands for sll r0,r0,0, meaning: Logically shift register 0 zero bits to the left and store the result in register 0. Writes to register 0 are ignored; it always contains 0.
MIPS-X	NOP	4	0x60000019	(extended opcode for add r0,r0,r0)
MIX	NOP	1 word	± * * * * 0	The * bytes are arbitrary, and can be anything from 0 to the maximum byte (required to be in the range 63-99). MIX uses sign-magnitude representation.
MMIX	SWYM	4	0xFD******	SWYM stands for "Sympathize with your machinery". The * digits can be chosen arbitrarily.
Motorola 68000 family	NOP	2	0x4E71	This synchronizes the pipeline and prevents instruction overlap.
Motorola 6809	NOP	1	0x12
MOS Technology 65xx (e.g. 6502)	NOP	1	0xEA	NOP consumes two clock cycles. Undefined opcodes in the NMOS versions of the 65xx family were converted to be NOPs of varying instruction lengths and cycle times in the 65C02.
PA-RISC	NOP	4	0x08000240	Opcode for OR 0,0,0.
	LDI 26,0	4	0x34000034	Palindromic NOP - that is, an instruction that executes as NOP regardless of whether byte order is interpreted as little-endian or big-endian. Some PA-RISC system instructions are required to be followed by seven palindromic NOPs.
PowerPC	NOP	4	0x60000000	(extended opcode for ori r0,r0,0)
PIC microcontroller	NOP	12 bits	0b000000000000	MOVW 0,W
RISC-V	NOP	4	0x00000013	ADDI x0, x0, 0
	C.NOP	2	0x0001	C.ADDI x0, 0. Only available on RISC-V CPUs that support the "C" (compressed instructions) extension.
SPARC	NOP	4	0x01000000	Stands for sethi 0, %g0 which zeroes the hardwired-to-zero %g0 register
Z80	NOP	1	0x00	There are some other instructions without any effect (and the same timing): LD A, A, LD B, B etc.
PDP-10	JFCL 0, (conventional) JUMP, SETA, SETAI, CAI, TRN, TLN	1 word	25500******* (octal)	Jump never Jump never, set nothing, skip never
PDP-11	NOP	16 bits	000240 (octal)	Clear none of the condition codes
VAX	NOP	1	0x01	Delay is dependent on processor type

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.

Code

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.

Ada

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.)

C and derivatives

The simplest NOP statement in C is the null statement, which is just a semi-colon in a context requiring a statement.

  ;

An empty block (compound statement) is also a NOP, and may be more legible:

  {}

In some cases, such as the body of a function, a block must be used, but this can be empty. In C, statements cannot be empty—simple statements must end with a ; (semicolon) while compound statements are enclosed in {} (braces), which does not itself need a following semicolon. Thus in contexts where a statement is grammatically required, some such null statement can be used.

The null statement is useless by itself, but it can have a syntactic use in a wider context, e.g., within the context of a loop:

while (getchar() != '\n') {}

alternatively,

while (getchar() != '\n')
      ;

or more tersely:

while (getchar() != '\n');

(note that the last form may be confusing, and as such generates a warning with some compilers or compiler options, as semicolon usually indicates an end of function call instruction when placed after a parenthesis on the end of line).

The above code continues calling the function getchar() until it returns a \n (newline) character, essentially fast-forwarding the current reading location of standard input to the beginning of next line.

Fortran

In Fortran, the CONTINUE statement is used in some contexts such as the last statement in a DO loop, although it can be used anywhere, and does not have any functionality.

JavaScript

The JavaScript language does not have a built-in NOP statement. Many implementations are possible:

• Use the ; empty statement or the {} empty block statement the same way as in the C and derivatives examples;
• Use the undefined or the null expression as a complete statement (an expression statement) when the previous methods are not allowed by the syntax.

Alternatives, in situations where a function is required, are:

• Use the Function.prototype() built-in function, that accepts any arguments and returns undefined;
• Use a NOP function available in a third-party library —see below;
• Define a custom NOP function, as in the following example (using the ES6 arrow function syntax):

const noop = () => {};

AngularJS

The AngularJS framework provides angular.noop function that performs no operations.

jQuery

The jQuery library provides a function jQuery.noop(), which does nothing.

Lodash

The Lodash library provides a function _.noop(), which returns undefined and does nothing.

Pascal

As with C, the ; used by itself can be used as a null statement in Pascal. In fact, due to the specification of the language, in a BEGIN / END block, the semicolon is optional before the END statement, thus a semicolon used there is superfluous.

Also, a block consisting of BEGIN END; may be used as a placeholder to indicate no action, even if placed inside another BEGIN / END block.

Python

The Python programming language has a pass statement which has no effect when executed and thus serves as a NOP. It is primarily used to ensure correct syntax due to Python's indentation-sensitive syntax; for example the syntax for definition of a class requires an indented block with the class logic, which has to be expressed as pass when it should be empty.

Shell scripting (bash, zsh, etc.)

The ':' [colon] command is a shell builtin that has similar effect to a "NOP" (a do-nothing operation). It is not technically an NOP, as it changes the special parameter $? (exit status of last command) to 0. It may be considered a synonym for the shell builtin 'true', and its exit status is true (0).

TeX macro language (ConTeXt, LaTeX, etc.)

The TeX typographical system's macro language has the \relax command. It does nothing by itself, but may be used to prevent the immediately preceding command from parsing any subsequent tokens.

NOP protocol commands

Many computer protocols, such as telnet, include a NOP command that a client can issue to request a response from the server without requesting any other actions. Such a command can be used to ensure the connection is still alive or that the server is responsive. A NOOP command is part of the following protocols (this is a partial list):

• telnet
• FTP
• SMTP
• X11
• POP3
• NNTP
• finger
• IMAP4
• BitTorrent

Note that unlike the other protocols listed, the IMAP4 NOOP command has a specific purpose—it allows the server to send any pending notifications to the client.

While most telnet or FTP servers respond to a NOOP command with "OK" or "+OK", some programmers have added quirky responses to the client. For example, the ftpd daemon of MINIX responds to NOOP with the message:

200 NOOP to you too!

Cracking

NOPs are often involved when cracking software that checks for serial numbers, specific hardware or software requirements, presence or absence of hardware dongles, etc. in the form of a NOP slide. This process is accomplished by altering functions and subroutines to bypass security checks and instead simply return the expected value being checked for. Because most of the instructions in the security check routine will be unused, these would be replaced with NOPs, thus removing the software's security functionality without altering the positioning of everything which follows in the binary.

Security exploits

The NOP opcode can be used to form a NOP slide, which allows code to execute when the exact value of the instruction pointer is indeterminate (e.g., when a buffer overflow causes a function's return address on the stack to be overwritten).

See also

• Computer architecture
• HLT (x86 instruction) – pauses the CPU
• Halt and Catch Fire – also pauses the CPU
• Identity function – the functional programming equivalent to NOP
• xyzzy (computing) – a command sometimes used instead of NOP
• IEFBR14 – mainframe tautology
• Filler text
• Comment (computer programming) – annotations generally for programmers that are ignored by compilers and interpreters