Win64Object – The Yasm Modular Assembler Project
wiki:Win64Object

The win64 or x64 object format generates Microsoft Win64 object files for use on the 64-bit native Windows XP x64/Vista x64/7 x64 platforms. Object files produced using this object format may be linked with 64-bit Microsoft linkers such as that in Visual Studio 2005 and 2008 in order to produce 64-bit PE32+ executables.