Module 4-
Control Unit Design
-Neha Surti
Hardwired Control Unit
Hardwired Control Unit
• The decoder/encoder is a combinational circuit that generates a set of required control
signals.
• A control step counter is used to keep track of the control steps.
• Each count of this counter corresponds to one control step.
• The required control signals are determined by the following information:
1. contents of the control step counter
2. contents of IR register
3. contents of the condition code flags
4. External input signals, like MFC and interrupt request.
Hardwired Control Unit
Hardwired Control Unit
• The step decoder generate a separate signal line for each step, or time slot, in the control
sequence.
• The instruction decoder decodes the instruction loaded in IR.
• The output of the instruction decoder consists of a separate line for each of the ‘m’
machine instruction.
• According to the code in the IR, only one line amongst all output lines of decoder is set
to 1 and all other lines are set to 0.
• The input signals to encoder are combined to generate the individual control signals like
add, read, etc.
• The End signal starts a new instruction fetch cycle by resetting the control step counter
to its starting value.
• When run=1, the counter to be incremented by one at the end of every clock cycle.
• When run=0, the counter stops counting, and this is needed whenever the WMFC signal
is activated.
Hardwired Control Unit
Advantages of Hardwired Control Unit:
• Fast because control signals are generated by combinational circuits.
• The delay in generation of control signals depends upon the number of gates.
Disadvantages of Hardwired Control Unit:
• More the control signals required by CPU, more complex will be the design of control
unit.
• Modification in control signal are very difficult i.e., it requires rearranging of wires in
the hardware circuit.
• Difficult to add new feature in existing design of control unit.
Hardwired Control Unit Design Methods
• State-table Method
• Delay-element Method
• Sequence-counter Method
State-table Method
Delay-element Method
• There is a specific time delay between activation of two control signals or group of
control signals.
• To ensure synchronous operation, the delay elements are implemented by D-flip flops
and controlled by a common clock signal.
Sequence-counter Method
Microprogrammed
Control Unit
Micro Instruction-Format
• Horizontal microinstruction
• Vertical microinstruction
Horizontal microinstruction
Vertical microinstruction
Wilkes's Microprogrammed Control Unit
Wilkes's Microprogrammed Control Unit
Wilkes's Microprogrammed Control Unit
Hardwired Vs Micro-programmed Control Unit
Hardwired Control Unit Microprogrammed Control Unit
Hardwired control unit generates the control signals needed for the processor Microprogrammed control unit generates the control signals with the help of
using logic circuits micro instructions stored in control memory
Hardwired control unit is faster when compared to microprogrammed control This is slower than the other as micro instructions are used for generating
unit as the required control signals are generated with the help of hardware signals here
Difficult to modify as the control signals that need to be generated are hard Easy to modify as the modification need to be done only at the instruction
wired level
Less costly than hardwired control as only micro instructions are used for
Costlier as everything has to be realized in terms of logic gates
generating control signals
It cannot handle complex instructions as the circuit design for it becomes
It can handle complex instructions
complex
Only limited number of instructions are used due to the hardware
Control signals for many instructions can be generated
implementation
Used in computer that makes use of Reduced Instruction Set Computers(RISC) Used in computer that makes use of Complex Instruction Set Computers(CISC)
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