With ASN1C 6.5.1 separate 64-bit kits for Windows will be available. For a while versions of ASN1C prior to 6.5.0 have included 64-bit libraries for Visual Studio 2005, Visual Studio 2008, and Visual Studio 2010. The 64-bit Windows software development kits for ASN1C 6.5.1 include these libraries also, but they also include a 64-bit version of the ASN1C compiler itself, which is something that wasn't offered before.
If you're familiar with the folder hierarchy of an ASN1C Windows installation prior to version 6.5.0, you've probably seen folders with names like c_vs2005_amd64, cpp_vs2008_amd64, c_vs2010_amd64, etc. In the 6.5.0 installation those amd64 folders aren't present. That's because 6.5.0 kits are 32-bit kits, and starting with 6.5.1 you can get either 32-bit ASN1C kits or 64-bit ASN1C kits.
The 32-bit kits will have folder hierarchies named c, cpp, c_vs2005, cpp_vs2005, c_vs2010, and cpp_vs2010 (among others). All of these hierarchies contain 32-bit libraries and DLLs. The c and cpp hierarchies contain Visual Studio 2008 libraries and DLLs starting with 6.5.0.
The 64-bit kits will have folder hierarchies named c_64, cpp_64, c_vs2005_64, cpp_vs2005_64, c_vs2010_64, and cpp_vs2010_64. The amd designation is gone. All of these hierarchies contain 64-bit libraries and DLLs.
Anyone who has a Windows ASN1C license and has active support can get a 64-bit run-time kit. So customers with active support who have Windows versions of ASN1C prior to 6.5.0 that have both 32-bit and 64-bit libraries can still have both sets of libraries when they move to 6.5.1.