Difference between revisions of "Instruction"
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('instruction set' is better equivalent for 'order code' than 'object code' (see EDSAC document)) |
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| − | An '''instruction''' is, at the highest level of abstraction, the smallest unit of computation which a programmer can direct the [[Central Processing Unit|CPU]] to perform. | + | An '''instruction''' is, at the highest level of abstraction, the smallest unit of computation which a [[programmer]] can direct the [[Central Processing Unit|CPU]] to perform. |
| − | In more practical terms, the CPU operates by fetching items from [[main memory]]; each item | + | In more practical terms, the CPU operates by fetching items from [[main memory]]; each item contains a single directive for the CPU (e.g. add the contents of one [[register]] to another, or move a unit of data from a register, to main memory) - those items are instructions. (An instruction is usually a [[word]], but on some CPUs, multiple instructions could be packed in each word, while on others, some instructions took multiple words.) |
| − | {{stub}} | + | Some early computers used the alternative term '''order''' for instruction (and '''order code''' for [[instruction set]]). This was more common in the UK; especially at Cambridge (on the [[EDSAC]], and descendants such as the [[LEO]]). In the US, uses included several [[Lincoln Laboratory]] computers, and the [[PDP-1]]. |
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| + | {{semi-stub}} | ||
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| + | ==External links== | ||
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| + | * [https://www.cl.cam.ac.uk/relics/orders.notes.html The EDSAC Order Code] | ||
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| + | [[Category: Computer Basics]] | ||
Latest revision as of 08:52, 6 November 2025
An instruction is, at the highest level of abstraction, the smallest unit of computation which a programmer can direct the CPU to perform.
In more practical terms, the CPU operates by fetching items from main memory; each item contains a single directive for the CPU (e.g. add the contents of one register to another, or move a unit of data from a register, to main memory) - those items are instructions. (An instruction is usually a word, but on some CPUs, multiple instructions could be packed in each word, while on others, some instructions took multiple words.)
Some early computers used the alternative term order for instruction (and order code for instruction set). This was more common in the UK; especially at Cambridge (on the EDSAC, and descendants such as the LEO). In the US, uses included several Lincoln Laboratory computers, and the PDP-1.
