Open Watcom Assembler

Open Watcom Assembler or WASM is an x86 assembler produced by Watcom and included as part of the Watcom C/C++ compiler.[1][2][3] Further development is being done on the 32- and 64-bit JWASM project,.[4] which more closely matches the syntax of Microsoft's assembler.[5]

Open Watcom Assembler
Original author(s)Open Watcom Assembler
Operating systemDOS for x86-based PCs, Microsoft Windows, Linux for x86-based PCs, OS/2 for x86-based PCs, FreeBSD for x86-based PCs.
Available inEnglish
Typex86 assembler

There are experimental assemblers for PowerPC, Alpha AXP, and MIPS.[6]

Technical details


  • Native support for output formats Intel OMF output formats
  • Supports Intel x86 (Pentium MMX, Pentium III-4, 3DNow!, SSE and SSE2) instruction sets.
  • Supports Microsoft macro assembler (MASM) 5 and 6 syntax (incomplete).[6]


There is an associated Watcom disassembler, wdis. The assembler does not have listing facilities; instead the use of wdis for generating listings is recommended.[7] wdis can read OMF, COFF and ELF object files and PE and ELF executables. It supports 16-bit and 32-bit x86 instruction set including MMX, 3DNow!, SSE, SSE2, and SSE3. Support for PowerPC, Alpha AXP, MIPS, and SPARC V8 instruction sets is also built in.[8]


JWasm is a fork of Wasm originated by Japheth with following features:

  • Native support for output formats Intel OMF (16/32-bit), MS Coff (32-bit and 64-bit), Elf (32-bit and 64-bit), Bin and DOS MZ.
  • Precompiled JWasm binaries are available for DOS, Windows and Linux. For OS/2 and FreeBSD, makefiles are supplied.
  • Supports Intel x86 (8086, 80186, 80286, 80386, 80486, Pentium, Pentium Pro), x86-64 instruction sets with SIMD (MMX, 3DNow!, SSE, SSE2, SSE3 and SSSE3, SSE4.1/2 (since Jwasm), AVX (since JWasm 2.06), VMX (Intel VT-x; AMD SVM, the latter though already implemented, currently still inactive) extensions (since JWasm 2.09)).
  • JWasm is written in C. The source is portable and has successfully been tested with Open Watcom, MS VC, GCC and more.
  • On Windows, JWasm can be used with both Win32Inc and Masm32. Since v2.01, it will also work with Sven B. Schreiber's SBS WALK32 Win32 Assembly Language Kit[9]
  • C header files can be converted to include files for JWasm with Japheth's own dedicated h2incX.
  • JWasm's source code is released under the Sybase Open Watcom Public License, which allows free commercial and non-commercial use.
  • Fully supports Microsoft macro assembler 6 syntax, all MASM 8 instructions sets.

Japheth ceased development of JWASM in January 2014, but others on the Masm32 forum[10] picked up where Japheth left off, adding support for AVX2 and developing support for AVX-512. JWASM has since then been named HJWASM, adding the prefix H in reference to Masm32 forum member Habran who started off this second WASM development continuation. As of May 20th 2017, HJWASM has once again been renamed, this time to UASM, for ease of pronunciation and greater awareness purposes.

The source is currently being hosted at Github[11][12]


  1. Randall Hyde. "WASM: The Open Watcom Assembler". Archived from the original on 2004-02-29. Retrieved 2017-09-11.CS1 maint: BOT: original-url status unknown (link)
  2. Leiterman, James (2005). "MASM vs. NASM vs. TASM vs. WASM". 32/64-bit 80x86 assembly language architecture. Wordware Publishing, Inc. p. 481. ISBN 978-1-59822-002-5. Retrieved 2010-02-01.
  3. Leiterman p482 on Google Books
  4. JWASM, a 32/64 bit assembler based on WASM with syntax similar to MASM. Archived 10 October 2014
  5. Fog, Agner (2009), Optimizing subroutines in assembly language (PDF) (2009-09-26 ed.), p. 13
  6. Open Watcom website: Assembler Archived 2006-07-15 at the Wayback Machine
  7. OpenWatcom: "No listing files are generated [by the assembler]. Producing full listings may be a waste of effort because wdis (the Open Watcom disassembler) does a very good job. However, it could be extremely helpful to produce a dump of the internal symbol table the way MASM does, especially for diagnostic purposes."
  8. Open Watcom website: Disassembler Archived 2006-07-15 at the Wayback Machine
  9. The 1996 "WALK32 consists of the following main components:
    • A full-featured PE (Portable Executable) file linker called W32Link.
    • A main include file, containing Win32 constant, type, and structure definitions.
    • Another include file, containing the application and DLL startup source code.
    • Segment and PE section management macros.
    • Macros related to Unicode support.
    • Several demo applications and DLL’s.
    • A collection of programming utilities for various purposes." walk32.doc in
  • wasm at
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