UASM

UASM is a free MASM-compatible assembler based on JWasm with these features:

native support for output formats Intel OMF, MS Coff (32/64-bit), Elf (32/64-bit), Binary, Windows PE (32/64-bit) and DOS MZ.
precompiled UASM binaries are available for Windows, Linux and OSX.
Instructions up to AVX2 and AVX512F are supported including all new extensions for VMX, MPX, AES, BND, F16C etc.
Support for MS Vectorcall on x64.
Support for Borland Register Calling Convention.
Full support for SystemV Calling Convention.
Integrated Macro Library with OO support.
Numerous new HLL features (as described in the extended manual).
UASM is written in C. The source is portable and has successfully been tested with Open Watcom, MS VC, GCC and more.
As far as programming for MS Windows is concerned, UASM can be used with both WinInc (32/64-bit) and Masm32 (32-bit).
C header files can be converted to include files for UASM with h2incX.
UASM's source code is released under the Sybase Open Watcom Public License, which allows free commercial and non-commercial use.
There's a bunch of source samples available - they are supplied with the precompiled binary packages.
JWasm started as a fork of Open Watcom's Wasm in March 2008. Today, the part of Wasm source lines still contained in JWasm is approximately 15%.
UASM is a continued evolution of JWasm.
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 Please visit http://www.terraspace.co.uk/uasm.html for more info.
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 UASM is released under the Sybase Open Watcom Public License version 1.0
 and under the Apache License, Version 2.0.