64-bit CPUs

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As with other new technologies, hype and misinformation about 64-bit CPUs have been readily available. Here are the differences between 32- and 64-bit CPUs:

• All of the 32-bit applications that you have now should still work on a 64-bit system.
• If you use a 64-bit OS, you'll be able to run your current 32-bit programs side-by-side with 64-bit programs, although you may have trouble with system-level stuff.
• Support for DOS-only and Microsoft Windows 3.x applications has been removed (http://support.microsoft.com/?kbid=896458&SD=tech) in Microsoft Windows XP 64 and will not work, although DOS-based programs which use 32-bit extenders (mostly games) may work. Other OSes may have their own design restrictions.
• Support for 32-bit drivers has been removed in Microsoft Windows XP 64; you must use 64-bit drivers.
• Many of your current programs will actually run a little faster, due to some 32-bit bottlenecks in the CPU being widened to 64 bits. For example, the CPU can fetch data from memory 8 bytes at a time instead of 4 bytes at a time.
• If you use a 64-bit OS, many of your current programs will actually run a little faster, due to some 32-bit bottlenecks in the OS being widened to 64 bits. For instance, the OS can move data into a hard drive buffer 8 bytes at a time instead of 4 bytes at a time.
• If you get 64-bit versions of your programs, they will generally run faster than the 32-bit (or older) versions, due to bottlenecks being widened to 64 bits. For instance, a graphics program can move images around 8 bytes at a time instead of 4 bytes at a time.
• In addition to being able to handle data in wider chunks, 64-bit CPUs have been given the ability to address more data. Most modern 32-bit CPUs have a limit of 4 gigabytes of physical memory and 64 gigabytes of virtual memory (although the practical limit seems to be 8 gigabytes).
  • 64-bit AMD CPUs are starting with a 1 terabyte (1024 gigabyte) physical memory limit.
  • 64-bit AMD CPUs are starting with a 256 terabyte (262,144 gigabyte) virtual memory limit.
  • The 64-bit IBM G5 used by Apple currently has a 4 terabyte (4096 gigabytes) physical memory limit.
  • 64-bit Intel CPUs are starting with a 16 terabyte (16,384 gigabyte) physical memory limit.
  • Intel and AMD 64-bit CPUs have a 4 petabyte (4096 terabytes) future physical memory limit.
  • Intel and AMD 64-bit CPUs have a 16 exabyte (16,384 petabytes) future virtual memory limit.
• Windows 2000 and Windows XP can allocate up to 4 gigabytes of memory to each application, but half (1G in certain situations (http://support.microsoft.com/?kbid=328882)) of that is set aside for system use (handling dialogs, icons, buttons, and so forth on behalf of the application). 4 gigabytes is the most memory that a 32-bit application can use. If you use a 64-bit Windows OS, 32-bit applications can use up to a full 4 gigabytes; the memory set aside for the OS can come out of the memory space that the application cannot access.

For the most part, 64-bit CPUs are mostly pluses, with few minuses. One minus is that the CPUs will not work on 32-bit motherboards (however, you may be able to find 32-bit CPUs that will work on 64-bit motherboards). This isn't a big problem, though, since most upgrades require motherboard replacement anyway. Prices do not differ much from 32-bit CPUs when comparing their effective speeds. Many new higher-end 32-bit systems are already at or near the 4 gigabyte memory limit. For this and other reasons, many vendors have started pushing for 64-bit systems, especially now that Intel has finally come out with 64-bit desktop CPUs.

For more information on how much a terabyte, petabyte, and exabyte are in real-world terms, see How many bytes for... (http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci944596,00.html).