Basic Concepts of Control (Digital Logic Fundamentals)

1. Design of Control Unit
The Control Unit is a fundamental component of a computer's central
processing unit (CPU), responsible for directing the operation of the
processor. Control unit generates timing and control signals for the
operations of the computer. The control unit communicates with ALU and
main memory. It also controls the transmission between processor,
memory and the various peripherals. It also instructs the ALU which
operation has to be performed on data.

2. Sequencing and Execution
State Machine Concept
Sequencing is implemented using a State Machine Concept, which is crucial
for the orderly processing of instructions within the Control Unit.
Hardwired or Microprogramming
The state machine logic is generally implemented using either Hardwired
Combinational logic or Microprogramming.
Control Signals
Execution is nothing but generating the necessary control signals.

3. Classification of Design of the Control Unit
Hardwired Control
The Hardwired Control organization involves the
control logic to be implemented with gates, flip-
flops, decoders, and other digital circuits.
Microprogrammed Control
The Microprogrammed Control organization, in
contrast, is implemented by using the programming
approach.

4. Hardwired Control Organization
1
Components
A Hard-wired Control consists of two
decoders, a sequence counter, and a
number of logic gates. 2 Instruction Register
An instruction fetched from the memory
unit is placed in the instruction register
(IR), which includes various components
for operation.
3
Operation Codes
The operation code in bits 12 through 14
are coded with a 3 x 8 decoder, and the
outputs are designated by symbols D0
through D7, at bit 15 is transferred to a
flip-flop designated by the symbol I, from
Bits 0 through 11 are applied to the
control logic gates.
4 Sequence Counter
The Sequence counter (SC) can count in
binary from 0 through 15, playing a crucial
role in the sequencing process.

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6. Characteristics of Hardwired Control Unit
1 Highest Speed
Provides the highest speed of execution.
2 Complex Logic
Uses complex sequencing & micro-operation
logic.
3 Difficult Design
Difficult to design and test.
4 Inflexible Design
Has an inflexible design.
5 Adding New Instructions
Difficult to add new instructions.
6 RISCs
It is implemented with hardwired control.
They are said to be one instruction per clock
cycle machines. Three-bus CPU structures,
Register to Register operations and
Hardwired control unit make it a reality.
7 Limitations
When the instructions set becomes complex like CISCs, the Hardwired Control Unit is unviable.

7. The Design Process of a
Hardwired Control Unit
When constructing a hardwired control logic, there are several methods
that can be used.
The Sequence Counter Method offers a practical approach for designing
a somewhat complex controller.
The Delay Element Method relies on timed delay elements to create the
sequence of control signals.
The State Table Method utilizes the classical state table approach to
design the controller.

8. Hardwired Control in RISC
1 One Instruction Per Cycle
RISCs are implemented with hardwired control and are known for executing
one instruction per clock cycle machines.
2 CPU Structure
Three-bus CPU structures and Register to Register operations are facilitated
by the Hardwired Control Unit.
3 Efficiency
The Hardwired Control Unit contributes significantly to the efficiency and
speed of RISC architectures.

9. Limitations of Hardwired Control Unit
1 Complexity Increases
The complexity of the design
increases as we require more
control signals to be generated
(need of more encoders &
decoders).
2 Difficult Modifications
Modifications in the control signals
are very difficult because it requires
rearranging of wires in the hardware
circuit.
3 Adding New Features
Adding a new feature is difficult &
complex.
4 Testing & Correcting Mistakes
Difficult to test & correct mistakes in
the original design.
5 Expensive
It is expensive.