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instruction code[in′strək·shən ‚kōd]
(in digital computers), a set of instructions that assigns algorithms for solving problems in a digital computer; the principal part of the machine language. Problem-solving programs are compiled according to definite rules with the aid of an instruction code. An instruction code is usually presented in the form of tables that give mnemonic designations of the instructions according to their structure, as well as declarations of format, restrictions on usage, and all the computer operations controlled by the instructions.
An instruction code is not the same as an operations system. Two computers that have the same operations systems may differ in their instruction codes—for example, in their instruction address or instruction content (the complex of operations that are combined by each operation code). The efficiency in solving different problems depends to a considerable extent on the instruction code’s capability of producing the necessary algorithms. Consequently, this is one of the fundamental parameters determining the structure of a digital computer. An instruction code is selected by simulation of the structural scheme of the planned computer, by experimental programming using the chosen code, and by evaluation and comparison of the results. In general-purpose digital computers of low and medium capacity, the number of different operations in the instruction code varies from 32 to 64, and in high-performance computers it is in the range of 100 or more.
In modern digital computers the instruction code can be replaced or reorganized within certain limits by using microprogrammed control and extended by connecting to the computer additional hardware, needed, for example, to process data in decimal notation when solving problems in economics.
An instruction code is an intermediate step between the programmer’s language and the computer’s performance in executing a problem-solving program. Therefore, the program for the solution is executed in two stages: translation into the instruction language and conversion of the instructions into a control sequence of signals. The two-stage process simplifies the structure of the computer.
A. V. GUSEV