Hardwired Control Unit

What is a Hardwired Control Unit?

A hardwired control is a method of generating control signals with the help of Finite State Machines (FSM). The control signals that are necessary for instruction execution control in the Hardwired Control Unit are generated by specially built hardware logical circuits, and we can’t change the signal production mechanism without physically changing the circuit structure.

Characteristics of Hardwired Control Unit

• Two decoders, sequence counter, and logic gates make up a Hardwired Control.
• The instruction register stores an instruction retrieved from the memory unit (IR).
• An instruction register consists of the operation code, the I bit, and bits 0 through 11.
• A 3 x 8 decoder is used to encode the operation code in bits 12 through 14.
• The decoder's outputs are denoted by the letters D0 through D7.
• The bit 15 operation code is transferred to a flip-flop with the symbol I.
• The control logic gates are programmed with operation codes from bits 0 to 11.
• The sequence counter (or SC) can count from 0 to 15 in binary.

Designing of Hardwired Control Unit

The following are some of the ways for constructing hardwired control logic that have been proposed:

• Sequence Counter Method - It is the most practical way to design a somewhat complex controller.
• Delay Element Method - For creating the sequence of control signals, this method relies on the usage of timed delay elements.
• State Table Method - The standard algorithmic approach to designing the Notes controller utilising the classical state table method is used in this method.

Working of a Hardwired Control Unit

The basic data for control signal creation is contained in the operation code of an instruction. The operation code is decoded in the instruction decoder. The instruction decoder is a collection of decoders that decode various fields of the instruction opcode.

Pros of Hardwired Control Unit

• Hardwired Control Unit is quick due to the usage of combinational circuits to generate signals.
• The amount of delay that can occur in the creation of control signals is dependent on the number of gates.
• It can be tweaked to get the fastest mode of operation.
• Quicker than a micro-programmed control unit.

Cons of Hardwired Control Unit

• As we require additional control signals to be created, the design becomes more complex (need for more encoders or decoders).
• Changes to control signals are challenging since they necessitate rearranging wires in the hardware circuit.
• It's difficult and time-consuming to add a new feature.
• It's difficult to evaluate and fix flaws in the initial design.
• It's a bit pricey.