Hardwired Control Unit | Computer Organization and Architecture | Saral Bhasha me

Hardwired Control Unit | Computer Organization and Architecture | Saral Bhasha me

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Computer Organization | Hardwired Control Unit
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.

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. The sequence of the operation carried out by this machine is determined by the wiring of the logic elements and hence named as “hardwired”.

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.
RISC architecture is based on hardwired control unit
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Micro-programmed Control Unit –

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.
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.
Design of Control Unit
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).
The component of an instruction register includes; I bit, the operation code, and bits 0 through 11.
The operation code in bits 12 through 14 are coded with a 3 x 8 decoder.
The outputs of the decoder are designated by the symbols D0 through D7.
The operation code at bit 15 is transferred to a flip-flop designated by the symbol I.
The operation codes from Bits 0 through 11 are applied to the control logic gates.
The Sequence counter (SC) can count in binary from 0 through 15.
Micro-programmed Control
The Microprogrammed Control organization is implemented by using the programming approach.

In Microprogrammed Control, the micro-operations are performed by executing a program consisting of micro-instructions.

The following image shows the block diagram of a Microprogrammed Control organization.

Design of Control Unit
The Control memory address register specifies the address of the micro-instruction.
The Control memory is assumed to be a ROM, within which all control information is permanently stored.
The control register holds the microinstruction fetched from the memory.
The micro-instruction contains a control word that specifies one or more micro-operations for the data processor.
While the micro-operations are being executed, the next address is computed in the next address generator circuit and then transferred into the control address register to read the next microinstruction.
The next address generator is often referred to as a micro-program sequencer, as it determines the address sequence that is read from control memory.

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