CPUs that process 64 bits as a single unit, compared to 8, 16 or 32. Today's fastest computers are 64-bit machines; however, they are not twice as fast as 32-bit computers, because the 64 bit "word size" relates to only one aspect of internal processing. The CPU's clock speed, along with the speed, size and architecture of the disks, memory and peripheral bus all play important roles in a computer's performance (see throughput). In many cases, the perceived difference can be negligible. See word. More Memory A major advantage to 64-bit operating systems is that they support more memory than a 32-bit OS, which is typically limited to 4GB. For example, while the 32-bit version of Windows 7 Professional handles 4GB, the 64-bit version supports up to 192GB. See PAE. A Lot of 32-Bit Software Although CPUs migrated to 64-bits years ago, the vast majority of people still run 32-bit operating systems and applications in their 64-bit personal computers. New versions of Windows come in 32-bit and 64-bit versions, and Apple upgraded OS X for the Mac entirely to 64 bits in 2009 (see Snow Leopard). The complete migration from 32-bit applications to their 64-bit counterparts may take many years. For example, people still run 16-bit DOS and Windows applications in their PCs that were written more than two decades ago. This is why some users opt to install the 32-bit version of Windows, because 64-bit Windows does not run 16-bit apps natively (see Windows XP Mode). For examples of 64-bit hardware, see Intel 64, AMD64, Opteron and Athlon. See 32-bit computing and bit specifications.