SS 1 First Term

Learning Objectives

• State the main characteristics of a computer.
• Describe the motherboard and the system unit.
• Explain the roles of the CPU, the ALU, the control unit and the registers.
• Describe the buses, RAM and ROM.

Characteristics of a Computer

A computer has several distinctive properties that set it apart from other machines and make it a powerful tool. These characteristics work together to handle tasks that would be impossible or impractical for humans to do alone. Understanding them helps learners appreciate why computers are used in nearly every area of modern life.

Speed - Computers process data and execute instructions at extraordinary speed, handling millions or even billions of operations every second. Modern CPUs operate at clock speeds of 3 to 5 GHz, meaning they can perform billions of cycles per second. Example: A school computer can process the results of 1,000 students and produce a complete report in just a few seconds.

Accuracy - Computers perform calculations and process data with a very high level of precision. When errors occur, they are almost always caused by incorrect input data or flawed programs, not by the computer itself. Example: A calculator will always return the correct sum - if you type 15 + 27, the answer is always 42.

Diligence - A computer never gets tired, bored, or distracted. It can perform the same task for hours or even days without any drop in quality or speed. Example: A security camera system monitored by a computer can watch every angle of a building continuously for 24 hours without a break.

Storage Capacity - Modern computers can store enormous amounts of data in both temporary memory (RAM) and permanent secondary storage such as SSDs and hard drives. This data can be saved, retrieved, and searched almost instantly. Example: A single laptop can hold thousands of documents, photos, and videos, and you can find any one of them in seconds using a search function.

Versatility - A computer can switch between completely different tasks with equal ease. The same machine can be used for word processing, playing music, browsing the internet, and running scientific simulations. Example: You can use the same laptop to write an essay in Word, listen to music, check your email, and run a spreadsheet all within the same hour.

Automation - Computers can perform long sequences of repetitive tasks automatically once a program is set in motion, without requiring a human to give a command at every step. This saves time and reduces the chance of human error. Example: A payroll program calculates salaries, deductions, and bonuses for every worker automatically each month once the correct data is entered.

Reliability - Given accurate input data and correctly written instructions, a computer will consistently produce the correct result every single time. Errors in output are almost always caused by faulty data or bugs in the software, not by the machine itself. Example: Every time you type 7 + 8 into a calculator, it returns 15 - the result never changes or becomes incorrect under normal conditions.

No Feelings - A computer operates purely on logic and electrical signals, not on emotions. Its performance is never affected by mood, stress, or tiredness. Example: A grading program applies the same marking criteria to every paper whether it is the first or the thousandth, without any bias or fatigue.

No IQ - A computer has no independent intelligence. It cannot think, reason, or solve new problems on its own. It follows exactly and only the instructions given to it by a programmer. Example: A computer cannot solve a new type of mathematics problem until a programmer writes new code to handle it - otherwise it will simply display an error or produce wrong output.

Secrecy (Data Security) - Computers protect sensitive data through authentication, passwords, and encryption so that unauthorised persons cannot access or understand the information. These security measures are built into operating systems, applications, and network protocols. Example: Banking systems encrypt every transaction so that even if the data is intercepted during transmission, it cannot be read or understood by anyone without the correct decryption key.

Now that we understand these key characteristics, let us examine the physical components that make them possible - the motherboard and the system unit.

The Motherboard and the System Unit

The motherboard is the main circuit board inside the system unit. Every major component connects to it either directly or through cables. The motherboard holds the CPU socket, memory slots, expansion slots, chipsets, power connectors, and ports for input and output devices. Think of the motherboard as the central crossroads of a town with roads leading to every building, each building being a component such as the CPU, RAM, or a graphics card.

The system unit is the case that houses the motherboard and other components. Inside the system unit, you will find the motherboard, CPU, RAM, power supply, storage drives, and sometimes a graphics card and other expansion cards. Each component has a specific role and connects to the motherboard through standardized slots and connectors.

The motherboard is called the backbone of the computer because it provides the physical connections that allow all components to communicate. Without the motherboard, the CPU, RAM, and storage devices cannot work together.

The CPU, ALU, Control Unit, and Registers

The Central Processing Unit (CPU) is the brain of the computer. Inside the CPU are three main parts, namely the Arithmetic and Logic Unit (ALU), the Control Unit, and Registers. The ALU performs arithmetic operations (add, subtract, multiply, divide) and logic operations (compare values). The Control Unit fetches instructions from memory, decodes them, and directs the other parts to act. Registers are very fast, tiny storage locations that hold data and instructions during processing.

The CPU repeats the machine cycle for every instruction, namely fetch, decode, execute, and store. In the fetch step, the control unit brings the instruction from memory. In the decode step, it works out what the instruction means. In the execute step, the ALU or another unit carries it out. In the store step, the result is saved to a register or memory. A modern CPU performs billions of these cycles per second, measured in gigahertz (GHz).

For example, to calculate 5 + 7, the control unit fetches the instruction 'add 5 and 7', decodes it, the ALU adds the numbers to make 12, and the result is stored in a register. The same four steps run for every operation, whether the computer is playing a video or drawing a graph.

Buses, RAM, and ROM

A bus is a shared pathway that carries data, addresses, and control signals between components. The three main types are the data bus (carries the data itself), the address bus (carries the location or address in memory), and the control bus (carries commands such as read and write). A wider bus can move more data at once; a 64-bit data bus carries 64 bits in a single trip.

RAM (Random Access Memory) is the main memory that holds data temporarily while the computer runs. It is volatile, meaning its contents are lost when power is off. ROM (Read Only Memory) is permanent memory that stores startup instructions. It is non-volatile, meaning its contents remain even without power. A common comparison is a work desk (RAM) where you spread out papers while working, versus a filing cabinet (ROM/other storage) where things are kept permanently.

When a program runs, data enters through input devices, travels along buses to the CPU, is processed, held temporarily in RAM, saved permanently to storage, and presented through output devices. Every action on a computer involves several of these stages cooperating in a single coordinated flow.

System Unit Characteristics and the Motherboard

The system unit is the box that houses the main working parts of the computer. Its key characteristics are its form factor (desktop tower or slim laptop profile), its power supply, its cooling system and the space it provides for expansion. The system unit is where processing actually happens. All other devices, keyboards, monitors and printers, connect to it so that data can enter, be processed and leave as output.

At the centre of the system unit is the motherboard, the main circuit board. Every major component, the CPU, RAM, storage controllers, expansion cards and ports, connects to the motherboard through sockets and slots. The motherboard provides the electrical pathways that allow all the parts to exchange data, and it is the physical backbone on which the whole system is built.

Because the motherboard is the common link for every component, its layout helps decide how much memory can be added, which processors it supports and how quickly the components can communicate. Older computers used many separate chips; modern motherboards integrate sound, network and graphics functions directly onto the board, which reduces cost and saves space.

Form factors range from desktop towers to slim laptop designs.

The motherboard carries the CPU socket, RAM slots and expansion slots.

Tracks on the motherboard carry data between components.

Integrated sound and networking reduce cost and space.

CPU and Control Unit Responsibilities

The Central Processing Unit (CPU) is responsible for executing the instructions of every program. The control unit is the director of this process. It fetches each instruction from memory, decodes what the instruction means and controls which units carry it out, making sure data moves between memory, the ALU and the registers in the correct order.

The responsibilities of the control unit include reading instructions in sequence, interpreting the operation code of each instruction, generating the control signals that tell the ALU when to add, subtract or compare, and synchronising the machine cycle with the system clock. It does not calculate values itself; it directs the parts that do.

The CPU as a whole is responsible for arithmetic and logic through the ALU, temporary storage through the registers and the movement of data along the buses. The control unit also responds to interrupts, signals from devices such as the keyboard requesting attention, so the processor can pause one task and handle the new request.

The control unit fetches, decodes and directs instructions.

It generates control signals but does not perform calculations.

The ALU performs the actual arithmetic and logic.

Registers hold operands and results during processing.

Interrupts let devices pause the CPU to request service.

ALU Operations and CPU Registers

The Arithmetic and Logic Unit (ALU) carries out all the calculations and comparisons the CPU needs. Its arithmetic operations include addition, subtraction, multiplication and division, while its logic operations include comparisons such as equal to, greater than and less than, plus the AND, OR and NOT operations used in digital circuits. When a program asks for 24 plus 18, the ALU performs the calculation and returns 42.

To work at full speed, the CPU keeps small high-speed storage areas called registers inside the processor. The Program Counter (PC) holds the address of the next instruction to be fetched. The Instruction Register (IR) stores the instruction currently being executed. The Memory Address Register (MAR) holds the address in memory that the CPU wants to read from or write to, and the Memory Data Register (MDR) holds the actual data being transferred between memory and the CPU.

Data moves through these registers during the machine cycle. The PC supplies the address of the next instruction, the address travels through the MAR to memory, and the fetched value arrives in the MDR and then in the IR. Registers are much faster than RAM because they sit directly on the processor chip, which is why the CPU uses them for its most frequent operations.

ALU arithmetic: add, subtract, multiply and divide.

ALU logic: compare values; AND, OR and NOT operations.

PC: address of the next instruction.

IR: the instruction currently being executed.

MAR: the memory address being accessed.

MDR: the data moving into or out of memory.

The System Bus Matrix

A bus is a set of parallel electrical pathways on the motherboard that carries information between the CPU, memory and devices. Because a complete system needs several kinds of information to travel at once, the computer uses a system bus made of three separate buses working together: the data bus, the address bus and the control bus.

The data bus carries the actual data being processed or stored. Its width, measured in bits (32-bit or 64-bit), states how many bits can travel side by side in one trip, so a 64-bit data bus moves twice as much data per transfer as a 32-bit bus. The address bus carries the memory addresses that the CPU wishes to access. Its width limits how much memory the CPU can use, because each combination of wires can represent one different address.

The control bus carries the command signals that coordinate the system. These signals tell memory and devices whether the CPU wants to read or write, and they carry the clock pulses that keep every part moving in step. Together, the three buses form the system bus matrix, the data highway that lets the CPU, RAM, ROM and input-output devices cooperate in a single coordinated machine.

Data bus: moves data; a wider bus transfers more at once.

Address bus: carries memory addresses; its width sets the memory limit.

Control bus: carries read and write commands plus clock signals.

The three buses together make up the system bus matrix.

Practical Work

1. Label a diagram of the system unit and the motherboard.
2. List the parts of the central processing unit and state the role of each.
3. Draw a table comparing RAM and ROM.
4. Trace how data travels along the system bus.

Scenario-Based Questions

1. A technician opens a desktop system unit and finds the CPU, RAM, graphics card, and storage drives all connected to one large circuit board. Name that board and explain why replacing it with a damaged board would stop the whole machine from working.
2. A student asks how the CPU manages to add 5 and 7. Using the ALU, the control unit, and registers, walk through the four steps of the machine cycle and state which part does each job.
3. A school is choosing new computers and the seller advertises a 64 bit data bus. Explain what the data bus carries, what a wider 64 bit bus means for data movement, and why the address bus and the control bus are also needed.
4. The lab assistant unplugs a computer in the middle of a lesson. The unsaved document vanishes, yet the startup instructions are still there when the machine is switched back on. Using RAM and ROM, explain the volatility of each and why this happens.
5. A student compares the motherboard to the central crossroads of a town. Using that comparison, trace what happens to a piece of data moving from the keyboard to the CPU and then out to the monitor, naming the pathway it travels along.
6. A phone repair workshop has two memory chips on the bench. One loses everything the moment power is removed; the other keeps its contents permanently. Identify which is RAM and which is ROM, and explain which chip must hold the boot instructions and why.
7. A technician explains that the CPU repeats the same four steps billions of times each second. Name those steps in order, and explain why this one repeated cycle is enough to handle both a lesson video and a drawn graph.

LEARNING OBJECTIVES: BY THE END OF THE LESSON STUDENTS SHOULD BE ABLE TO:

Rationale: A computer has several distinctive properties that set it apart from other machines and make it a powerful tool. Understanding characteristics of a Computer, the Motherboard and the System Unit, the CPU, ALU, Control Unit, and Registers, buses, RAM, and ROM helps learners use digital technology confidently, safely and responsibly.

Pre-requisite Knowledge: In Week 2, learners studied History and Evolution of Computing, including early Calculating Devices, charles Babbage and Ada Lovelace, the First Electronic Computers. The knowledge from that week is used again in this lesson.

Learning Material/Teaching Aid: Whiteboard, markers, computer or laptop where available, printed notes and charts on Computer Hardware Architecture, practical task sheets, learners' notebooks and pens.

Reference Materials: Digital Technology for Senior Secondary Schools (SS 1); NERDC Digital Technology Curriculum; Digital Technology SS 1 First Term Notes (Week 3); First Term examination and Continuous Assessment question papers.

LESSON DEVELOPMENT

Assignment: i. A technician opens a desktop system unit and finds the CPU, RAM, graphics card, and storage drives all connected to one large circuit board. Name that board and explain why replacing it with a damaged board would stop the whole machine from working. ii. A student asks how the CPU manages to add 5 and 7. Using the ALU, the control unit, and registers, walk through the four steps of the machine cycle and state which part does each job.