Computer hardware: Class 9 Computer Science

Key points

A computer is an electronic machine that accepts data, processes it under the control of instructions, stores it and gives out information. Data means raw facts such as numbers and names. Information is data that has been processed so that it has meaning.

Hardware means the physical parts of a computer that you can see and touch, such as the keyboard, monitor, mouse and system unit. Software means the programs that tell the hardware what to do. Hardware cannot do useful work without software.

The computer works in four basic steps called the information processing cycle:

• Input: data and instructions enter the computer.
• Processing: the CPU works on the data.
• Storage: data and results are kept for later use.
• Output: results are shown or printed for the user.

Input devices send data and instructions into the computer. Common examples are:

• Keyboard: enters text, numbers and commands using keys.
• Mouse: a pointing device used to move a cursor and click.
• Scanner: converts printed pictures or pages into digital form.
• Microphone: records sound.
• Webcam and digital camera: capture pictures and video.
• Touch screen: lets the user touch the display to give commands.

Output devices show the result of processing to the user:

• Monitor: shows text and pictures on a screen. This is called soft copy output.
• Printer: produces output on paper. This is called hard copy output. Inkjet and laser printers are common types.
• Speakers: produce sound.
• Projector: shows the screen on a wall or a large screen.

The system unit is the box that holds the main parts of the computer. Inside it are the motherboard, the CPU, RAM, the hard disk or SSD, the power supply and expansion cards. The motherboard is the main circuit board. All other parts are connected to it.

The CPU (Central Processing Unit) is the brain of the computer. It has three main parts:

• Control Unit (CU): directs and coordinates all activities, and fetches and decodes instructions.
• Arithmetic Logic Unit (ALU): performs arithmetic (add, subtract, multiply, divide) and logic operations (comparisons such as greater than or equal to).
• Registers: very small, very fast storage places inside the CPU that hold the data being used at that moment.

The speed of a CPU is measured in hertz. One hertz is one cycle per second. Modern processors are measured in gigahertz (GHz). A processor with more cores can work on more than one task at the same time.

Memory is divided into main memory and secondary storage:

• RAM (Random Access Memory): temporary memory that holds the programs and data in use. It is volatile, which means its contents are lost when power is off. Both reading and writing are possible.
• ROM (Read Only Memory): permanent memory that keeps the start-up instructions (firmware). It is non-volatile, so it keeps its contents without power.
• Cache memory: a small, very fast memory between the CPU and RAM that stores frequently used data.

Secondary storage keeps data permanently, even when the computer is switched off:

• Hard disk drive (HDD): magnetic disks with moving parts, large and cheap.
• Solid state drive (SSD): uses flash memory with no moving parts, faster and quieter.
• Optical discs (CD, DVD): data is read with a laser.
• USB flash drive and memory card: small portable flash storage.

A bit (binary digit) is the smallest unit of data and is either 0 or 1. Eight bits make one byte, which stores one character. Larger units are kilobyte (KB), megabyte (MB), gigabyte (GB) and terabyte (TB). In most school books, 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB and 1 TB = 1024 GB.

Ports let you connect devices to the computer:

• USB port: connects keyboards, mice, printers, flash drives and phones.
• HDMI port: carries video and sound to a monitor or television.
• Audio ports: for headphones and microphones.
• Ethernet (LAN) port: connects the computer to a wired network.

Care of hardware: keep the computer in a clean, dry place, use a UPS or surge protector against power cuts and voltage spikes, shut down properly, and do not eat or drink near the machine.

Follow a simple input-process-output-storage example: a student types an essay with a keyboard, the CPU executes the editor’s instructions using RAM, the display presents the text, and saving writes the file to persistent storage. RAM and storage are not interchangeable.

Study tip: Learn the lists of devices by grouping them into input, output and storage, and give one use for each. For storage conversions, remember that you multiply by 1024 when going to a smaller unit and divide by 1024 when going to a larger one. In differences questions, write two or three clear points for each side.

Definitions and flashcards

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Short questions with model answers

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Common mistakes

• Saying that RAM is permanent storage. RAM is temporary and loses its data when power is off.
• Confusing a bit with a byte. A byte is 8 bits, and a bit is a single 0 or 1.
• Calling the monitor, the keyboard or the mouse the computer. The computer is the system unit with its processing parts.
• Listing a touch screen only as an output device. It works as both an input and an output device.
• Mixing up the roles of the control unit and the ALU. The control unit directs, the ALU calculates and compares.

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