Difference between revisions of "Word"
(Expan discussion somewhat; cover machine 'width' too) |
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| − | In the widest sense, a '''word''' is an ordered group of bits considered together. | + | In the widest sense, a '''word''' is an ordered group of bits considered together. Different computer architectures have different word sizes. A word may denote data processed by the [[Central Processing Unit|CPU]], an [[instruction]], an [[address]], or the unit by/in which [[main memory]] is accessed. In early and simple computers, these were to a large extent the same width. However, contemporary practice is generally to give each [[byte]] in main memory its own address. Instructions are also now often smaller than the [[arithmetic logic unit|ALU]] width; [[physical address|physical]] and [[virtual address]]es are different sizes, etc. |
Words are now usually a power-of-two multiple of an 8-bit byte in size, but in early computers, other word sizes were common. 36 bits was a very popular size (for reasons of accuracy of physical computations, the principal early [[application]] for computers) on early [[mainframe]]s, such as the [[IBM 704]] and its descendants; the [[Honeywell 6000 series]]; and the [[PDP-10]] family. However, small word sizes were common too, e.g. the 12-bit words of the [[PDP-8 family]]. | Words are now usually a power-of-two multiple of an 8-bit byte in size, but in early computers, other word sizes were common. 36 bits was a very popular size (for reasons of accuracy of physical computations, the principal early [[application]] for computers) on early [[mainframe]]s, such as the [[IBM 704]] and its descendants; the [[Honeywell 6000 series]]; and the [[PDP-10]] family. However, small word sizes were common too, e.g. the 12-bit words of the [[PDP-8 family]]. | ||
Latest revision as of 11:42, 27 October 2025
In the widest sense, a word is an ordered group of bits considered together. Different computer architectures have different word sizes. A word may denote data processed by the CPU, an instruction, an address, or the unit by/in which main memory is accessed. In early and simple computers, these were to a large extent the same width. However, contemporary practice is generally to give each byte in main memory its own address. Instructions are also now often smaller than the ALU width; physical and virtual addresses are different sizes, etc.
Words are now usually a power-of-two multiple of an 8-bit byte in size, but in early computers, other word sizes were common. 36 bits was a very popular size (for reasons of accuracy of physical computations, the principal early application for computers) on early mainframes, such as the IBM 704 and its descendants; the Honeywell 6000 series; and the PDP-10 family. However, small word sizes were common too, e.g. the 12-bit words of the PDP-8 family.
Computer 'widths'
Machines' 'widths' are often considered to be the machine's word-size, but the discussion is complicated, and made even more confusing, because there is no universally accepted standard for what sets a machine's 'width'. Indeed, for most suggested benchmarks, one can find a machine which violates it:
- address space (KA10; 18 bits - 36-bit machine)
- instruction size (Motorola M68000 family; 16 bits - 32-bit machine)
- bus width (Motorola MC68000; 16 bits - 32-bit machine)
- ALU width (LSI-11; 8 bits - 16-bit machine)
Thus, the 'word' size is now usually used as the measure of the machine's 'size'.
