General Register organization & Components of CPU and their functions:

May 26, 2019

CPU or Central processing unit is the brain of the computer system. Functions of CPU varies from data processing to controlling input-output devices. Each and every instruction no matter how complex or simple, it has to go through the CPU. In this article we will learn various components of CPU and their functions.

So, let us say we press a key on the keyboard and it instantly appears on our computer monitor so the CPU of the computer is what makes this possible. The central processing unit is also responsible for storing data or information, intermediate results and instructions in the memory system. It also controls the operations of all other parts of the computer system.

Functions of a CPU:

CPU generally performs the arithmetical and logical operations, controlling of different input-output devices. These operations are performed based on some predefined algorithms and instructions normally referred as computer programs.

A computer program is a set of instructions written by a human to perform a specific operation by the CPU. A computer program is normally stored in the memory unit of the Central Processing Unit.

A CPU mainly consists of ALU (Arithmetic & Logic Unit), Control Unit and Memory Unit. These 3 units are the primary components of a CPU. Various functions of CPU and operations are generally performed by these 3 units are described below.

Components of CPU and their functions :

Memory unit(storage component):

The primary job of the memory unit is to store data or instructions and intermediate results. Memory unit supplies data to the other units of a CPU. In Computer Organization, memory can be divided into two major parts primary memory and secondary memory. Speed and power and performance of a memory depends on the size and type of the memory.

When an instruction is processed by the central processing unit, the main memory or the RAM (Random Access Memory) stores the final result before it is sent to the output device. All inputs and outputs are intermediate and are transmitted through the main memory.

Control unit (Control Component)

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.

Arithmetic and logic unit(Execution Component)

ALU can also be subdivided into 2 sections namely, arithmetic unit and logic unit. It is a complex digital circuit which consists of registers and which performs arithmetic and logical operations. Arithmetic sections perform arithmetic operations like addition, subtraction, multiplication, division etc. All other Complex operations can also be performed by repetition of these above basic operations.

The logic unit is responsible for performing logical operations such as comparing, selecting, matching and merging of different data or information.

So basically ALU is the major part of the computer system which handles different calculations. Depending on the design of ALU it makes the CPU more powerful and efficient.

A decoder is a combinational logic circuit that converts binary information from the n coded inputs to a maximum of 2ⁿ unique outputs. They are used in a wide variety of applications, including data demultiplexing, seven segment displays, and memory address decoding.

A mutliplexer (Mux) is a device used to select a single line of input from multiple input lines using control signals. In this diagram, D0 to D3 are input data lines and Y is the output.

General Register organization

Generally CPU has seven general registers. Register organization show how registers are selected and how data flow between register and ALU. A decoder is used to select a particular register.The output of each register is connected to two multiplexers to form the two buses A and B. The selection lines in each multiplexer select the input data for the particular bus.

The A and B buses form the two inputs of an ALU.The operation select lines decide the micro operation to be performed by ALU. The result of the micro operation is available at the output bus. The output bus connected to the inputs of all registers, thus by selecting a destination register it is possible to store the result in it.

A bus organization for seven CPU register

EXAMPLE:

• To perform the operation R3 = R1+R2 We have to provide following binary selection variable to the select inputs.

1. SEL A : 001 -To place the contents of R1 into bus A.

2. SEL B : 010 - to place the contents of R2 into bus B

3. SEL OPR : 10010 – to perform the arithmetic addition A+B

4. SEL REG or SEL D : 011 – to place the result available on output bus in R3.

Register and multiplexer input selection code

Binary code	SELA	SELB	SEL-D or SEL-REG
000	Input	Input	---
001	R1	R1	R1
010	R2	R2	R2
011	R3	R3	R3
100	R4	R4	R4
101	R5	R5	R5
110	R6	R6	R6
111	R7	R7	R7

Operation with symbol

Operation selection code	Operation	symbol
0000	Transfer A	TSFA
0001	Increment A	INC A
0010	A+B	ADD
0011	A-B	SUB
0100	Decrement A	DEC
0101	A AND B	AND
0110	A OR B	OR
0111	A XOR B	XOR
1000	Complement A	COMA
1001	Shift right A	SHR
1010	Shift left A	SHL

What is CONTROL WORD?

• The combined value of a binary selection inputs specifies the control word.

• It consist of four fields SELA,SELB,and SELD or SELREG contains three bit each and SELOPR field contains four bits thus the total bits in the control word are 13-bits.

SEL A

SELB

SELREG OR SELD

SELOPR

FORMATE OF CONTROL WORD

1. The three bit of SELA select a source registers of the a input of the ALU.

2. The three bits of SELB select a source registers of the b input of the ALU.

3. The three bits of SELED or SELREG select a destination register using the decoder.

4. The four bits of SELOPR select the operation to be performed by ALU.

CONTROL WORD FOR OPERATION R2 = R1+R3