CPUs that process 64 bits as a single unit, compared to 8, 16 or 32. Today's fastest computers are 64-bit machines; however, that does not mean they are twice as fast as 32-bit computers, because the 64 bit "word size" is only one aspect of internal processing. The CPU's clock speed, along with the speed of storage, RAM and input/output (the 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 MemoryA major advantage of 64-bit operating systems is their support for more RAM than a 32-bit OS, which is typically limited to 4GB. For example, 64-bit Windows 10 Pro supports up to 512GB. See PAE.A Lot of 32-Bit SoftwareAlthough CPUs migrated to 64-bits years ago, many 32-bit applications are still running in their 64-bit personal computers. Windows comes in both 32-bit and 64-bit versions, but Apple upgraded the Mac OS entirely to 64 bits in 2009.The complete migration from 32-bit applications to their 64-bit counterparts may take a long time. For example, people still run 16-bit DOS and Windows applications in their 32-bit Windows PCs that were written more than two decades ago. This is why some users 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.