64-bit

64-bit means using computer words containing sixty-four bits. This adjective often refers to the number of bits used internally by a computer's CPU. E.g. "The Sun UltraSPARC is a 64-bit processor". Its external data bus or address bus[?] may be narrower, and the term is also often used to describe the size of these buses as well. Many current 32-bit machines use 64-bit buses for instance. The term may also refer to the size of an instruction in the computer's instruction set or to any other item of data.

64-bit processor architectures include:

• the DEC Alpha architecture
• Intel's IA-64 architecture
• AMD's x86-64 architecture
• Sun UltraSPARC architecture

See also:

• computer memory

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64-bit is a term used to describe a computer architecture based around an arithmetic and logic unit (ALU), registers, and data bus which are 64 bits wide.

64-bit processors are quite common, e.g. Digital Alpha, versions of Sun SPARC, and the IBM AS/4000[?]. The PowerPC and Intel are expected to move to 64 bits at their next generation - PPC 620[?] and Intel's IA-64.

A 64-bit address bus allows the processor to address 18 million gigabytes as opposed to the mere 4 gigabytes allowed with 32 bits. Floating point calculations can also be more accurate.

Often mixed architectures are used: with 32-bit integer/addresses and 64-bit floats.

Taking full advantage of a 64-bit CPU requires a A 64-bit operating system, but backward-compatible architectures can also run a 32-bit OS. For example, processors based on the AMD Hammer architecture can run Intel x86 compatible software, whereas processors based on IA64 architecture need to use software emulation.

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Partly based on material from FOLDOC, used with permission.