Difference between revisions of "Instruction"
(Add order code. Maybe also ILLIAC and UNIVAC.) |
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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.) | 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 instructions, and '''order code''' for [[Object code|machine code]]. | + | Some early computers used the alternative term '''order''' for instructions, and '''order code''' for [[Object code|machine code]]. 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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* [[Instruction set]] | * [[Instruction set]] | ||
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| + | ==External links== | ||
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| + | * [https://www.cl.cam.ac.uk/relics/orders.notes.html The EDSAC Order Code] | ||
[[Category: CPU Basics]] | [[Category: CPU Basics]] | ||
Revision as of 19:06, 5 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 instructions, and order code for machine code. 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.
