Computer Architecture
Topic name:-Design of control unit, Central Processing Unit: Introduction, General Register Organization
Introduction:-
Control Unit
It is the unit which controls all the operations of the different units but does not carry out any actual data processing operation. Control unit transfers data or instruction among different units of a computer system. It receives the instructions from the memory, interprets them and sends the operation to various units as instructed.
Control unit is also responsible for communicating with all input and output devices for transferring or receiving the instruction from the storage units. So, the control unit is the main coordinator since it sends signals and find the sequence of instructions to be executed.
Central Processing Unit
A central processing unit (CPU) is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logical, control and input/output (I/O) operations specified by the instructions.
General Register Organization
When we are using multiple general purpose registers, instead of single accumulator register, in the CPU Organization then this type of organization is known as General register based CPU Organization. In this type of organization, computer uses two or three address fields in their instruction format. Each address field may specify a general register or a memory word. If many CPU registers are available for heavily used variables and intermediate results, we can avoid memory references much of the time, thus vastly increasing program execution speed, and reducing program size.
Design of Control Unit:-
Control unit can be designed by two methods which are given below:
1. Hardwired Control Unit
2.Microprogrammed Control Unit
Hardwired Control Unit:-
It is implemented with the help of gates, flip flops, decoders etc. in the hardware. The inputs to control unit are the instruction register, flags, timing signals etc. This organization can be very complicated if we have to make the control unit large.
If the design has to be modified or changed, all the combinational circuits have to be modified which is a very difficult task.
Advantages And Disadvantages Of Hardwired Control Unit
Advantages
• It is faster than the microprogrammed control unit.
• It can be optimized to produce the fast mode of operation.
Disadvantages
• Instruction set, the control logic is directly implemented.
• It requires a more cheap area, therefore, it is costlier control unit.
Microprogrammed Control Unit:-
It is implemented by using programming approach. A sequence of micro operations is carried out by executing a program consisting of micro-instructions. In this organization any modifications or changes can be done by updating the micro program in the control memory by the programmer.
Advantages And Disadvantages Of Microprogrammed Control Unit
Advantages
• The decoders and sequencing logic unit of microprogrammed control unit are very simple pieces of logic, compared to the hardwired control unit.
• More flexible to accommodate with new instruction.
Disadvantages
• Slower than a hardwired unit of comparable technology.
Difference Between Hardwired And Microprogrammed Control Unit
Hardwired control unit
It is a circuitry approach.
It works well for simple instructions.
Implementing hardwired structure requires a cost.
Microprogrammed control unit
This control unit is implemented by programming.
It works well for complex instructions also.
Implemented microprograms is not costly.
Central Processing Unit:-
The central processing unit or CPU is a part of the computer that performs a bulk of the data processing operations. The CPU is made up of three major parts — the control unit, memory unit and the arithmetic and logic unit (ALU). The memory unit consists of a set of registers that stores intermediate data and is used during the execution of the instructions. The ALU performs the required micro-operations for executing instructions. The control unit supervises the transfer of information among the registers and instructs the ALU to perform the required micro-operations. The CPU generates a variety of operations stated by the instructions format that are integrated in a computer system. The instruction formats, addressing modes, instruction sets and the general organisation of the CPU register defines the computer structure. This computer structure is nothing but the computer architecture. The instruction set provides the specifications for the design of the CPU. CPU designing refers to a task of selecting a particular hardware for implementing the machine’s instructions. The computer programmer must be aware of the register set, memory structure and the instruction operations while programming in a machine or assembly language.
General Register Organization:-
In a computer system, memory locations are important while performing instructional operations. Memory locations are the pointers and counters that store temporary return addresses and temporary results of operations. Memory access is the most time consuming process in a computer system — a more convenient solution for this, is to use processor registers to store intermediate values.
A common bus is used when a large number of registers are used in the CPU. To perform a variety of micro-operations and direct data transfers, registers communicate with each other. Therefore, it is compulsory to give a common unit that can perform all the arithmetic operations, logic operations and shift micro-operations in a computer processor.
Figure 1.4 shows a bus organisation for the seven CPU registers. The output of each register is connected to the two multiplexers. It forms two buses, called A and B. The selection lines from each multiplexer selects the input data from each register, for the particular bus.
Conclusion:-
Control Unit is the part of the computer’s central processing unit (CPU), which directs the operation of the processor. It is the responsibility of the Control Unit to tell the computer’s memory, arithmetic/logic unit and input and output devices how to respond to the instructions that have been sent to the processor.
Hardwired Control Unit implemented with the help of gates, flip flops, decoders etc.
Microprogrammed Control Unit implemented by using programming approach.
When we are using multiple general purpose registers, instead of single accumulator register, in the CPU Organization then this type of organization is known as General register based CPU Organization. In this type of organization, computer uses two or three address fields in their instruction format.
The central processing unit or CPU is a part of the computer that performs a bulk of the data processing operations. The CPU is made up of three major parts — the control unit, memory unit and the arithmetic and logic unit (ALU).
References:-
1. Design of Control Unit https://www.studytonight.com/computer-architecture/design-of-control-unit, accessed on 12/02/2021 at 10:00pm.
2. Design of Control Unit https://www.geeksforgeeks.org/introduction-of-control-unit-and-its-design/, accessed on 12/02/2021 at 10:15pm.
3. Difference-between Hardwired and Microprogrammed Control Unit https://www.geeksforgeeks.org/difference-between-hardwired-and-microprogrammed-control-unit/, accessed on 12/02/2021 at 10:30pm.
4. Advantages and Disadvantages of hardwired control unit https://enemsproject.wordpress.com/2016/01/19/advantages-of-hardwired-control-unit-over-the-micro-program-control-unit/, accessed on 12/02/2021 at 10:45pm.
5. General Register Organization https://moonwelfarefoundation.blogspot.com/2019/05/general-register-organization.html, accessed on 13/02/2021 at 11:15pm.
