Nasm is the Netwide Assembler, an open-source x86/x64 assembler. It aims at being portable, modular and at having a simple syntax.
NASM is the Netwide Assembler, an open-source x86/x64 assembler. It aims at being portable, modular and at having a simpler syntax than the most commonly used open-source assembler gas. It supports a whole range of output formats (including ELF, PE/COFF), plain binary (a.out) and supports Intel 64, 32, 16 and 8 bit syntaxes.
For x86 asm info in general, see the many links to reference manuals/docs, optimization / performance guides, tools, and debugging tips in the x86 tag wiki
See also:
The NASM homepage
The NASM manual
an older version of the NASM Appendix A that has text descriptions with every instruction entry, along with the CPU they were introduced in (8086, 186, 386, etc.) But it only includes MMX and older; the current version of the appendix stripped the text because SSE2/AVX/etc. have so many instructions.
https://yasm.tortall.net/ YASM is a NASM-compatible assembler with some nice features (e.g. long NOPs by default), but development has stalled and it doesn't support AVX512.
x264 has a very large set of NASM macros that attempt to abstract the calling conventions of x86_32, win64, linux64, and also do CPU feature-level checking. (e.g. to declare a function/block as SSSE3, and catch accidental usage of an SSE4.1 instruction).
It's very intrusive and makes your source code look significantly different from normal x86 asm (macros for register names). It's licensed separately (ISC, not GPL) so it can be used in other projects.
One copy of it can be found in the libvpx (VP8/9 video codec) source tree. x264 itself also has a copy, and see those projects for DSP functions using it.
