If you're porting your application to x64, and you use much in the way of __asm in your x86 code, you're likely to start looking at ml64 - the 64 bit version of Masm. The reason you're likely to do this is that the x64 compiler doesn't support __asm blocks...

My role for the past 6 months (and the next several) I'm charged with effectively "going down with the ship" WRT the current generation of code gen & tools for Visual C++. This includes the linker, Masm, ml64, DIA, c2, pdb, and a few other miscellaneous...