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What is Hardwired Control Unit?


Hardwired Control Unit, often abbreviated as HCU, represents a pivotal component in the architecture of computer systems. It functions by utilizing Finite State Machines (FSM) to generate control signals, a process commonly referred to as hardwired control. This method involves physically integrating various electronic components such as flip-flops, gates, and drums to construct a sequential logic circuit. The term "hardwired controller" stems from the intricate wiring and circuitry required to facilitate its operation, distinguishing it from other control mechanisms in computing systems

Hardwired Control Unit: What Is It?

Using Finite State Machines (FSM) to generate control signals is known as a hardwired control. The Hardwired Control Unit's hardware logical circuits are specifically designed to generate the control signals required for instruction execution control. It is not possible to alter the signal production process without also altering the circuit topology.

What is Hardwired Control Unit?

How a Hardwired Control Unit Works?

The fundamental information required to generate a control signal is contained within an instruction's operation code. This operation code is decoded by the instruction decoder. Different fields of the instruction opcode are decoded by a set of decoders collectively known as the instruction decoder.

As a result, only a limited number of output lines from the instruction decoder carry active signals. These active signals are received by the computer's executive units via connections to the inputs of a matrix. This matrix combines the decoded signals from the instruction opcode with outputs from the matrix to generate signals indicating the successive states of the control unit and signals from external sources, such as interrupt signals. The construction of these matrices mirrors that of programmable logic arrays.

What is Hardwired Control Unit?

Generation of a Signal

Instead of being generated at a single point in time, control signals for instruction execution must be generated over the entire time range that corresponds to the instruction execution cycle. The control unit uses this cycle's structure to determine the proper order in which to arrange the internal states.

Several signals are returned to the inputs of the subsequent control state generator matrix by the control signal generator matrix. Depending on the rectangular patterns that the quartz generator normally provides, this matrix combines these signals with the timing signals produced by the timing unit. Every time a new instruction reaches the control unit, it starts the process of fetching it. As long as the computer's timing signals and other input signals, such flags and state information, remain unaltered, instruction decoding enables the control unit to enter the initial state pertinent to the new instruction execution. The status of the control unit changes in response to modifications in any of the signals mentioned before.


This results in the formation of a new appropriate input for the control signal generator matrix. The next control state, which deals with the response to the external signal (such as interrupt processing), is entered by the control unit when an external signal, such as an interrupt, occurs. The computer's state variables and flags are used to select the proper states for each cycle of instruction execution

The final states of the cycle are control states that start retrieving the subsequent instruction from the program. This involves reading the instruction word into the computer's instruction register and sending the program's counter content to the location of the main memory buffer register. When the running instruction is the stop instruction, which ends program execution, the control unit enters an OS state where it waits for the next user direction.

What is Hardwired Control Unit?


Because combinational circuits are used to create signals, Hardwired Control   Units operate quickly.

The number of gates determines how much delay can happen when control signals are created.

To acquire the fastest mode of operation, it can be adjusted.

faster than a control unit with microprogramming.


The design gets more complicated (more encoders or decoders are needed) as more control signals must be produced.

Modifications to control signals are difficult since they require reconfiguring the hardware circuit's wires.

Introducing a new feature takes work and effort.

Evaluating and correcting errors in the original design is challenging.

It costs a little amount.


  • A Hardwired Control consists of a sequence counter, two decoders, and logic gates.
  • An instruction read from the memory unit (IR) is stored in the instruction register.
  • Bits 0 through 11 together with the operation code and I bit make up an instruction register.
  • Bits 12 through 14 of the operation code are encoded using a 3 x 8 decoder.
  • The letters D0 through D7 stand for the decoder's outputs.
  • The flip-flop with symbol I receives the bit-15 operation code.
  • Operation codes ranging from bits 0 to 11 are put into the control logic gates.
  • The binary count of the sequence counter, or SC, ranges from 0 to 15.


Some methods for building hardwired control logic that have been suggested are as follows:

Method of Sequence Counter - It is the most useful method for creating a moderately intricate controller.

Method of Delay Element - This approach uses timed delay components to provide the sequence of control signals.

Method of State Tables - This solution uses the traditional algorithmic methodology to create the Notes controller using the classical state table method.