The Microsoft Macro Assembler (MASM) is an x86 assembler that uses the Intel syntax for MS-DOS and Microsoft Windows. Beginning with MASM 8.0 there are two versions of the assembler - one for 16-bit and 32-bit assembly sources, and another (ML64) for 64-bit sources only.
MASM is maintained by Microsoft, but since version 6.12 has not been sold as a separate product, it is instead supplied with various Microsoft SDKs and C compilers. Recent versions of MASM are included with Microsoft Visual Studio.
Read more about Microsoft Macro Assembler: History, Object Module Formats Supported By MASM, Assemblers Compatible With MASM