Ensuring a Program Written for 32-bit Windows is Compatible with 64-bit Windows

Question

While it's my understanding that there's no fundamental reason a program written for 32-bit hardware / OSs not to run on 64-bit hardware / OSs, in practice, I've found many programs intended for 32-bit versions of Windows that will not work on 64-bit versions of Windows. Examples include a number of popular security utilities (most products from Norton and Check Point's Zone Alarm) and several games (I've been trying to get Grand Theft Auto 4 to run for a few weeks now, but to no avail - of course, that might be related to any number of other problems related to GTA4, but that's neither here nor there).

I've heard that a program's incompatibility might result from something as simple as not wanting to run from the "Program Files (x86)" folder, but what are some of the other reasons? Why would a virus scanner or firewall written for a 32-bit system not run on a 64-bit system? Why would a game not run when everything is theoretically backwards-compatible?

Answer 1

There is a lot of misinformation on this thread.

When a 32-bit application is run on 64-bit windows:

• Most of the compatibility problems come when the application tries to install a kernel-mode driver. A 32-bit driver can't be installed on the 64-bit OS. This is amost certainly the problem with a firewall. It's trying to hook into the TCP/IP driver stack.
• THERE IS NO EMULATOR! The 32-bit object code is executed by the cpu completely natively at full speed.
• There is no support for old 16-bit code. This broke a lot of installers.
• Accessing the right folders is generally not a problem. When a 32-bit program opens a file in, say %windir%\system32\, the OS automagically redirects it to %windir%\syswow64. The same for certain parts of the registry. There are a few potential gotchas here, but they're generally along the lines of assuming that various WINAPI Get...Directory() functions return the same strings that they did in Windows 95.
• Whether it was compiled 10 years ago or just yesterday, then C/C++ pointers are still 32-bits (4 bytes) and all of the code that just assumed that -- including SendMessage()! -- still works. The 8-byte pointer issue doesn't come into the picture until you start converting to 64-bit compilers.