SS 2 First Term

Learning Objectives

• Describe the parts of a computer system and the IPO model.
• Explain the role of the processor in a computer.
• Describe the system bus and the fetch-execute cycle.
• Trace the movement of data and instructions in a cycle.

Review of Computer System and IPO Model

A computer system is made up of four essential parts that must work together for the machine to be useful. Hardware refers to all the physical parts, including the CPU, RAM, hard drive, keyboard and monitor. Software refers to the programmes and instructions that tell the hardware what to do, such as the operating system and applications like Microsoft Word. Data consists of raw facts, such as numbers, text, images and sound, that the computer processes into useful information. Users and procedures are the people who operate the computer and the rules they follow. Without any one of these four parts, the computer cannot function properly. Hardware without software is just an inert machine, while software without hardware has nothing to run on.

The IPO model explains how a computer processes data. Input is when data enters the system, for example when barcodes are scanned at a supermarket checkout or when a student types on a keyboard. Process is when the CPU works on that data, such as adding prices together or sorting a list of names. Output is when the result is produced, for example when a receipt is printed or a result appears on screen. Storage is when data or results are saved for later use, such as when the day's sales figures are written to a hard drive. The centre of the process stage is always the CPU, which is the main topic of this week's study.

Hardware is organised by function. Input devices include the keyboard, mouse, scanner and microphone. The processing device is the CPU itself. Output devices include the monitor, printer and speakers. Storage devices include the hard drive, solid-state drive and flash drive. Software is divided into system software, such as the operating system, and application software, such as a word processor or web browser. This review of SS1 topics provides the foundation for understanding how the CPU actually works inside the machine.

Computer Architecture and the CPU

Computer architecture is the design and structure of a computer's internal components and how they are organised to work together. It describes the CPU, memory, buses and the way instructions move between them. The most widely taught model is the Von Neumann architecture, in which the CPU fetches instructions and data from the same memory, processes them, and stores results back. This is the classic model that most modern computers follow.

The CPU consists of three main components. The Arithmetic Logic Unit (ALU) performs arithmetic operations such as addition, subtraction, multiplication and division, and logical operations such as AND, OR, NOT and comparisons like greater than, less than and equal to. When a spreadsheet computes a class average, the ALU performs the addition and division step by step. The Control Unit (CU) does not perform calculations itself. Instead, it directs the flow of work by fetching instructions from memory, decoding them to understand what they mean, and coordinating the ALU, memory and input/output devices to carry them out. It also controls the system clock that synchronises all operations.

Registers are extremely fast storage locations inside the CPU, much faster than main memory but very small in size. The Program Counter (PC) holds the address of the next instruction to be executed. The Memory Address Register (MAR) holds the address of the memory location being read from or written to. The Memory Data Register (MDR) holds the actual data or instruction being transferred. The Accumulator (ACC) stores the result of the most recent arithmetic or logical operation. The Instruction Register (IR) holds the current instruction being executed.

The System Bus and the Fetch-Execute Cycle

A bus is a set of parallel wires that carries information between the CPU, memory and other devices. The data bus carries the actual data being transferred between components. The address bus carries the memory addresses that tell the CPU where data is going or coming from. The control bus carries control signals such as read and write commands that coordinate the operations. The three buses work together so that the CPU can locate data, move it and control the timing of each transfer.

The CPU carries out every instruction using a repeated cycle known as the fetch-execute or machine cycle. In the fetch step, the CPU reads the next instruction from memory, using the PC to supply the address and the MAR and MDR to transfer it. In the decode step, the Control Unit interprets what the instruction means. In the execute step, the ALU or other component carries out the instruction. In the store step, the result is written back to memory or a register. The PC is then updated to point to the next instruction and the entire cycle repeats. This cycle runs millions or billions of times per second, and the clock speed measured in gigahertz (GHz) tells how many cycles the CPU can perform each second.

For example, when the program must add two numbers 5 and 3, the CPU first fetches the instruction to add 5 and 3 from memory. The Control Unit decodes it as an addition operation. The ALU performs the addition and produces 8. The result is stored in the accumulator or written back to memory. All the CPU components work together in this process. The CU fetches and decodes, the ALU calculates, registers hold temporary values, and buses carry addresses, data and control signals. This co-ordinated behaviour of components is what computer architecture means in practice.

Fetch-execute cycle means the repeated steps of fetching, decoding and executing an instruction.

System bus means the set of wires that carries data, addresses and control signals.

Practical Work

1. Draw the input, process and output model and label each stage.
2. List the three buses and state what each carries.
3. Describe the steps of the fetch-execute cycle in order.
4. Name the parts of the processor and one job of each.

Scenario-Based Questions

1. During a practical class, a student is asked to trace what happens inside the CPU when it processes an instruction that adds the numbers 5 and 3. Walk through the fetch, decode, execute and store steps of the fetch-execute cycle, and state which CPU component (ALU, Control Unit, registers or buses) performs each job.
2. A school technician explains that the CPU in Lab A runs at 3.5 GHz while Lab B runs at 2.8 GHz. What does clock speed measure, and would Lab A always finish every task faster? Give a reason.
3. Two students argue about whether the ALU or the Control Unit is more important in the CPU. Compare the roles of the ALU and the Control Unit and explain why both are needed.
4. A technician notes that the data bus and the address bus carry different kinds of information. Explain what the data bus carries, what the address bus carries and how the control bus co-ordinates the transfer between the CPU and memory.
5. During a practical test a student is asked to trace an ADD instruction, using the two numbers 5 and 3, from memory to execution. Describe each step of the fetch-execute cycle and state what the registers PC, MAR, MDR and IR are used for at each stage.
6. A computer science teacher says most personal computers follow the Von Neumann architecture, in which the CPU fetches instructions and data from the same memory. Explain what this design says about where instructions and data are stored and why it is the most widely taught model.
7. A new desktop has a clock speed of 4.0 GHz. Explain what 4.0 GHz means in terms of the fetch-execute cycle and state how many fetch-execute cycles the CPU can carry out in one second.

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

Rationale: A computer system is made up of four essential parts that must work together for the machine to be useful. Understanding review of Computer System and IPO Model, computer Architecture and the CPU, the System Bus and the Fetch-Execute Cycle helps learners use digital technology confidently, safely and responsibly.

Pre-requisite Knowledge: Learners have just entered SS 2 and already use digital devices such as phones, radios, televisions and computers at home or in the community. They can switch a device on, make a call, send a message and identify common parts such as the screen, keyboard and mouse.

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

Reference Materials: Digital Technology for Senior Secondary Schools (SS 2); NERDC Digital Technology Curriculum; Digital Technology SS 2 First Term Notes (Week 1); First Term examination and Continuous Assessment question papers.

LESSON DEVELOPMENT

Assignment: i. A learner asks what computer architecture means and why one model is taught more than any other. Define computer architecture, describe the Von Neumann architecture, and explain why the model matters. ii. During a practical class, a student is asked to trace what happens inside the CPU when it processes an instruction that adds the numbers 5 and 3. Walk through the fetch, decode, execute and store steps of the fetch-execute cycle, and state which CPU component (ALU, Control Unit, registers or buses) performs each job.