Open Watcom Assembler

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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

Open Watcom Assembler or WASM is an x86 assembler produced by Watcom, based on the Watcom Assembler found in Watcom C/C++ compiler and Watcom FORTRAN 77.[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]

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

Technical details[edit]


  • 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]

WASM forks[edit]


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 (or rather, was out of contact) of JWASM in January 2014 with version 2.12pre, but others on the Masm32 forum[10] picked up where Japheth left off. Japheth reappeared on github (


HJWasm, adding the prefix H in reference to Masm32 forum member Habran who started off this second WASM development continuation. Version 2.13pre was originally announced in 2016.[11] New features include:

  • Instructions:
  • SIMD:
  • MMX: MOVQ and added in 2.13, to supplement MOVD.
  • AVX2: VGATHERDPD, VGATHERQPD, VGATHERDPS, VGATHERQPS, VPGATHERDD, VPGATHERQD, VPGATHERDQ, VPGATHERQQ, VEX-encoded general purpose instructions added in 2.13. Remaining instructions added in 2.16.
  • AVX-512: VCMPxxPD, VCMPxxPS, VCMPxxSD, VCMPxxPD, VCMPxxSS, AVX-512F set, EVEX-encoded instructions added in 2.13; VMOVQ added 2.13, to supplement MOVD. Remaining instructions added in 2.16.
  • Random Number Generator: RDRAND, RDSEED added in 2.13.
  • half-precision conversions: F16C(VCVTPH2PS, VCVTPS2PH) added in 2.13.
  • Intel MPX: Added in 2.31.
  • Registers: RIP, ZMM registers added in 2.13.
  • OO language extension added in 2.25.


HASM is a renamed version of HJWASM, starting in version 2.33. The name was used following a MASM Forum discussion thread that originally proposed a replacement name. The name HASM was proposed by forum member habran in Reply #6,[12] and was finalized at the end of discussion thread at Reply #33.[13] No known features are added in HASM's release cycle.


The name was actually used in version 2.33 (dated 2017-05-20) at Terraspace ltd's product page,[14] but it was only announced in version 2.34.[15] Changes to HJWASM includes:[16]

  • Instruction sets: RDPID added in 2.38.
  • Addressing modes: 64bit absolute immediate (2.37)
  • Identified types
  • Record types: fully supports registers and up to 32bit record fields in 2.41.
  • Support for 128bit: Added in 2.42, inline declaration with the type added in 2.43.1 / .2.
  • Support of typedef chain on return types added in 2.46.8.
  • m512 built-in types added in 2.47.
  • Console colour coding: Added for Windows, OSX and Linux in 2.43.1 / .2.
  • Function calling: C-style function calling added in 2.46.


  1. ^ Randall Hyde. "WASM: The Open Watcom Assembler". Archived from the original on 2012-03-02. Retrieved 2017-09-11.{{cite web}}: 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. ^ a b 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
  10. ^ UASM Assembler Development
  11. ^ HJWasm Releases
  12. ^ A New Name? (thread page 1)
  13. ^ A New Name? (thread page 3)
  14. ^ UASM (2.33)
  15. ^ UASM 2.33 Release
  16. ^ UASM ChangeLog

External links[edit]