Controller Design: Microprogrammed and Hardwired

The purpose of the control unit in a computer is to begin sequences of micro-operations. The number of various types of micro-operations that are present in a given system is finite. The complexity of the computer system is obtained from the number of sequences of micro-operations that are performed. To execute an instruction, the control unit of the CPU must generate the required control signal in the proper sequence. There are two approaches used for generating the control signals in proper sequence as Hardwired Control unit and Micro-Programmed Control unit.

1. Hardwired Control Unit :

When the control signals are produced by hardware using conventional logic design method, the control unit is termed as hardwired. The control hardware can be viewed as a state machine that charges from one state to another in every clock cycle, depending on the contents of the instruction register, the condition codes and the external inputs. The outputs of the state machine are control signals. Now we can also say that the sequence of the operation carried out by this machine is determined by the wiring of the logic elements and hence named as "hardwired". In hardwired control the fixed logic circuits that correspond directly to the boolean expressions are used to generate the control signals. Hardwired control is faster than micro-programmed control. A controller that uses this approach can operate at high speed.

2. Micro-programmed Control :

The concept of micro-programming is an refined and systematic method for controlling the micro-operation sequences in a computer system. The control signals associated with operations are stored in special memory units inaccessible by the programmer as Control Words. Control signals are generated by a program are similar to machine language programs. Micro-programmed control unit is slower in speed because of the time it takes to fetch microinstructions from the control memory.

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