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an associated set of microinstructions in digital computers. Each microinstruction indicates the machine micro-operations or micro-orders, the address of the next microinstruction, the duration of the microinstruction itself, and special actions relating to test operations. One microprogram can call out another as a microsubroutine. By changing the sequence and composition of the microinstructions—that is, by altering the structure of a microprogram—the system of instructions for a digital computer can be changed, thus adapting it to a certain class of problems or providing program compatibility with another digital computer.
Microprograms are usually stored in a specialized memory, which responds more rapidly than a direct-access memory. The length of a microprogram is usually 10–100 microinstructions, and a microinstruction requires 16–100 or more binary digits. The length of a microprogram in small digital computers is 256–1,024 16-digit words; in medium and large computers it is 1,024–8,196 words of 50 to 100 digits each.
REFERENCESBoulaye, G. Mikroprogrammirovanie. Edited by M. D. Pebart. Moscow, 1973. (Translated from French.)
A. V. GUSEV
microcodeA set of elementary instructions in a complex instruction set computer (CISC). The microcode resides in a separate high-speed memory and functions as a translation layer between the machine instructions and the circuit level of the computer. Microcode enables the computer designer to create machine instructions without having to design electronic circuits. Writing microcode is called "microprogramming," and the microcode for a given computer is called a "microprogram."
RISC computers do not use microcode, which is the reason why RISC compilers generate more instructions than CISC compilers.
Source Code to Machine Code to Microcode
When software is written, the source code is converted into machine instructions by assemblers and compilers. At execution time, the machine instructions are converted into microinstructions, and the microinstructions cause transistors to open and close in the circuits. See microinstruction, CISC and RISC.