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Central Processing Unit
CPU is the hardware part of a computer that carries out the instruction of a computer
program by performing the basic arithmetic, logical and input/output operations of the
system. CPU is the brain of the computer where most operations, calculations and processes
take place.
Two main components of a CPU
1. The arithmetic logic unit (ALU) – it performs arithmetic and logical operation.
ALU carries out arithmetic operations on integer (whole number) and real (with a
decimal point) operands.
2. The control unit (CU) – it extracts instruction from the computer memory, decodes
and executes them and also transferring them to the ALU when necessary. The
CU controls the fetching of instructions from the main memory and the subsequent
execution of these instructions.
Control unit Functions
1. The control unit directs the entire computer system to carry out stored program
instructions.
2. The control unit must communicate with both the arithmetic logic unit and main
memory.
3. The control unit uses the instruction contained in the Instruction Register to decide
which circuits need to be activated.
4. The control unit co-ordinates the activities of the other two units as well as all
peripheral and auxiliary storage devices linked to the computer.
5. The control unit instructs the arithmetic logic unit which arithmetic operations or
logical operation is to be performed.
6. The control unit is literally in control.
Functions of Arithmetic Logic Unit
1.
2.
3.
4.
The arithmetic logic unit executes arithmetic and logical operations.
Arithmetic operations include addition, subtraction, multiplication and division.
Logical operations compare numbers, letters and special characters.
Comparison operations test for three conditions:
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equal-to condition in which two values are the same
less-than condition in which one value is smaller than the other
greater-than condition in which one value is larger than the other
5. Relational operations (=, <, >) are used to describe the comparison operations used
by the arithmetic logic unit.
6. The arithmetic logic unit performs logic functions such as AND, OR and NOT
Memory Unit
The memory unit is the part of the computer that holds data and instructions for processing.
Although it is closely associated with the CPU. In actual fact, it is separate from it. Memory
unit associated with CPU is also called Primary Storage and main storage, internal storage
and main memory. When we load software from a floppy disk, hard disk or CD-ROM, it is
stored in the main memory.
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The Memory Unit is the part of the computer that holds data and instructions for
processing.
Although it is closely associated with the CPU, in actual fact it is separate from it.
Memory associated with the CPU is also called primary storage, primary memory,
main storage, internal storage and main memory.
When we load software from a floppy disk, hard disk or CD-ROM, it is stored in the
Main Memory.
There are two types of primary memory (RAM) and (ROM).
RAM – stands for Random Access Memory. This is the main storage and is the place where
the programs and software we load gets stored.
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This is really the main store and is the place where the programs and software we
load gets stored. When the Central Processing Unit runs a program, it fetches the
program instructions from the RAM and carries them out.
If the Central Processing Unit needs to store the results of calculations it can store
them in RAM.
Random Access Memory can have instructions READ from it by the CPU and also it
can have numbers or other computer data WRITTEN to it by the CPU.
The more RAM in your computer, the larger the programs you can run.
When we switch a computer off, whatever is stored in the RAM gets erased.
ROM – stands for Read Only Memory. The CPU can read instructions from read only memory.
ROM comes with instructions permanently stored inside and these instructions cannot be
over written by the computer CPU. ROM is used for storing special sets of instruction which
the computer needs when start up. When we switch the computer off, the content of the
ROM does not become erased but remain stored permanently. Therefore, it can’t be
deleted.
The CPU can only fetch or read instructions from Read Only Memory (or ROM). ROM comes
with instructions permanently stored inside and these instructions cannot be over-written
by the computer’s CPU.
ROM memory is used for storing special sets of instructions which the computer needs when
it starts up.
When we switch the computer off, the contents of the ROM does not become erased but
remains stored permanently. Therefore it is non-volatile
Types of memory
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Primary memory (main memory)
Secondary memory (auxiliary memory)
Types of secondary memory
1. Internal memory
2. External memory
Types of storage devices
1.
2.
3.
4.
5.
Floppy disk
Hard disk
Flash drive
Computer disk (CD)
Digital video disk (DVD)
Logic Circuits
Logic gates are physical devices that perform logical operations on one or more logic inputs
to produce an output. They are mainly diodes or transistors though can also be constructed
using electromagnetic rays. A logic gate is an elementary building block of a digital circuit.
Most logic gates have two inputs and one output. At any given moment, every terminal is
in one of the two binary conditions low (0) or high (1), represented by different voltage
levels. Both the input and output carry the binary valves ‘O’ which mean low or false ‘
I’ meaning high or true.
Types of logic gates
1. AND gate: Suppose we have two inputs AND gate A and B, the A.B denotes A and B
which are both connected to the AND gate. This gate gives a high output if all inputs
are high.
INPUT (A)
0
1
0
1
INPUT (B)
0
0
1
1
OUTPUT (A and B)
0
0
0
1
2. NOT gate: Also called an inverter, the Not gate implements the logical negation.
The Not gate outputs a voltage representing the opposite logic level of its input i.e.
when the input is 0, the output will be 1 and vice versa.
INPUT (A)
0
1
OUTPUT (B)
1
0
3. OR gate: The OR gate gives a high output, If at least one of the input is high. For a
2 input circuit with input A and B then A+B denote that A and B are both connected
to the OR gate.
INPUT (A)
INPUT (B)
0
0
1
0
0
1
1
1
Uses of standard Logic Circuit
OUTPUT (A and B)
0
1
1
1
1. Logic gates are building blocks of hardware electronic components.
2. It is used in activation of door bells.
3. The AND gates use to combine multiply signals
4. The NOT gate is used in building a switch.