High Quality Content by WIKIPEDIA articles! FASM (flat assembler) in computing is an assembler. It supports programming in Intel-style assembly language on the IA-32 and x86-64 computer architectures. It claims high speed, size optimizations, operating system (OS) portability, and macro abilities. It is a low-level assembler and intentionally uses very few command-line options (see `SSSO`). It is free and open source software.