Here are some useful links if you ever need to use the Microsoft Macro Assembler (MASM), which assembles and links one or more assembly-language source files.
(e.g. if building the zlib compression library)
ml64.exe is the assembler that accepts x64 assembly language.
http://msdn.microsoft.com/en-us/library/hb5z4sxd.aspx
The ML and ML64 command line reference guide can be found at
http://msdn.microsoft.com/en-us/library/s0ksfwcf.aspx
I also came across a useful discussion in a forum on MSDN, How do I get Visual Studio 2008 to use ML64.EXE instead of ML.EXE in an x64 Build?
http://social.msdn.microsoft.com/Forums/lv-LV/vcgeneral/thread/1cdb9aa7-eead-4d73-9318-18ffb4e45e21