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NOP (code)

From Wikipedia, the free encyclopedia

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

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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.[1]

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.

Code

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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

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In Ada, the null statement serves as a NOP.[18] 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

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

Most C compilers generate no code for null statements, which has historical and performance reasons.

  ;

An empty block (compound statement) is also a NOP, and may be more legible, but will still have no code generated for it by the compiler.

  {}

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

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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

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

  • Use the ; empty statement[19] 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;[20]
  • 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

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

jQuery

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The jQuery library provides a function jQuery.noop(), which does nothing.[21]

Lodash

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The Lodash library provides a function _.noop(), which returns undefined and does nothing.[22]

Pascal

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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

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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.)

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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).[23][24][25]

TeX macro language (ConTeXt, LaTeX, etc.)

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The TeX typographical system's macro language has the \relax command.[26] It does nothing by itself, but may be used to prevent the immediately preceding command from parsing any subsequent tokens.[27]

NOP protocol commands

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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):

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:[28]

200 NOOP to you too!

Cracking

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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

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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

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References

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  1. ^ a b c d e "Motorola 68000 Programmer's Reference Manual" (PDF).
  2. ^ a b c d "Intel 64 and IA-32 Architectures Software Developer's Manual: Instruction Set Reference A-Z". Retrieved 2012-03-01.
  3. ^ AMD, "Am29050 Microprocessor User's Manual" (PDF). 1991. pp. 223, 257.
  4. ^ a b ARM, ARM Architecture Reference Manual, ARMv7-A and ARMv7-R edition, ARM DDI 0406C.d, 29 Mar 2018, section A8.8.120 on page 511. Archived on 27 Sep 2023.
  5. ^ a b ARM, ARM Software Development Toolkit Version 2.50 Reference Guide, ARM DUI 0041C, Nov 1998, see sections 5.5.6 and 5.6.4 for NOPs in ARM/Thumb assembly. Archived on 1 Jun 2022.
  6. ^ a b ARM, ARM Compiler toolchain Assembler Reference v5.03, NOP instruction. Archived on 16 Mar 2025.
  7. ^ a b O. Smith, StrongARM multiply speed, 12 Dec 1996. Archived on 16 Mar 2025.
  8. ^ Acorn Computers, Acorn Assembler Release 2, May 1991. MOVNV listed on page 218. Archived on 25 Oct 2018.
  9. ^ ARM, ARM610 32 Bit RISC Microprocessor, ARM DDI 0004D, Aug 1993, page 20
  10. ^ ARM, ARM Architecture Reference Manual, ARM DDI 0100D, Feb 2000, section 3.2.1 on page 69. Archived on 16 Feb 2025.
  11. ^ "IBM System/360 Model 91 Functional Characteristics" (PDF). Systems Reference Library. IBM. p. 16. Retrieved 17 January 2025.
  12. ^ a b PA-RISC 2.0 Architecture (PDF). Hewlett-Packard. 1995. pp. 2–21, 7–103. Archived from the original (PDF) on June 21, 2020.
  13. ^ Power ISA version 3.1C (PDF). OpenPOWER Foundation. 26 May 2024. Book 1 section 3.3.13 on pages 97-99; book 2 section 3.2 on page 987 and section 4.3.3 on page 1005; book 3 section 5.4.3 on page 1100, section 9.2.1 on page 1244 and section 11.3 on page 1253. Archived from the original (PDF) on 10 September 2024.
  14. ^ POWER8 Processor User's Manual for the Single-Chip Modul, version 1.3 (PDF). IBM. 16 March 2016. Section 10.1.8 on page 209. Archived (PDF) from the original on 28 December 2018.
  15. ^ The RISC-V Instruction Set Manual, Volume 1: User-Level ISA, version 2.2 (PDF). RISC-V Foundation. 7 May 2017. p. 79.
  16. ^ Weaver, D. L.; Germond, T., eds. (1994). The SPARC Architecture Manual, Version 9 (PDF). Prentice Hall. ISBN 0-13-825001-4. Archived from the original (PDF) on 2012-01-18. Retrieved 2014-01-09. Note that NOP is a special case of the SETHI instruction, with imm22 = 0 and rd = 0.
  17. ^ LoongArch Reference Manual, Volume 1: Basic Architecture, version 1.10 (PDF). Loongson. Section 2.2.1.10 on page 23 and appendix B on page 190.
  18. ^ Ada Reference Manual — null statements. "The execution of a null_statement has no effect."
  19. ^ MDN JavaScript reference – empty statement. "The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one."
  20. ^ ECMAScript Language Specification – Edition 5.1 – Properties of the Function Prototype Object
  21. ^ jQuery.noop() from jQuery API documentation
  22. ^ "Lodash Documentation". lodash.com. Retrieved 2017-12-15.
  23. ^ Advanced Bash-Scripting Guide > Chapter 3. Special Characters
  24. ^ bash manpage > SHELL BUILTIN COMMANDS
  25. ^ zsh manpage (zshbuiltins) > SHELL BUILTIN COMMANDS
  26. ^ Bausum, David (2002). "TeX Primitive Control Sequences". TeX Reference Manual. Kluwer Academic Publishers. Retrieved 1 April 2020. According to The TeXbook, 'TeX does nothing' when it encounters \relax. Actually, \relax may tell TeX, 'This is the end of what you've been doing'.
  27. ^ TeX wikibook – relax
  28. ^ "ftpd.c". Retrieved 2016-06-19.