YASM is a complete rewrite of the NASM assembler under the "new" BSD License
(some portions are currently under the GNU Lesser General Public License
(LGPL)). Yasm currently supports the x86 and AMD64 instruction sets, accepts
NASM and GAS assembler syntaxes, outputs binary, ELF32, ELF64, COFF, Mach-O
(32 and 64), RDOFF2, Win32, and Win64 object formats, and generates source
debugging information in STABS, DWARF 2, and CodeView 8 formats.

WWW: https://yasm.tortall.net/