Control Memory in Computer Architecture

Control Memory in Computer Architecture – In a digital computer, the function of the control unit is to initiate sequences of microoperations. When the control signals are generated by hardware using conventional logic design techniques, the control unit is said to be hardwired. Microprogramming is a second alternative for designing the control unit of a digital computer.

The control function that specifies a micro-operation is a binary variable. When it is in one binary state, the corresponding micro-operation is executed. A control variable in the opposite binary state does not change the state of the registers in the system. The active state of a control variable may be either the 1 state or the 0 state, depending on the application. In a bus-organized system, the control signals that specify micro-operations are groups of bits that select the paths in multiplexers, decoders, and arithmetic logic units.

The control unit initiates a series of sequential steps of micro-operations. During any given time, certain micro-operations are to be initiated, while others remain idle. The control variables at any given time can be represented by a string of 1’s and 0’s called a control word. As such, control words can be programmed to perform various operations on the components of the system.

A control unit whose binary control variables are stored in memory is called a microprogrammed control memory. Each word in control memory contains a microinstruction. The microinstruction specifies one or more micro-operations for the system. A sequence of microinstructions constitutes a microprogram. Since alterations of the microprogram are not needed once the control unit is in operation, the control memory can be a read-only memory (ROM).

The content of the words in ROM is fixed and cannot be altered by simple programming since no writing capability is available in the ROM. ROM words are made permanent during the hardware production of the unit. The use of a microprogram involves placing all control variables in words of ROM for use by the control unit through successive read operations. The content of the word in ROM at a given address specifies a microinstruction.

The general configuration of a microprogrammed control unit is demonstrated in the block diagram given below. The control memory is assumed to be a ROM, within which all control information is permanently stored. The control memory address register specifies the address of the microinstruction and the control data register, which holds the microinstruction read from memory.