nop

NOP, short for No Operation, is an instruction used in many processor architectures that, by design, does nothing to the machine state other than advancing the program counter. In practice, executing a NOP has no effect on registers or memory and serves primarily as a time placeholder, a means to align code, or to reserve space for future changes.

In assembly language, NOPs are used to create deliberate timing delays, fill delay slots after branches, and align code to certain boundaries. Some assemblers treat NOP as a pseudo-instruction that expands to one or more real instructions when needed to preserve timing or alignment.

Implementation varies by architecture. Some CPUs provide a dedicated NOP instruction; others implement a harmless operation whose effect is observable only in timing. Typical examples include:

- x86: a common single-byte NOP is 0x90; there are longer, multi-byte no-ops formed by sequences of instructions that compile to the same effect.

- ARM: a canonical NOP is MOV r0, r0 (in ARM mode), with Thumb-2 also providing dedicated NOP encodings.

- MIPS: NOP is commonly represented as sll $zero, $zero, 0, effectively a no-op.

- RISC-V: addi x0, x0, 0 is used as a NOP.

Pseudo-instructions and compilers: Many assemblers offer NOP as a pseudo-instruction. Optimizers and linkers may insert NOPs to improve scheduling, alignment, or to avoid hazards in pipelined CPUs.

Security note: In exploit development, the term NOP sled refers to a long sequence of NOP-like instructions used to increase the chance that the processor’s flow lands in the shellcode payload.