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The CPU is described as the computer’s ‘brain’ that monitors all the computer operations.  It is the unit inside the computer where all software instructions, math and logic operations are carried out.

In Microcomputers, the CPU is implemented in a single silicon device called a Microprocessor (Computer chip), which is made by combining a very large no. of transistors together using a technology referred to as Very Large Scale Integration (VLSI).


  • The CPU is one of the most expensive components of the Motherboard & also a very delicate piece of equipment.
    • The CPU has a label, such as “486” or “Pentium” to show its type, and the chip manufacturer’s logo on it.
    • It has a large fan (known as the Heat sink) screwed on top of it, that keeps the CPU cool while the system is on.  This is because the CPU gets very hot when it is in operation.

Functions of the Processor (Central processing unit).

  1. To control the use of the Main memory in storing of data & instructions.
  2. To control the sequence of operations within the computer.
  3. To give commands to all parts of the system.
  4. To carry out data processing.

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Basic features (functional elements) of a Microprocessor system.

The Processor is made up of 3 major/ main components;

  • Control Unit (CU).                                
  • Arithmetic Logic Unit (ALU).              
  • Main Memory Unit (MMU).                 

Other components include;

  • Registers.
    • Accumulator.
    • Buses.

Note. The elements making up the Computer system communicate through electronic paths called Buses.  The buses carry data, instructions, information, control commands and power between the communicating elements.


The CU acts as the manager of the computer.  Its main work is to control, supervise & co-ordinate all the activities of the various units of the computer, enabling the machine to perform useful tasks.

The CU is usually described as the Nerve centre of a computer system.  It co-ordinates & controls the activities of the different components of the computer system in the same way that the brain directs the actions of the body.

The CU carries out the fetching, decoding & execution of the instructions.  It fetches/ selects the required instruction from Main storage, stores it in a no. of special Registers, interprets the instructions, and causes the instruction to be executed by sending appropriate signals to the appropriate hardware devices.

In order to execute an instruction, the CU must do the following:

  • Fetch (get) the instruction from the Memory.
  • Decode the instruction, i.e., determine what the instruction is saying.  For example, whether to ADD or COMPARE two numbers.
  • Get the data required by the instruction.  E.g., the two numbers.
  • Activate the right circuits for the instruction to be obeyed, e.g. call into action the circuits which perform addition.

Note.  The CU automatically repeats this cycle of operations until either it is instructed to stop or the last instruction has been executed.

In order to enable the Control Unit to carry out the various functions, certain registers are used.

It controls the order in which the instructions are carried out. 

It contains the address of the next instruction in the computer program to be executed.

For the next instruction to be executed, it must be brought from the memory into the Processor.  When an instruction has been fetched from memory, the Program Counter is increased by 1, and is then ready to find the next instruction.

Therefore, the Sequence Control register sequentially points to the address of the instruction to be carried out, reads it into the Instruction Register, and automatically moves to the next instruction in the processing sequence.

  • Instruction Register.

It stores a copy of the instruction being processed.

Note. An instruction has 2 parts;

  • Operation part, e.g., Multiplication.
    • Address part.

The Instruction Register is connected to the Instruction Decoder.

  • Instruction Decoder.

It decodes (interprets) the instruction received from the Instruction Register & sends signals to the control switches of the computer.

  • Address Registers.

Each location in a memory has its own address, which allows us to get directly to any program instruction or item of data stored within the memory.

Address – A label, name or a number identifying a storage location, or a device from which information is received or to which it is transmitted.

Address Registers are 16-bit registers used for the storage of addresses.  They are connected to the Address Bus

The Address part of the instruction goes to the Address Register, which retrieves the required data item in the address indicated and copies it in the ALU so that the operation instruction may be executed.

Address modification – the process of changing the address part of a machine instruction by means of coded instructions.

  • System Clock.

In order to be able to fetch & execute instructions, the CU uses a timing signal provided by a System Clock attached to the CPU.  The Clock is an electronic system that is used to control the Processor on when to fetch the next instruction from the Main memory.

The Clock sorts out all the internal paths inside the Processor to make sure that data gets from the right place & goes to the right place.

The clock is an Oscillator that generates timing pulses (at a frequency of several Megahertz) to synchronize the computer’s operations & ensure that the operation occurs at the right time.

The CU performs 1 step of the instruction in 1 clock pulse.  The speed of processing will depend on:

  • CPU Frequency, i.e., time duration of 1 clock pulse.
  • The no. of steps involved in executing a full instruction.
  • The Access time of the computer, i.e., the time required to access any one memory address.

Functions of the Control unit.

  1. It co-ordinates & controls various parts of the computer system, namely; Main memory, ALU & the Peripheral devices.
  2. It maintains order & controls all the operations or activities inside the Processor. 

The CU controls the activities of all the other units of the computer by using the appropriate control signals.  For example, it instructs the ALU on which arithmetic or logical operations are to be performed.

  • It connects the required circuits to enable the ALU to process the data in storage, as specified by the program.
  • It directs sequence of operations, i.e., it generates synchronization signals & manages commands exchanged between the ALU, I/O units & the Memory.
  • It retrieves and interprets instructions from the Main storage & makes the computer to execute these instructions by giving commands to the rest of the computer elements.
  • It controls the transfer of unprocessed data to the Main storage & results from the Main storage.
  • It stores the results in the memory.
  • It determines the location of the memory that contains the next instruction to be retrieved.
  • It monitors the CPU operations & identifies problems, such as equipment malfunction, illogical instructions or erroneous data finding its way into the system, and flashes them on the computer screen.


This is the part of the Processor that performs all the arithmetic operations, such as adding or multiplying figures, & logical operations needed to solve a particular problem.

For example, if two numbers are to be added or multiplied, this is done by the ALU.

The ALU also performs Logic Functions, e.g., AND, NOT, OR, TRUE, FALSE & IF.

Functions of the ALU.

The ALU has 2 main functions:

  1. It carries out the Arithmetic operations.
  2. It performs logical operations and comparison on data.

Other minor functions include;

  • It performs output of variable (logical) functions.
  • It performs branching of prefixed conditions.

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  • It is a storage device in the A.L.U, where all the answers from both arithmetic & logical operations are stored temporarily before being transferred to the memory.
  • A Register in the ALU in which operands are placed, and in which arithmetical results are formed.


A computer bus consists of a set of parallel wires connecting the Processor to the other devices of the computer. 

  • It is a communication path/medium within the computer that allows different elements of the computer to communicate with each other.
  • An electrical connection linking internal sections of a computer, such as Processor, Expansion cards, and peripherals such as Keyboard and other Input devices.
  • A parallel collection of conductors that carry data and control signals from one unit to another.

There are 3 major types of computer buses:

  • Data Bus.

It is a bi-directional (2-way) bus that carries information & data to and from the Processor, i.e., it carries data from the Processor to the memory during Write operations & from memory to the Processor during Read operations.

The Data bus usually carries data for processing. 

It consists of 4, 8 or 16 lines each carrying 1 bit at a time.

Note. The Data Bus determines the Bus Width of the microprocessor.  Its size indicates the

       moving capability of information of the chip.

  • Address Bus.

It is a unidirectional (1-way) bus from the Processor to the external devices.  It usually contains the address of the memory location or device to be acted on by the Processor (i.e. it conveys addresses).

The Address bus is wide (usually between 4 – 32 lines) to enable it to address as many devices as possible.

The size of the Address Bus tells you how much memory the chip can handle.

  • Control Bus.

It’s a unidirectional (1-way) bus that carries command (i.e. timing & control) signals from the Processor.  These signals are necessary to coordinate the activities of the entire system.  For example, the command for the Printer to prepare to receive data is a control signal from the processor.

They usually have 3 – 10 lines.

Note. Buses are implemented as actual communication lines.  They may be Internal buses, which are usually laid down as a circuit on the chip itself, or they may be External buses, implemented as cables.


There are 2 basic factors (parameters/ considerations) used to classify Microprocessors.

  1. Clock speed.
  2. Width of the Data Bus.

Clock Speed.

Every Microprocessor has a clock that drives its operation.  Clock speed is the speed at which the processor runs.

Microprocessor speed refers to its Clock speed, which is measured in Megahertz (MHz – 1 million cycles per second).

The Clock speed varies from one processor to another.  Microprocessors with faster clocks perform operations much faster compared to those with slower clocks.  Therefore, the speed of a microprocessor gives its power – the higher the speed, the more powerful the microprocessor.

Note. The speed at which a computer can process data is also affected by the speed at which the memory can work.

Bus Width.

The size of the Data Bus determines the Bus Width of a microprocessor.  It indicates the moving capability of information of the chip. 

Higher Bus widths provide higher computer performance.  For example, fetching a 16-bit instruction from memory using a Data bus width of 16 bits would require a single fetch operation, whereas an 8-bit Data bus would require 2 cycles to fetch the same instruction; hence slowing the execution of the instruction.


  • Microprocessors.
  • RISC (Reduced Instruction Set Computer) processors.
  • CISC (Complete Instruction Set Computer) processors.


The Microcomputers use Microprocessors, which usually have all the required functions on one chip.

Some of the manufacturers who make microprocessor chips include;

  • Intel Inc., AMD (American Device Manufacturers) & Cyrix who manufacture microprocessors for IBM compatible microcomputers.
  • Motorolla for Apple computers.


The following are some of the Microprocessors manufactured by Intel Inc:

Processor            Year      Speed       No. of             Data    Size of         Maximum       No. of

      (MHz)       Transistors     Bus      Internal      addressable    memory 

  width   Registers    memory            Address

  (Bits)    (Bits)                                   Lines     

8088                    1979      5 – 8                __               8         16               1 MB                    20

80286                  1982      10              130,000          16         “                16 MB                  24

80386                  1985      12 -16        275,000          32       32               4 GB                    32

80486                  1989      33 – 66       1 Million           “          “                  “                                      “

Pentium 75          1993      90 –200     3 Million         64         “                  “                                      “

Pentium Pro        1995      180 –200   5,500,000         “          “                 64 GB                  36

Pentium MMX   1997       166 –233   7,500,000         “          “                  “                                      “

Pentium II Xeon 1998                  233 –300         “                  “          “                  “                                      “

Pentium III         1999                  500 –550         “                  “          “                  “                                      “

Pentium MMX has Multimedia instructions such as 3-D bitmap manipulation built in them.  The Pentium III Multimedia instructions are 4 times more powerful than those of Pentium MMX Microprocessors.


Larger computers such as Supercomputers, Mainframes & Minicomputers use Processors made of separate, high speed sophisticated components.  That is, the CPU components in large computers are not all on one chip.

RISC Processors.

The RISC processors are used in manufacture of Minicomputers.

RISC machines are much faster than Micro-processor based machines, i.e. in RISC machines, the time required to execute an operation is much shorter compared to the time a Micro-processor would take to execute the same operation.

RISC chips generally use 128 or fewer instructions to execute their tasks.  This means that, the processor has few instructions to locate.  They can execute these instructions more quickly & are therefore very fast.

Again, each instruction is exactly 32 bits long.  So, the processor does not waste any power determining where the instructions begin or end.  This makes RISC processors more superior.

They are also cheaper to produce.

Most of the 64-bit RISC machines are manufactured by: –

  • Digital Equipment Corporation (DEC).
  • Sun Microsystems.
  • Hewlett-Packard, and
  • IBM (International Business Machine Corporation).

CISC Processors.

CISC chips use between 200 – 300 instructions.  Therefore, the processor has more instructions to look up. 

The instructions are usually between 8 -120 bits long.  This means that, a CPU devotes at least part of its circuit time determining where instructions begin and end, making them run slowly.

Factors that affect the performance of a Processor.

  • Overheating.
    • Incorrect configuration (construction / arrangement).
    • Failed components.
    • Running the processor at the wrong speed.
    • Jammed or clogged or too small heat-sink / cooling fan.
    • Incompatibility.
    • Processor inserted the wrong way.

Exercise (a).

  1. Draw a well-labeled diagram showing the components of a computer Hardware model.
  2. Compare with the aid of a diagram, the physical appearance and the electronic components of a Microcomputer.
  3. (a). What is the Central Processing Unit?

(b). What parts make up the Central processing unit?

  • (a). Define the acronym A.L.U.

(b). Identify TWO functions of the above unit.

  • Write short notes on the following:
  • Control unit.
  • Arithmetic logic unit (A.L.U).
  • Main memory.
  • (a). Define Registers.

(b). List THREE types of Registers and state their functions.

  • (a). What is a Computer Bus?

(b). List 3 types of computer buses and explain their functions.

  • Explain briefly the Microprocessor operation cycle.
  • State TWO factors that determine the type of processor.

Exercise (b).

  1. Name and explain 3 main parts/ components of the Processor.
  2. What is the function of registers in a processor?
  3. List the electronic components of a Microcomputer.
  4. Draw a block diagram for a simple Microcomputer showing its four main parts.  Say what each part does.
  5. Describe the functions of each of the following in a Processor:
    1. Control unit.
    1. Arithmetic Logic unit.
    1. Accumulator.
    1. Internal Registers.
  • Draw a carefully well labeled diagram showing the architecture of a typical Microprocessor.
  • Explain in a sentence the function of each of the following components in a Control unit.
  • Clock.
  • Program Counter.
  • Instruction Register.
  • Name the 3 stages/ phases of the computer processing cycle.

Exercise (c).

  1. (a). Give the meaning of the initials CPU, and state its other name.

(b). Describe the 3 main components of the CPU.

(c). State two functions of the Central processing unit.

  • Outline the FIVE main processes under which data undergoes to become information.

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