nasm

NASM, short for Netwide Assembler, is an open-source assembler for x86 and x86-64 processors. It targets 16‑bit real mode, 32‑bit protected mode, and 64‑bit long mode, and can emit object files for a range of formats, including ELF, COFF/PE, Mach‑O, and flat binary. NASM is widely used in systems programming, operating system kernels, bootloaders, and other performance-critical code across Linux, Windows, macOS, and other Unix-like platforms.

NASM was created by Simon Tatham in 1996 and is maintained by a community of volunteers. It is distributed under the Netwide License (NWL), a permissive open-source license.

The assembler uses Intel syntax by default and provides a rich macro preprocessor and a variety of directives for defining constants, macros, sections, and symbols. It supports 16/32/64-bit modes via a bits directive and can include external files, manage relocations, and emit relocatable object code suitable for modern linkers.

NASM's support for multiple output formats and its portable design make it popular for low-level programming tasks, including OS development and bootloaders. It is widely available on major platforms and is commonly installed via package managers or built from source.