Overview of 32-bit and 64-bit objects

It is important to generally understand how 32-bit and 64-bit objects are related in a computing environment, since there can be several dependencies between them that can affect everything from what hardware you purchase to how you deploy your applications.

32-bit and 64-bit hardware
32-bit computer hardware uses 32 bits to represent memory addresses and to process instructions and data. 64-bit hardware uses 64 bits to do the same thing. In general, 32-bit operating systems run on 32-bit hardware and 64-bit operating systems run on 64-bit hardware, although it is possible to run 32-bit operating systems on some 64-bit hardware.


32-bit and 64-bit operating systems
Operating systems consist of a kernel that interfaces directly with the hardware and operating system libraries. Operating systems come with either a 32-bit or 64-bit kernel, or, in some cases, both. In general, 32-bit operating system kernels can exploit four gigabytes of real memory (the physical RAM shared by the operating system and the running applications), whereas 64-bit operating system kernels can exploit more than this. Of course, some 32-bit operating system kernels can exploit more than four gigabytes of memory, but don't do this as well as 64-bit kernels. 64-bit kernels may be required on some operating systems to run 64-bit applications, which cannot be run if a 32-bit kernel is in use. This is the case with all UNIX operating systems, with the exception of AIX. On AIX, a particular exception, you can run 32-bit and 64-bit applications with either a 32-bit or 64-bit kernel; however, to prevent scalability problems it is better to use the 64-bit kernel even when running 32-bit applications.
Operating system libraries are important, because they are required to build and run applications. To build 32-bit applications, you have to link to 32-bit system libraries. To build 64-bit applications you have to link to 64-bit system libraries. Having the 64-bit system libraries available in a particular operating system doesn't necessarily mean that the system is actually capable of running 64-bit applications. This is often true of the 32-bit Windows operating system, which allows you to cross-compile and link 64-bit applications without being able to run them. This is also true of the UNIX platforms, because there are cases where you can install 64-bit-capable versions of operating systems on 32-bit hardware and cases where 32-bit kernels are incapable of running 64-bit applications. Essentially you can consider a 32-bit operating system to be one that only has the capacity to run 32-bit applications, whereas 64-bit operating systems can also run 64-bit applications, although this capability requires the use of 64-bit hardware and might additionally require the use of a 64-bit operating system kernel.

32-bit and 64-bit applications
32-bit applications are compiled such that memory addresses are 32-bit (four bytes) in size. These applications can directly exploit up to four gigabytes of virtual memory - the memory potentially available for use on a computer. This virtual memory constraint is present regardless of the amount of real memory (RAM) available on the system to be shared between the operating system and other applications. 64-bit applications on the other hand are compiled such that memory addresses are 64-bits (eight bytes) in size and can use more than four gigabytes of virtual memory without restriction. Operating systems typically also impose additional virtual memory restrictions on applications, and the theoretical maximum virtual memory per application may be as little as 1-2 gigabytes, even though it may have 32-bit addressing capability.
When you compile an application on a 32-bit or 64-bit platform, by default it is generally compiled to run on that particular platform. You can create 64-bit applications on 32-bit operating systems and 32-bit applications on 64-bit operating systems with some compilers by using special compiler-specific compile options and by appropriately linking to 32-bit or 64-bit libraries where appropriate.

32-bit applications can generally be run on both 32-bit and 64-bit operating systems, although many operating systems cannot run 64-bit applications if they employ a 32-bit kernel.