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Computer Systems

Computing Stage 9 Chapter Notes

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Computer Systems
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In this chapter

  1. 1The Operating System and Utility Programs
  2. 2High-Level Languages and Translators
  3. 3The Fetch-Decode-Execute Cycle
  4. 4Data Storage Units and Sound Digitisation
  5. 5Building with Logic Gates
  6. 6Machine Learning and Modern Industry

1. The Operating System and Utility Programs

The operating system (OS) is the master software that manages your computer's hardware and software resources. Think of it as the government of the computer. It handles tasks like memory management, process scheduling, file management, and providing a user interface. Utility programs are system software designed to help analyse, configure, optimise or maintain a computer. They perform specific tasks that support the OS, such as virus scanning, file compression, and disk defragmentation.

Key term

Operating System (OS): Software that manages all computer hardware and software, acting as an interface between the user and the computer hardware.

Examiner insight

Examiners look for specific examples of OS functions (like memory management or providing a user interface) and distinct examples of utilities (like defragmentation or anti-virus), not just a general definition.

Common pitfall

Confusing application software (like a word processor) with system software (like the OS or a device driver). System software runs the computer; application software performs user tasks.

Fun fact

The first version of the Android mobile OS, released in 2008, was codenamed 'Astro Boy'. All subsequent public releases were named alphabetically after desserts, starting with 'Cupcake'.

Worked example 14 marks

Describe two tasks carried out by an operating system.

  1. 1
    1. Memory Management: The OS allocates and manages the computer's main memory (RAM). It ensures that different programs and processes do not interfere with each other's memory space, keeping track of which parts of memory are currently being used and by what.
  2. 2
    1. File Management: The OS provides a system for storing and organising files on storage devices like hard drives. It handles the creation, deletion, renaming, and copying of files and folders, and controls access permissions to ensure data security.

Worked example 24 marks

Explain the purpose of a device driver and disk defragmentation software.

  1. 1

    Device Driver: A device driver is a utility program that acts as a translator between the operating system and a specific hardware device (like a printer or graphics card). It tells the OS how to communicate with and control the device, allowing the hardware to function correctly.

  2. 2

    Disk Defragmentation: This utility reorganises files on a magnetic hard disk drive. Over time, parts of a single file can become scattered across different physical locations on the disk (fragmented), slowing down access. Defragmentation software moves these parts together, improving file access speed.

Recap

  • The operating system manages all hardware and software resources.
  • Key OS functions include memory, process, file, and input/output management.
  • Utility programs perform specific maintenance tasks to support the OS.
  • Device drivers allow the OS to communicate with hardware peripherals.
  • Security software protects the system from malware and unauthorised access.
  • Disk defragmentation improves performance by reorganising fragmented files on a hard drive.

Quick check

  1. What type of software is an anti-virus program?1 mark
  2. Name one function of a computer's operating system.1 mark

2. High-Level Languages and Translators

Computers only understand machine code, which is a sequence of binary 1s and 0s. Humans write programs in high-level languages (like Python or Java) because they are closer to human language and easier to understand. A language translator is a program that converts this high-level source code into low-level machine code that the processor can execute. There are two main types: compilers and interpreters.

Key term

Translator: A program that converts source code written in a high-level programming language into low-level machine code for the processor to execute.

Examiner insight

Marks are often awarded for clearly contrasting the features of compilers and interpreters, especially regarding how they translate the code (all at once vs. line-by-line) and what they produce (executable file vs. direct execution).

Common pitfall

Stating that interpreters are 'slower' without explaining why. The slowness comes from translating each line every time the program is run, whereas a compiler does the translation only once.

Fun fact

The first compiler was created by Grace Hopper in 1952 for the A-0 programming language. Her work was fundamental to the development of COBOL, one of the earliest high-level languages.

Worked example 16 marks

Compare a compiler and an interpreter, stating one advantage of each.

  1. 1

    A compiler translates the entire source code into machine code all at once, before the program is run. It creates a separate executable file.

  2. 2

    An interpreter translates the source code one line at a time, executing each line as it goes. It does not create a separate executable file.

  3. 3

    Advantage of a compiler: The resulting executable file runs very fast because the translation has already been done. The source code is also kept private from the end-user.

  4. 4

    Advantage of an interpreter: It is easier to debug code because errors are reported line-by-line as they are found, stopping execution at the exact point of the error. This is useful during development.

Recap

  • High-level languages are human-readable, while low-level machine code is computer-readable.
  • Translators convert high-level source code into executable machine code.
  • A compiler translates the entire program at once, creating a standalone executable file.
  • An interpreter translates and executes the program one line at a time.
  • Compiled code generally runs faster than interpreted code.
  • Interpreted code is often easier to debug and more portable across different computer architectures.

Quick check

  1. Which type of translator produces a .exe file?1 mark
  2. During software development, which translator is often preferred for its debugging capabilities?1 mark

3. The Fetch-Decode-Execute Cycle

The central processing unit (CPU) continuously performs a sequence of operations called the fetch-decode-execute cycle. This is the fundamental process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions. This cycle is repeated for every instruction in a program. Key registers involved are the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), and Current Instruction Register (CIR).

Key term

Fetch-Decode-Execute Cycle: The fundamental operation cycle of a CPU where it fetches an instruction from memory, decodes it, and then executes it.

Examiner insight

To get full marks, students must describe the role of specific registers (PC, MAR, MDR, CIR) within each stage of the cycle, not just give a vague overview.

Common pitfall

Mixing up the roles of the MAR and MDR. Remember: MAR holds the ADDRESS of the memory location, while MDR holds the DATA or INSTRUCTION that is being transferred.

Fun fact

Modern CPUs can perform billions of fetch-decode-execute cycles per second. A 3 GHz processor can theoretically perform 3 billion cycles every second!

Worked example 16 marks

Explain the 'fetch' and 'decode' stages of the fetch-decode-execute cycle, mentioning the registers involved.

  1. 1

    Fetch Stage:

  2. 2
    1. The Program Counter (PC) holds the memory address of the next instruction to be fetched.
  3. 3
    1. This address is copied from the PC to the Memory Address Register (MAR).
  4. 4
    1. The instruction located at the address in the MAR is copied from RAM to the Memory Data Register (MDR) via the data bus.
  5. 5
    1. The Program Counter is incremented to point to the next instruction.
  6. 6

    Decode Stage:

  7. 7
    1. The instruction in the MDR is copied to the Current Instruction Register (CIR).
  8. 8
    1. The Control Unit decodes the instruction in the CIR to determine what operation needs to be performed.

Recap

  • The fetch-decode-execute cycle is the basic operation of a CPU.
  • Fetch: The CPU retrieves the next instruction from memory.
  • Decode: The CPU interprets the instruction to see what needs to be done.
  • Execute: The CPU performs the required action.
  • The Program Counter (PC) stores the address of the next instruction.
  • The Memory Address Register (MAR) holds the memory address for the current read/write operation.

Quick check

  1. Which register is incremented during the fetch stage?1 mark
  2. What is the purpose of the Current Instruction Register (CIR)?1 mark

4. Data Storage Units and Sound Digitisation

All data in a computer system, from text to images to sound, is stored as binary numbers (bits). A bit is a 0 or a 1. These are grouped into larger units to measure file sizes and storage capacity. The main units are byte (8 bits), kilobyte (KB), megabyte (MB), gigabyte (GB), terabyte (TB), and petabyte (PB). Each is 1000 times larger than the previous for exam purposes. For sound, an analogue sound wave is converted into digital data through a process called sampling. The amplitude (height) of the wave is measured at regular intervals (the sample rate), and this measurement is stored as a binary number.

1 byte = 8 bits

1 kilobyte (KB) = 1000 bytes

1 megabyte (MB) = 1000 kilobytes

1 gigabyte (GB) = 1000 megabytes

1 terabyte (TB) = 1000 gigabytes

Key term

Digitisation: The process of converting information, such as an analogue sound wave, into a digital format represented by binary 1s and 0s.

Examiner insight

When converting storage units, show your working clearly. For sound digitisation, examiners reward answers that link sample rate and bit depth to the quality and file size of the resulting audio file.

Common pitfall

Using 1024 instead of 1000 for conversions. While technically 1 KiB = 1024 bytes, for exam purposes, using the metric approximation of 1000 is usually expected unless specified otherwise.

Fun fact

A single petabyte (PB) can hold about 13.3 years of HD-TV video. The entire written works of mankind, from the beginning of recorded history, would be approximately 50 petabytes.

Worked example 13 marks

A music file is 5 MB. How many bytes is this? How many bits?

  1. 1

    To convert MB to bytes, we multiply by 1000 twice (MB -> KB -> Bytes).

  2. 2

    5 MB = 5 * 1000 KB = 5,000 KB

  3. 3

    5,000 KB = 5,000 * 1000 Bytes = 5,000,000 bytes.

  4. 4

    To convert bytes to bits, we multiply by 8.

  5. 5

    5,000,000 bytes * 8 bits/byte = 40,000,000 bits.

Worked example 24 marks

Explain how an analogue sound wave is digitised.

  1. 1

    The process is called sampling.

  2. 2

    The amplitude (height) of the continuous analogue sound wave is measured at regular, fixed time intervals.

  3. 3

    The frequency of these measurements is called the sample rate.

  4. 4

    Each measurement (sample) is then rounded to the nearest available value (quantisation) and converted into a binary value. A sequence of these binary values represents the original sound.

Recap

  • The smallest unit of data is a bit (0 or 1).
  • 8 bits make a byte.
  • Storage units increase in size: byte, kilobyte (KB), megabyte (MB), gigabyte (GB).
  • To convert to a smaller unit, multiply by 1000. To convert to a larger unit, divide by 1000.
  • Analogue sound is continuous; digital data is discrete.
  • Sound is digitised by sampling the amplitude of the wave at regular intervals.

Quick check

  1. How many megabytes are in 2.5 gigabytes?1 mark
  2. What is the name of the process used to convert analogue sound to digital?1 mark

5. Building with Logic Gates

Logic gates are the fundamental building blocks of digital circuits. They are electronic devices that perform a basic Boolean logic operation on one or more binary inputs to produce a single binary output. The main gates are NOT, AND, OR, and XOR. These can be combined to create complex logic circuits that perform calculations, make decisions, and store data. A Boolean expression is an algebraic way to represent a logic circuit.

A AND B is written as A . B or AB

A OR B is written as A + B

NOT A is written as Ā

A XOR B is written as A ⊕ B

Key term

Logic Gate: An electronic circuit that performs a logical operation on one or more binary inputs to produce a single binary output.

Examiner insight

Examiners require students to draw the standard symbols for logic gates accurately. When drawing a circuit from an expression, work methodically from the inputs towards the final output, paying attention to brackets which indicate the order of operations.

Common pitfall

Confusing the OR and XOR gates. An OR gate outputs 1 if AT LEAST ONE input is 1. An XOR (Exclusive OR) gate outputs 1 only if the inputs are DIFFERENT (one is 1 and the other is 0).

Worked example 14 marks

Draw the logic circuit for the Boolean expression X = (A AND B) OR (NOT C).

  1. 1
    1. Start with three inputs from the left: A, B, and C.
  2. 2
    1. Draw an AND gate. Connect inputs A and B to it.
  3. 3
    1. Draw a NOT gate. Connect input C to it.
  4. 4
    1. Draw an OR gate to the right of the first two gates.
  5. 5
    1. Connect the output of the AND gate as one input to the OR gate.
  6. 6
    1. Connect the output of the NOT gate as the second input to the OR gate.
  7. 7
    1. Label the final output of the OR gate as X. (A diagram would be drawn showing the gates connected as described).

Recap

  • Logic gates perform Boolean operations on binary inputs.
  • The main gates are NOT, AND, OR, and XOR.
  • A NOT gate has one input and inverts it (0 becomes 1, 1 becomes 0).
  • An AND gate outputs 1 only if all inputs are 1.
  • An OR gate outputs 1 if any input is 1.
  • Logic circuits are created by combining logic gates to solve problems.

Quick check

  1. An AND gate has two inputs, both are 1. What is the output?1 mark
  2. An OR gate has two inputs, both are 0. What is the output?1 mark

6. Machine Learning and Modern Industry

Machine learning (ML) is a type of artificial intelligence (AI) that allows computer systems to learn and improve from experience without being explicitly programmed. Instead of following a fixed set of instructions, an ML model is 'trained' on large amounts of data to recognise patterns. It can then make predictions or decisions about new, unseen data. Computerisation is the process of introducing computers to automate tasks previously done by humans. This has had a massive impact on industry and manufacturing, bringing both benefits (like increased efficiency and precision) and risks (like job displacement and high initial costs).

Key term

Machine Learning: The ability of a computer system to automatically learn and improve from experience (data) without being explicitly programmed.

Examiner insight

When discussing machine learning, provide specific, real-world examples (e.g., spam filtering, recommendation engines) rather than just a general definition. For computerisation, a balanced answer that discusses both advantages and disadvantages is required for top marks.

Common pitfall

Treating 'Machine Learning' and 'Artificial Intelligence' as the same thing. Machine learning is a subset of AI. AI is the broader concept of machines being able to carry out tasks in a way we would consider 'smart'.

Fun fact

Netflix's recommendation engine, which uses machine learning to suggest what you should watch next, is estimated to be worth over $1 billion per year to the company by reducing customer cancellation.

Worked example 14 marks

Describe two different applications where machine learning is used.

  1. 1
    1. Email Spam Filtering: Machine learning models are trained on vast numbers of emails labelled as 'spam' or 'not spam'. They learn to identify patterns and features common in spam emails (e.g., certain keywords, sender reputation). When a new email arrives, the model predicts whether it is spam and moves it to the junk folder if necessary.
  2. 2
    1. Medical Image Analysis: ML algorithms can be trained on thousands of medical scans (like X-rays or MRIs) that have been diagnosed by expert radiologists. The system learns to detect anomalies, such as tumours or fractures. It can then assist doctors by highlighting potential areas of concern in new patient scans, often with high accuracy.

Worked example 24 marks

Explain one benefit and one risk of the computerisation of a traditional car manufacturing plant.

  1. 1

    Benefit: Increased Precision and Consistency. Robots on an assembly line can perform tasks like welding, painting, and component fitting with a level of precision and consistency that is difficult for humans to match over long shifts. This leads to higher quality products with fewer defects.

  2. 2

    Risk: Job Displacement. The introduction of robots and automated systems to perform tasks previously done by human workers can lead to significant job losses on the factory floor. This can cause social and economic problems for the displaced workers and their communities if they are not retrained for new roles.

Recap

  • Machine learning allows computers to learn from data without explicit programming.
  • ML is used in recommendation engines, spam filters, voice assistants, and medical diagnosis.
  • Computerisation is the automation of tasks using computers.
  • Benefits of computerisation in industry include increased efficiency, consistency, and safety.
  • Risks include high initial cost, job displacement, and security vulnerabilities.
  • A balanced view considering both pros and cons is important.

Quick check

  1. Is machine learning a type of AI, or is AI a type of machine learning?1 mark
  2. State one advantage of using robots in a factory instead of humans.1 mark

End-of-chapter exercise

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

  1. Define 'systems software' and give one example.2 marks
  2. A file is 20,000 bytes. Convert this size to kilobytes (KB). Show your working.2 marks
  3. Explain the difference between a high-level language and a low-level language, and state why translators are necessary.4 marks
  4. Describe the 'execute' stage of the fetch-decode-execute cycle and name two registers that are essential for the cycle to function.5 marks
  5. Draw a logic circuit for the Boolean expression Y = NOT (A OR B). Also, provide the truth table for this circuit.5 marks
  6. Explain how increasing the sample rate and the bit depth affects a digitised sound file.4 marks
  7. A software company is releasing a new video game. Should they distribute the compiled version or the interpreted version to customers? Justify your answer with three distinct points.6 marks
  8. Compare and contrast the roles of the Memory Address Register (MAR) and the Memory Data Register (MDR) within the fetch-decode-execute cycle.4 marks
  9. 'Computerisation of traditional industries has more risks than benefits.' Discuss this statement, providing two arguments for and one argument against.6 marks
  10. Name and describe two types of utility program that help manage a computer system.4 marks

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