Here are some (partial) answers that might help you:
- The code you posted is 16-bit assembly for DOS, so an x64 assembler is not going to help you.
- If you are actually looking for a 64-bit capable assembler for the Windows platform, there are several. Among them would be NASM and Microsoft's MASM, which (depending on the version) comes with the DDK and/or Visual Studio.
- I don't know if this is of interest to you but there are also some 64-bit compilers which (unlike VC++ x64) allow inline assembly (e.g. Delphi x64).