Before we start discussing the topic of developing 64-bit program code, let us speak about backward compatibility of 64-bit Windows versions, with 32-bit applications. Backward compatibility is arranged through the mechanisms implemented in WoW64.
WoW64 (Windows-on-Windows 64-bit) is a subsystem of Windows operating system, which allows you to execute 32-bit applications on all the 64-bit versions of Windows.
The WoW64 subsystem does not support the following programs:
Different processor architectures have a bit different WoW64. For example, the 64-bit Windows version developed for Intel Itanium 2 processor, employs WoW64 to emulate x86 instructions. This emulation is rather resource-intensive in comparison to WoW64 for Intel 64 architecture, because the system has to switch from the 64-bit mode to compatibility mode when executing 32-bit programs.
WoW64 on Intel 64 (AMD64 / x64) does not require instruction emulation. In this case the WoW64 subsystem emulates only the 32-bit environment through an additional layer between a 32-bit application, and the 64-bit Windows API. In some places this layer is thin, in others a bit thicker. For an average program, you may expect 2% performance penalty because of this layer. For some programs, it can be larger. Two per cent is not very much, but keep in mind that 32-bit applications work a bit slower under the 64-bit Windows than in the 32-bit environment.
Compilation of 64-bit code does not only allow you to avoid using WoW64, but also gives you an additional performance gain. This is explained by architectural modifications in the microprocessor, such as an increased number of general-purpose registers. For an average program, you may expect a 5-15% performance gain after recompilation alone.
Because of the WoW64 layer, 32-bit programs are less efficient in the 64-bit environment than in their native 32-bit one. However, simple 32-bit applications can still get one benefit of being executed in the 64-bit environment. Maybe you know that a program built with the switch "/LARGEADDRESSAWARE:YES" can allocate up to 3 Gbytes of memory, if a 32-bit Windows is launched with the switch "/3gb". Well, the same 32-bit program built on a 64-bit system can allocate almost 4 Gbytes of memory (in practice it is usually about 3.5 Gbytes).
The WoW64 subsystem isolates 32-bit programs from 64-bit ones by redirecting calls to files and the register. It helps to keep 32-bit programs from accidentally accessing the data of 64-bit ones. For example, a 32-bit application that launches a DLL file from the catalogue "%systemroot%\System32" can accidentally address a 64-bit DLL, which is incompatible with the 32-bit program. To avoid this, the WoW64 subsystem redirects the access from the folder "%systemroot%\System32" into the folder "%systemroot%\SysWOW64". This redirection helps you avoid compatibility errors because the 32-bit application will need a special DLL file created to work with 32-bit applications.
To learn more about the mechanisms of file system and register redirection see MSDN section "Running 32-bit Applications".
It is impossible to load a 32-bit DLL from a 64-bit process, and execute its code. It is impossible due to the design of 64-bit systems. It is impossible fundamentally. And no tricks and undocumented means will help you. To do this you will have to load and initialize WoW64, not to mention the kernel structures. Actually, it means that a 64-bit process must be made 32-bit "on the fly". This topic is described more thoroughly in the post "Why can't you thunk between 32-bit and 64-bit Windows?". The only thing we can recommend is to create a surrogate process, and work with it through the COM technology. You may read about it in the article "Accessing 32-bit DLLs from 64-bit code".
But it is quite easy to load resources from a 32-bit DLL into a 64-bit process. You may do it specifying the flag LOAD_LIBRARY_AS_DATAFILE when calling LoadLibraryEx.
It would be quite natural if the Microsoft Company were to stimulate the move to 64-bit systems, by gradually canceling the support of 32-bit programs in some versions of the Windows operating system. Of course it will be a very slow process, but the first steps in this direction have been already made.
Many administrators know about a relatively new installation and operation mode of the server version of the operating system, called Server Core. It is that very mode the participants of "Windows vs Linux" wars have been speaking of for a long time. One of the reasons that adherents of using Linux on servers referred to was the capability to install the server operating system without graphical interface (GUI). But here, there is such a capability in Windows Server too. Now, if you install the system in this mode, you will get only the command line without user interface.
This capability (Server Core installation), appeared in Windows Server 2008. Yet in Windows Server 2008 R2 there is another innovation that brings the 64-bit future even closer. Support of 32-bit applications is now optional, and you may enable or disable it when installing Windows Server 2008 R2 (Server Core). Moreover, this option is disabled by default. So when trying to launch a 32-bit application in Server Core mode, you will get a message telling you that it is impossible. Of course you may add 32-bit application support:
start /w ocsetup ServerCore-WOW64
In the usual (Full Installation) mode, execution of 32-bit applications is enabled by default, but not in Server Core.
The tendency is obvious. It will be more and more rational to create 64-bit versions of applications in time, as they will be able to work on more operating system versions.
Alexey Pahunov's Russian blog is also a very interesting source of information on WoW64: http://blog.not-a-kernel-guy.com/. Alexey is a worker for the Microsoft Company, and he personally participates in developing the WoW64 subsystem.
The rightholder of the course "Lessons on development of 64-bit C/C++ applications" is OOO "Program Verification Systems". The company develops software in the sphere of source program code analysis. The company's site: http://www.viva64.com.