The operations performed by a computer are controlled by a computer program. A computer program consists of instructions to the electronics of the computer system to process data in some manner. Program instructions are stored in main computer storage as machine language instructions which the electronic circuits of the computer system can interpret and execute. A Knowledge of the basic characteristics of machine language instructions is useful in understanding how a computer operates.
A machine language instruction is composed of: 1) An operation code; 2) Values indicating the number of characters to be processed by the instruction; 3) The addresses of the data to be used in the processing.
The operation code is a unique value which is typically stored in a single byte. This unique value indicates to the computer electronics what operation is to occur. For example, the letter of the alphabet “A” stored as the operation code might indicate that an addition operation is to occur. The letter “0” might mean that division is to take place.
The number of characters to be processed must be included in a computer instruction so that the electronic circuitry of the processor unit will reference the proper number of digits in the field to be processed. For example, if a four digit field were to be added to another four digit field, the number of characters specified in the instruction for each field would be four.
The locations of the fields involved in the operation must be specified in the instruction. This enables the circuitry to identify where in storage the data to be processed is located.
Although the formats of computer instructions vary a great deal between different computer systems, the basic elements of operation code, number of characters to process, and the addresses of the data to be processed will almost always be present.
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