Day Secondary School Independence Layout Enugu State, Nigeria. Senior Secondary Class 2 Online Note

Day Secondary School Independence Layout Enugu State, Nigeria.
SS2 Online Note
From week2-4

By Corper Francis Obinna Eyisi

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What is central processing Unit?

This is part of a computer in which operations are controlled and executed. That means CPU is the hardware device in a computer that executes all of the instructions from the software. The CPU also known as (Central Processing Unit) is the part of a computer system that is commonly referred to as the “brains” of a computer. It is also known as the processor or microprocessor. The CPU is responsible for executing a sequence of stored instructions called a program. This program will take inputs from an input device, process the input in some way and output the results to an output device.Meanwhile,CPUs aren’t only found in desktop or laptop computers, but many electronic devices now rely on them for their operation such as Mobile phones and Tablets PC.

What is Arithmetic and logic unit (ALU)?
This is a unit in a computer which carries out arithmetic and logical operations. An arithmetic-logic unit (ALU) could also be defined as the part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands(means the quantity on which an operation is to be done). in computer instruction words.

What is control Unit?

The control unit is a component of a computer’s central processing unit(CPU) that directs operation of the processor. It tells the computer’s memory, arithmetic/logic unit and input and output devices how to respond to a program’s instructions.

What is Registers?

This could be a particular part of the range of computer instrument to enter or record on an official list or directory. Example are software and computer instrument that detect and show a reading automatically.In a computer, a register is one of a small set of data holding places that are part of a computer processor.

Functions of ALU, control unit and Registers


• An arithmetic logic unit (ALU) is a digital circuit used to perform arithmetic and logic operations. It represents the fundamental building block of the central processing unit (CPU) of a computer. Some of the arithmetic operations are as follows: addition, subtraction, multiplication and division while some of the logical operations are as follows: comparison between numbers, letter and or special characters. The ALU is also responsible for the following conditions: Equal-to conditions, Less-than condition and greater than condition.
• This part of the CPU is the one that is in charge of all the operations being carried out.

• It is responsible to direct the system to execute instructions.

• It helps in communication between the memory and the arithmetic logical unit.

• It also aids in the loading of data and instructions residing in the secondary memory to the main memory as required.

Control Units

• The control unit co-ordinates the activities of the other two units as well as all peripherals and auxiliary storage devices linked to the computer.
• A control unit can be described as a sort of circuitry that supervises and controls the path of information that runs over the processor and organizes the various activities of those units that lie inside it.
• The control unit directs the entire computer system to carry out stored program instructions

• The control unit must communicate with both the arithmetic logic unit (ALU) and main memory.

• The control unit instructs the arithmetic logic unit that which logical or aithmethic operation is to be performed.

• It carries out many tasks such as decoding, fetching, handling the execution and finally storing the results.

• It controls the execution of instructions in a sequential order.

• It guides the flow of data through the different parts of the computer.

• It interprets instructions.

• It regulates the time controls of the processor.

• It sends and receives control signals from various peripheral devices.

• A control unit is a major computer part which helps in the functioning of the central processing unit and in turn runs the whole computer.


A register is the lowest level of data storage in a computer. Registers also have the lowest read/write time of all types of memory in your computer, making them idea to perform small,repeated calculations. CPU register refers to a memory allocation inside a CPU (central processing unit) Its main function is to provide a temporary storage area for dynamic information access that is stored on the hard drive. This makes access to mathematical functions easier since they are preloaded into the CPU for instant

Memory Unit.

What is memory Unit ?

It is a temporary storage area in the computer memory . It holds the data and instructions that the Central Processing Unit (CPU) needs. Before a program can be run, the program is loaded from some storage medium into the memory.

Types of Memory Unit?

We have primary storage or Memory which is also known as RAM and Secondary memory devices which includes magnetic disks like hard drives and floppy disks ; optical disks such as CDs and CDROMs .

Primary memory and secondary memory

The Memory hierarchy is very crucial operation functionality in the Computer and can be categorized into Primary and Secondary Memory. Both these memories vary in the speed, cost and capacity. Primary Memory is considered as a main memory that is accessed directly by the computer, so as to store and retrieve information. Secondary Memory is considered as an external or additional memory, this memory is not directly accessed by the CPU because, the Secondary Memory is an external storage device, It can be used as a permanent memory, because even if the computer is turned off we can retrieve the information.

Description of Primary memory and secondary memory

• Primary memory is physically made up of Random Access Memory (RAM) which is otherwise known as volatile memory. A typical personal computer will have between 1 – 4 Gigabytes of RAM outside the main CPU. The CPU itself will also contain volatile memory in the form of registers and cache while secondary memory which is otherwise known as non-volatile memory can operate outside the main CPU in a way that input/ output channels are used to access the secondary device such as hard disk.

Differences between primary and secondary memory

• Unlike primary memory, secondary memory is not directly accessed through CPU. The accessing of the primary memory through CPU is done by making use of address and data buses, whereas input/ output channels are used to access the secondary memory.

• The primary memory is embedded with two types of memory technologies; they are the RAM (Random Access Memory) and ROM (Read Only Memory). The secondary memory is accessible in the form of Mass storage devices such as hard disk, memory chips, Pen drive, floppy disk storage media, CD and DVD.

• Primary memory is volatile in nature, while secondary memory is non volatile. The information that is stored in the primary memory cannot be retained when the power is turned off. In case of secondary memory, the information can be retrieved even if the power is turned off because the data will not be destructed until and unless the user erases it.

• When the data processing speed is compared between the primary and secondary memory, the primary memory is much faster than the secondary memory. In the cost perspective, the primary memory is costlier than the secondary memory devices. Because of this reasons most of the computer users install smaller primary memory and larger secondary memory.

Unit of Storage.

A storage unit is a fixed unit of physical memory allocated to certain data. A storage sequence is a sequence of storage units. The size of a storage sequence is the number of storage units in the storage sequence. A storage unit can be numeric, character, or unspecified. Example 1g hard of drive .

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