Microsoft Macro Assembler

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Commonly known as just MASM, this is a macro assembler for 16-bit development of OS/2 and DOS software and drivers, that had some limited support for 32-bit code generation and MS Windows development. MASM version 5.1 was supplied with some versions of the IBM developer toolkit and the Device Driver Source Kit (DDK).

MASM is also an informal standard as an assembly language format.


Early versions of MASM were generic 8088, 8087 and 8086 assemblers and could generate code for any system based on these processors not just DOS, later versions also supported 80186, 80286, NEC V20 and so on.

IBM did however market the same product under the name IBM Macro Assembler that only supported the generation of DOS executables.


  • 5.0 (1987)
  • 5.1 (1988) - was the first version with native OS/2 host support. The IBM ALP has a compatibility mode (M510) for MASM 5.1.
  • 5.1a (1989)
  • 5.1b
  • 6.0 - added high level language constructs such as IF, WHILE and REPEAT alongside some further loop controls, also the first MASM version to support LR offsets, fully flat 32 bit memory and the full Intel 80486 instruction set. The engine had also been re-written so it now performed all passes in memory rather than off disk and supports DPMI memory management, but no longer runs in severely memory constrained DOS systems. Version 6 also added some automatic memory range fixes.
  • 6.0a
  • 6.0b (1992) - Last version that supports OS/2 fully, can generate flat 32-bit code for OS/2, DOS (using a supplied DOS extender) and Windows. See Publications below.
  • 6.1 was the first version to support Windows PE executables.
  • 6.11 can be patched to support OS/2 (see above), it is also the last DOS-compatible version of MASM.
    • The masm4os2 package contains a version of DOSXNT.EXE that makes the MASM 6.11d package compatible with OS/2.
  • 6.12 and later were Win32 console executables.
  • Link386 or compatible linker for generating OS/2 32-bit files,
  • 16-bit files can be handled by the supplied linker.

Known issues

As with other Microsoft tools that support both OS/2 and DOS environments you may run into problems if you try to use those tools on Microsoft Windows systems that support the running of OS/2 1.x programs (Windows NT and 2000), as the system tries to run the DOS executable as an OS/2 program and fails. Either make sure that only the DOS executables are installed or if they are already installed or you migrated the install by hand, precede every command with the FORCEDOS command.

License and status

  • Closed source commercial software
  • Author: Microsoft



  • Charles Petzold: OS/2 Kernel Programming - Contains an example OS/2 16 bit MASM assembly program. - PC Magazine 1987