NASM

NASM is an open-source assembler for the x86 and x86-64 architectures. It translates human-readable assembly language into machine code that can be executed by a processor. NASM produces object files, which must be linked using a linker—such as ld (the GNU linker)—to create executable binaries.

root@dev:~$ nasm -f elf64 filename.asm -o filename.o
 
 * the -f refers to "format"
    - use elf64 to assemble a 64-bit assembly code
    - use elf to assemble a 32-bit code