Q. Can I run 32-bit Windows applications under 64-bit Windows 7?
A. Yes. Virtually any 32-bit Windows application that is supported on Windows XP can run unmodified under 64-bit Windows. This is made possible by a technology known informally as "Win32 on Win64" (WOW for short), which translates 32-bit API calls from a legacy Win32 executable into 64-bit API calls that can be serviced by the native subsystems of 64-bit Windows 7. The net result is that 32-bit applications run seamlessly on 64-bit Windows and, thanks to optimizations in current generation Intel and AMD CPUs, at or near full speed. The few exceptions to the WOW compatibility rule usually involve applications that rely on one or more proprietary legacy 32-bit device drivers that have no equivalent 64-bit versions.
Note that the WOW concept is really nothing new. A similar technique was employed by the earliest versions of Windows NT to support legacy 16-bit Windows 3.xx applications.
Q. When I install a 32-bit application under 64-bit Windows 7, I can't see its registry entries. Why is this?
A. The 64-bit versions of Vista and Windows 7 include the WOW translation layer for running 32-bit applications (see description above). In addition to translating API calls, 64-bit Windows isolates registry changes made by 32-bit applications and redirects them to a special sub-key within the appropriate registry hive.
For example, a 32-bit application that updates a key within the HKEY_LOCAL_MACHINE\Software structure will automatically have its changes redirected to the Wow6432Node key underneath the primary Software key. Expanding this key will reveal all of the 32-bit application registry keys and values that have been automatically redirected by WOW.
Q. Is there a performance advantage to using 64-bit Windows 7?
A. That depends. If you're working with large files or running applications that consume a great deal of memory, then 64-bit Windows will typically give you a slight performance advantage over 32-bit Windows running on identical hardware. This is true even when using 32-bit applications. That's because the OS and device drivers themselves benefit from the 64-bit wide registers of an Intel or AMD CPU running in 64-bit extended mode. In addition, because 64-bit Windows 7 supports more physical RAM than 32-bit Windows 7 (192GB on non-Home versions vs. 4GB for any 32-bit flavor), you can easily expand your PC's capabilities well beyond what is possible in a 32-bit world.
Q. Why does 64-bit Windows use more RAM than 32-bit Windows?
A. Any 64-bit OS will consume more memory than its 32-bit equivalent. This is due to the nature of 64-bit code: It uses larger internal structures that necessarily take up more space, both in RAM and on the hard disk. It's no surprise that the ISO image for the 64-bit versions of Vista and Windows 7 are roughly 50 to 70 percent larger than the equivalent 32-bit ISOs, or that 64-bit Windows shows 20 to 30 percent higher physical memory utilization after initial boot-up.
Q. Are there security advantages to using 64-bit Windows 7 vs. 32-bit Windows 7?
A. Yes. Many of the widely publicized kernel "hardening" initiatives Microsoft debuted with Vista are specific to the 64-bit flavor -- things like hardware-backed Data Execution Prevention and PatchGuard. Also, 64-bit Windows Vista and Windows 7 require device drivers to be digitally signed by their authors, making it tougher for hackers and root-kit developers to install their exploits covertly as kernel-mode drivers.
Q. Can I upgrade from a 32-bit flavor of Windows to 64-bit Windows 7?
A. No. Microsoft's upgrade process does not currently support moving between processor architectures. In order to upgrade from a 32-bit version of Windows you'll need to perform a "clean" installation of 64-bit Windows 7, then migrate your applications and data to the new OS.
This story, "Which Windows 7 Is Right for You -- 32-Bit or 64-Bit?" was originally published by InfoWorld.