Microsoft Macro Assembler

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 & 32-bit assembly sources, and another (ML64) for 64-bit sources only.

MASM is maintained by Microsoft, but since version 6.12 it 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.

Notable applications compiled using MASM are RollerCoaster Tycoon which was 99% written in assembly language and built with MASM.

The earliest versions of MASM date back to 1981. They were sold either as the generic "Microsoft Macro Assembler" for all x86 machines or as the OEM version specifically for IBM PCs. By Version 4.0, the IBM release was dropped. Up to Version 3.0, MASM was also bundled with a smaller companion assembler, ASM.EXE. This was intended for PCs with only 64k of memory and lacked some features of the full MASM, such as the ability to use code macros.

MS-DOS versions up to 4.x included Microsoft's LINK utility, which was designed to convert intermediate OBJ files generated by MASM and other compilers; however, as users who did not program had no use of the utility, it was moved to their compiler packages.

Version 4.0, released October 1985, added support for 286 instructions.

Version 5.0, released August 1987, supported 386 instructions, and also shorthand mnemonics for segment descriptors (.code, .data, etc.), but it could still only generate real mode executables.

Through version 5.0, MASM was available as an MS-DOS application only. Versions 5.1 and 6.0 were available as both MS-DOS and OS/2 applications.