The Control Unit is classified into two major categories: Hardwired Control Microprogrammed Control Hardwired Control The Hardwired Control organization involves the control logic to be implemented with gates, flip-flops, decoders, and other digital circuits. The following image shows the block diagram of a Hardwired Control organization. A Hard-wired Control consists of two decoders, a sequence counter, and a number of logic gates. An instruction fetched from the memory unit is placed in the instruction register IR.
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Hardwired, Microprogrammed Control Unit What is a Hardwired Control Unit Hardwired Control Unit is implemented using various electronic components such as combinational logic units and gates. The circuit uses a fixed architecture. If the instruction set is changed, the wiring should also be changed.
As it is hardwired, the instruction set is constant and does not change. Figure 1: Basic Structure of the Computer Usually, these control units execute faster.
However, Hardwired Control Units are difficult to modify and implement. It is also difficult to add new features to the existing design. Therefore, it has minimum flexibility. A control variable refers to a binary variable that specifies micro-operations.
A control word is a set of ones and zeros in a control variable. The control memory contains control words. Microprogrammed Control Unit has a sequence of microinstructions stored in control memory. These microinstructions refer to a control word that resides in control memory, containing control signals for execution of micro-operations. Overall, these control units have a simple structure.
Therefore, it is easier to design, implement and test. Moreover, Microprogrammed Control Units are easier to modify. Difference Between Hardwired and Microprogrammed Control Unit Definition Hardwired Control Unit is a unit that uses combinational logic units, featuring a finite number of gates that can generate specific results based on the instructions that were used to invoke those responses. Microprogrammed Control Unit is a unit that contains microinstructions in the control memory to produce control signals.
Speed The speed of operations in Hardwired Control Unit is fast. The speed of operations in Microprogrammed Control Unit is slow because it requires frequent memory accesses.
Modification To do modifications in a Hardwired Control Unit, the entire unit should be redesigned. In Microprogrammed Control Unit, modifications can be implemented by changing the microinstructions in the control memory.
Therefore, Microprogrammed Control Unit is more flexible. Handling Complex Instructions Also, it is difficult for Hardwired Control Unit to handle complex instructions, but is easier for the Microprogrammed Control Unit to handle complex instructions.
Instruction set Size In additon to the above differences, the Hardwired Control Unit uses a small instruction set while theMicroprogrammed Control Unit uses a large instruction set. Control Memory Also, there is no control memory usage in Hardwired Control Unit but, on the other hand, Microprogrammed Control Unit uses control memory.
Conclusion In conclusion, the main difference between Hardwired and Microprogrammed Control Unit is that a Hardwired Control Unit is a sequential circuit that generates control signals while a Microprogrammed Control Unit is a unit consisting of microinstructions in the control memory to generate control signals.
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Computer Organization | Hardwired v/s Micro-programmed Control Unit
There are two approaches used for generating the control signals in proper sequence as Hardwired Control unit and Micro-programmed control unit. Hardwired Control Unit — The control hardware can be viewed as a state machine that changes 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 the control signals. Fixed logic circuits that correspond directly to the Boolean expressions are used to generate the control signals.
Difference between Hardwired and Micro-programmed Control Unit | Set 2
Comparatively slow What are Control Signals? It decides what operation has to be performed, what must be the sequence of the operations performed by the processor, in what time an operation must be executed and so on. What is Hardwired Control Unit? In simple words, the hardwired control unit generates the control signals to execute the instructions in a proper sequence and at the correct time. The hardwired control unit is created with the hardware; it is a circuitry approach. It is designed for the RISC style instruction set. A hardwired circuit organization is shown in the figure below.
Difference Between Hardwired and Microprogrammed Control Unit