This article provides a description of popular assemblers and their architecture-specific differences.
GCC generates assembly code and invokes GNU Assembler (also known as "gas"), which is part of GNU Binutils, to convert the assembly code into machine code. The GCC driver is also capable of accepting assembly input files. Due to GCC's widespread use, GNU Assembler is arguably the most popular assembler.
Within the LLVM project, the LLVM integrated assembler is a library that is linked by Clang, llvm-mc, and lld (for LTO purposes) to generate machine code. It supports a wide range of GNU Assembler syntax and can be used as a drop-in replacement for GNU Assembler.
On the Windows platform, the Microsoft Macro Assembler (MASM) is widely used.