Cambridge Lower Secondary CheckpointStage 8

Computer Systems

Computing Stage 8 Chapter Notes

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Computer Systems
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In this chapter

  1. 1The CPU and the Fetch-Decode-Execute Cycle
  2. 2Primary Memory: RAM and ROM
  3. 3Systems Software: The Operating System
  4. 4Binary and Hexadecimal Number Systems
  5. 5Logic Gates and Logic Circuits
  6. 6Data Storage and Compression

1. The CPU and the Fetch-Decode-Execute Cycle

The Central Processing Unit (CPU), often called the 'brain' of the computer, is responsible for processing all data and instructions. It contains several key components: the Control Unit (CU) which directs the flow of data, the Arithmetic Logic Unit (ALU) which performs calculations and logical comparisons, and registers which are small, high-speed memory locations. To process instructions, the CPU continuously performs the Fetch-Decode-Execute cycle.

Key term

Fetch-Decode-Execute Cycle: The fundamental process by which a CPU retrieves a program instruction from memory, determines what the instruction means, and then carries out that action.

Examiner insight

Examiners award higher marks for answers that correctly name and describe the roles of the key registers involved in the Fetch-Decode-Execute cycle, such as the Program Counter (PC) and Instruction Register (IR).

Common pitfall

Students often confuse the roles of the Memory Address Register (MAR) and the Memory Data Register (MDR), or forget to mention that the Program Counter (PC) is incremented during the fetch stage.

Fun fact

The clock speed of a CPU, measured in Gigahertz (GHz), represents how many Fetch-Decode-Execute cycles it can perform per second. A 4 GHz CPU can theoretically perform 4 billion cycles every second.

Worked example 16 marks

Describe the three stages of the Fetch-Decode-Execute cycle.

  1. 1
    1. Fetch: The CPU fetches the next instruction from its address in main memory (RAM). The address is held in the Program Counter (PC). This instruction is copied into the Instruction Register (IR). The PC is then incremented to point to the next instruction.
  2. 2
    1. Decode: The Control Unit (CU) decodes the instruction held in the IR. It interprets what needs to be done and what other parts of the CPU (like the ALU) are needed to carry it out.
  3. 3
    1. Execute: The decoded instruction is performed. This could involve a calculation by the ALU, a data transfer to or from memory, or a change to the Program Counter for a jump instruction.

Worked example 24 marks

Explain the role of the Arithmetic Logic Unit (ALU) and the Control Unit (CU).

  1. 1

    ALU: The ALU performs all arithmetic operations (like addition, subtraction) and all logical operations (like AND, OR, NOT comparisons). For example, it would be used to check if one value is greater than another.

  2. 2

    CU: The Control Unit coordinates all the activities within the CPU. It sends control signals to other components, manages the timing of operations, and directs the flow of data between the CPU and other parts of the computer.

Recap

  • The CPU processes instructions using the Fetch-Decode-Execute cycle.
  • The Control Unit (CU) directs operations, and the Arithmetic Logic Unit (ALU) performs calculations.
  • Registers are small, fast storage locations within the CPU.
  • The Program Counter (PC) holds the memory address of the next instruction to be fetched.
  • The cycle repeats billions of times per second in a modern processor.

Quick check

  1. What are the three main components of a CPU?3 marks
  2. Which register holds the address of the next instruction?1 mark

2. Primary Memory: RAM and ROM

Primary memory is the only memory that the CPU can access directly. It is crucial for the computer's operation. There are two main types: Random Access Memory (RAM) and Read-Only Memory (ROM). RAM is used to temporarily store the operating system, currently running programs, and any data the user is working on. ROM is used to permanently store essential startup instructions.

Key term

Volatility: A characteristic of computer memory that describes whether it retains its data when electrical power is removed.

Examiner insight

Clear comparisons are rewarded. When asked to compare RAM and ROM, students must address multiple distinct points like volatility, purpose, and read/write capability to achieve full marks.

Common pitfall

A common mistake is simply stating that RAM is 'fast' and ROM is 'slow'. While RAM is generally faster, both are significantly faster than secondary storage. The key differences are volatility and read/write capability.

Worked example 16 marks

Compare and contrast RAM and ROM. You should mention volatility, purpose, and read/write capability.

  1. 1
    1. Volatility: RAM is volatile, meaning its contents are lost when the power is turned off. ROM is non-volatile, so it retains its data without power.
  2. 2
    1. Purpose: RAM stores the OS, active programs, and data currently in use. ROM stores the BIOS/UEFI, which are the initial instructions needed to boot the computer.
  3. 3
    1. Read/Write: RAM is read/write memory, meaning its contents can be changed continuously. ROM is read-only, meaning its contents are permanent and cannot be altered by the user.

Recap

  • Primary memory is accessed directly by the CPU and includes RAM and ROM.
  • RAM (Random Access Memory) is volatile, read/write, and stores programs and data currently in use.
  • ROM (Read-Only Memory) is non-volatile, read-only, and stores the computer's startup instructions (BIOS).
  • Turning off the computer erases everything in RAM but does not affect ROM.
  • More RAM generally allows a computer to run more applications simultaneously without slowing down.

Quick check

  1. Which type of primary memory is volatile?1 mark
  2. What is the name for the startup program stored in ROM?1 mark

3. Systems Software: The Operating System

The Operating System (OS) is the most important software on a computer. It is a type of systems software that manages all the computer's hardware and software resources. It acts as an intermediary between the user and the computer hardware, providing a platform on which application programs can run. Without an OS, a computer is just a collection of unusable electronic parts.

Key term

Operating System (OS): Systems software that manages computer hardware and software resources and provides common services for computer programs.

Examiner insight

Examiners look for specific, well-explained examples of OS functions. A good answer goes beyond a simple list, explaining *why* a function like memory management is necessary.

Common pitfall

Confusing the operating system with application software. A word processor is an application; the OS is the system software that allows the word processor to run, access the keyboard, and save files.

Worked example 16 marks

Describe three functions of a modern operating system.

  1. 1
    1. Memory Management: The OS allocates and manages the computer's main memory (RAM). It ensures that each running program has its own protected memory space and handles the swapping of data to virtual memory when RAM is full.
  2. 2
    1. Process Management: The OS manages how the CPU's time is shared between multiple running programs (processes). It schedules which process gets to run and for how long, creating the illusion that many things are happening at once.
  3. 3
    1. User Interface: The OS provides a way for the user to interact with the computer. This is typically a Graphical User Interface (GUI) with windows, icons, menus, and a pointer, but can also be a Command Line Interface (CLI).

Recap

  • The OS is essential systems software that manages all hardware and software.
  • Key functions include memory management, process management, file management, and device management.
  • The OS provides a user interface (e.g., GUI) for the user to interact with the computer.
  • It handles security by managing user accounts and permissions.
  • Examples of operating systems include Windows, macOS, Linux, iOS, and Android.

Quick check

  1. Name two functions of an operating system.2 marks
  2. What does GUI stand for?1 mark

4. Binary and Hexadecimal Number Systems

Computers do not understand human language or numbers directly. At their most basic level, they operate using the binary number system (base-2), which has only two digits: 0 and 1. Each digit is a 'bit'. To make long binary strings easier for humans to read, we often use the hexadecimal number system (base-16). Hexadecimal uses the digits 0-9 and the letters A-F (representing 10-15). Crucially, one hexadecimal digit represents exactly four binary digits (a nibble).

Denary to Binary: Use place values (..., 128, 64, 32, 16, 8, 4, 2, 1) or repeated division by 2.

Binary to Hexadecimal: Group binary into 4-bit nibbles (from the right) and convert each nibble to its hex equivalent.

Hexadecimal to Denary: Multiply each digit by its place value (..., 256, 16, 1) and sum the results.

Key term

Hexadecimal: A base-16 number system using digits 0-9 and letters A-F, often used in computing as a shorthand representation for binary.

Examiner insight

Examiners require you to show your working for any number conversion questions. Simply writing the final answer, even if correct, may not earn full marks.

Common pitfall

When converting from binary to hex, students sometimes group the bits from the left instead of the right. Always start grouping into nibbles from the right-hand side.

Fun fact

Hexadecimal is used to define colours in web pages (e.g., #FF0000 is pure red), and to represent MAC addresses for network devices.

Worked example 12 marks

Convert the denary number 181 into an 8-bit binary number.

  1. 1
    1. Write down the 8-bit place values: 128, 64, 32, 16, 8, 4, 2, 1.
  2. 2
    1. Start from the left. 128 fits into 181. Place a 1. Remainder: 181 - 128 = 53.
  3. 3
    1. 64 does not fit into 53. Place a 0.
  4. 4
    1. 32 fits into 53. Place a 1. Remainder: 53 - 32 = 21.
  5. 5
    1. 16 fits into 21. Place a 1. Remainder: 21 - 16 = 5.
  6. 6
    1. 8 does not fit into 5. Place a 0.
  7. 7
    1. 4 fits into 5. Place a 1. Remainder: 5 - 4 = 1.
  8. 8
    1. 2 does not fit into 1. Place a 0.
  9. 9
    1. 1 fits into 1. Place a 1. Remainder: 1 - 1 = 0.
  10. 10
    1. The resulting binary number is 10110101.

Worked example 23 marks

Convert the hexadecimal number 4F into denary.

  1. 1
    1. Identify the place values for the two digits. The rightmost digit is the 1s (16^0) column, and the next is the 16s (16^1) column.
  2. 2
    1. The digit '4' is in the 16s column. The digit 'F' is in the 1s column.
  3. 3
    1. Convert 'F' to its denary equivalent, which is 15.
  4. 4
    1. Calculate the total: (4 * 16) + (15 * 1) = 64 + 15 = 79.
  5. 5
    1. Therefore, the denary equivalent of 4F is 79.

Recap

  • Computers use the binary (base-2) system of 0s and 1s.
  • Hexadecimal (base-16) is a shorthand for binary, using digits 0-9 and letters A-F.
  • One hexadecimal digit represents a 4-bit binary pattern (a nibble).
  • You must be able to convert between denary, binary, and hexadecimal.
  • Always show your working clearly in conversion questions.

Quick check

  1. What is the denary value of the binary number 1010?1 mark
  2. Convert the hexadecimal digit 'C' to denary.1 mark

5. Logic Gates and Logic Circuits

Logic gates are the fundamental building blocks of all digital circuits, including the CPU. Each gate takes one or more binary inputs (0 or 1) and produces a single binary output based on a simple logical rule. The main gates are AND, OR, and NOT. Other common gates include NAND, NOR, and XOR. By combining these gates, we can build complex logic circuits that perform calculations and make decisions.

AND (A . B): Output is 1 only if input A AND input B are both 1.

OR (A + B): Output is 1 if input A OR input B (or both) are 1.

NOT (¬A): Output is the inverse of the single input.

XOR (A ⊕ B): Output is 1 if the inputs are different.

Key term

Truth Table: A table that exhaustively lists all possible input combinations and the corresponding logical output for a given logic gate or circuit.

Examiner insight

When drawing logic circuits, use the standard, recognised symbols for each gate. When completing truth tables, be systematic to ensure you cover all possible input combinations (2^n rows for n inputs).

Common pitfall

Confusing the symbols for the AND gate (D-shaped) and the OR gate (curved input side). Also, misunderstanding the XOR gate, which is true only when inputs are different, not when one or more are true.

Worked example 14 marks

A car has a warning system. An alarm (Q) sounds if the driver's seatbelt is not fastened (A=0) AND the engine is on (B=1). Write the Boolean expression for Q and draw the logic circuit.

  1. 1
    1. Translate the logic: 'seatbelt not fastened' is NOT A. 'Engine is on' is B. The alarm sounds if (NOT A) AND B are true.
  2. 2
    1. Write the Boolean expression: Q = (NOT A) AND B. In standard notation, this is Q = ¬A . B
  3. 3
    1. Draw the logic circuit: Start with input A. Connect it to a NOT gate. The output of the NOT gate becomes one input to an AND gate.
  4. 4
    1. The other input to the AND gate is B.
  5. 5
    1. The output of the AND gate is the final output, Q.

Worked example 24 marks

Complete the truth table for the expression Q = A OR (NOT B).

  1. 1
    1. Set up the table with columns for inputs A and B, an intermediate column for NOT B, and a final column for Q.
  2. 2
    1. List all possible input combinations for A and B: 00, 01, 10, 11.
  3. 3
    1. Fill in the 'NOT B' column by inverting the 'B' column: 1, 0, 1, 0.
  4. 4
    1. Fill in the 'Q' column by applying the OR rule to the 'A' column and the 'NOT B' column.
  5. 5

    Row 1 (A=0, B=0): Q = 0 OR 1 = 1

  6. 6

    Row 2 (A=0, B=1): Q = 0 OR 0 = 0

  7. 7

    Row 3 (A=1, B=0): Q = 1 OR 1 = 1

  8. 8

    Row 4 (A=1, B=1): Q = 1 OR 0 = 1

Recap

  • Logic gates (AND, OR, NOT, XOR) are the building blocks of digital circuits.
  • A truth table shows the output of a logic circuit for all possible inputs.
  • Boolean algebra is a mathematical way to express logic.
  • Logic circuits are used to perform calculations and make decisions in a computer.
  • You must know the standard symbols for each logic gate.

Quick check

  1. An OR gate has two inputs, 0 and 1. What is the output?1 mark
  2. Which logic gate is also known as an inverter?1 mark

6. Data Storage and Compression

Secondary storage is non-volatile memory used to store data and programs for the long term. Unlike RAM, it retains data when the power is off. The main types are Hard Disk Drives (HDDs), Solid-State Drives (SSDs), and Optical Disks. To save space on these devices and speed up data transfer over networks, we use data compression. There are two main types: lossless, which preserves all data, and lossy, which removes some data to achieve smaller file sizes.

Key term

Lossless Compression: A data compression method that reduces file size without losing any information, allowing the original data to be perfectly reconstructed.

Examiner insight

For questions on compression, examiners look for an understanding of the trade-off between file size and quality, and the ability to apply this to a given scenario (e.g., emailing photos vs archiving documents).

Common pitfall

Stating that lossy compression 'ruins' a file. It reduces quality, but this is often an acceptable trade-off and the result is still perfectly usable for its intended purpose (e.g., streaming a video).

Worked example 14 marks

Compare the characteristics of a Solid-State Drive (SSD) and a Hard Disk Drive (HDD).

  1. 1
    1. Technology: HDDs use spinning magnetic platters and a read/write head on a mechanical arm. SSDs use non-volatile flash memory chips with no moving parts.
  2. 2
    1. Speed: SSDs have much faster read/write speeds and access times because there are no moving parts to position.
  3. 3
    1. Durability: SSDs are more durable and resistant to shock and impact, making them ideal for laptops. HDDs are fragile due to their mechanical nature.
  4. 4
    1. Cost: HDDs are typically cheaper per gigabyte, offering larger storage capacities for less money. SSDs are more expensive per gigabyte.

Worked example 23 marks

A programmer is creating a backup of their source code. Explain which type of compression, lossy or lossless, they should use and why.

  1. 1
    1. The programmer must use lossless compression.
  2. 2
    1. Source code is text. Every single character, space, and symbol is critical. If even one character is changed or removed, the code will fail to compile or will contain bugs.
  3. 3
    1. Lossless compression (like in a ZIP file) guarantees that when the file is uncompressed, it is an exact, bit-for-bit copy of the original, preserving the integrity of the code.

Recap

  • Secondary storage is non-volatile and includes HDDs, SSDs, and optical disks.
  • SSDs are faster and more durable than HDDs but are also more expensive.
  • HDDs offer higher capacity for a lower cost but are slower and more fragile.
  • Data compression reduces file sizes for storage and transmission.
  • Lossless compression is reversible and used for text and programs (e.g., ZIP).
  • Lossy compression is irreversible and used for media like images and audio (e.g., JPEG, MP3).

Quick check

  1. Give one advantage of an SSD over an HDD.1 mark
  2. Would you use lossy or lossless compression to store a high-quality master audio recording? Why?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Describe the purpose of the Program Counter (PC) and the Memory Address Register (MAR) within the CPU during the Fetch-Decode-Execute cycle.4 marks
  2. Convert the denary number 93 into an 8-bit binary number. You must show your working.2 marks
  3. Convert the hexadecimal number D4 to a denary number. You must show your working.3 marks
  4. A security system for a vault has two sensors, A and B. The alarm, Q, should sound if sensor A is triggered AND sensor B is NOT triggered. Write the Boolean expression for this system and draw the corresponding logic gate circuit.4 marks
  5. Compare a Solid-State Drive (SSD) with a Hard Disk Drive (HDD), stating two differences in their characteristics or operation.4 marks
  6. Explain the difference between systems software and applications software, giving one clear example of each.4 marks
  7. A student is editing a high-resolution 4K video for a project. Explain why having a large amount of RAM is important for this task.3 marks
  8. Explain the difference between lossy and lossless compression. Give a suitable example use for each type.4 marks
  9. A logic circuit is represented by the Boolean expression X = (A NOR B) AND C. Draw a truth table for this expression.6 marks
  10. Describe three key functions of a modern operating system, explaining why each is necessary for a computer to be used effectively.6 marks

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