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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:

See also:

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.

Partly based on material from FOLDOC, used with permission.

All Wikipedia text is available under the terms of the GNU Free Documentation License

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