Computer Systems — Diagnostic Tests

Unit Tests

UT-1: Hardware Components

Question:

(a) Describe the function of the following components in a computer system: CPU (Central Processing Unit), RAM (Random Access Memory), hard disk drive, and motherboard.

(b) Explain the difference between RAM and ROM. State one use of each.

(c) The CPU has three main components: the ALU, the control unit, and registers. Describe the function of each.

(d) A computer has a clock speed of $3.2\,\text{GHz}$. Explain what this means and describe how clock speed affects processor performance.

Solution:

(a)

• CPU (Central Processing Unit): The “brain” of the computer that processes data and instructions. It fetches, decodes, and executes instructions from programs.
• RAM (Random Access Memory): Volatile memory used to store data and instructions that the CPU needs to access quickly. It loses its contents when power is turned off.
• Hard disk drive: A non-volatile storage device that stores the operating system, applications, and user data permanently. Data persists when power is off.
• Motherboard: The main circuit board that connects all components (CPU, RAM, storage, graphics card, etc.) and provides communication pathways (buses) between them.

(b) RAM is volatile (loses data when power is off), allows both read and write operations, and is used for temporary storage of data the CPU is currently working with. ROM (Read Only Memory) is non-volatile (retains data without power), typically allows only read operations, and stores permanent data such as the BIOS/firmware that boots the computer.

(c)

• ALU (Arithmetic Logic Unit): performs arithmetic operations (addition, subtraction, multiplication, division) and logical operations (AND, OR, NOT, comparison).
• Control Unit: coordinates and manages the activities of the CPU. It fetches instructions from memory, decodes them, and sends signals to other components to execute them. It controls the timing and sequence of operations.
• Registers: small, high-speed storage locations within the CPU that hold data and instructions being processed. Examples include the program counter (PC), accumulator (ACC), memory address register (MAR), and memory data register (MDR).

(d) A clock speed of $3.2\,\text{GHz}$ means the CPU executes $3.2 \times 10^9 = 3{,}200{,}000{,}000$ clock cycles per second. Each clock cycle represents one basic operation (or part of an operation) performed by the CPU. A higher clock speed means more instructions can be executed per second, so the processor performs faster. However, clock speed alone does not determine overall performance; the number of cores, cache size, and the instruction set architecture also play significant roles.

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UT-2: Software and Operating Systems

Question:

(a) Explain the difference between system software and application software. Give two examples of each.

(b) Describe the main functions of an operating system.

(c) Explain the terms “open source software” and “proprietary software.” Give one example of each and state an advantage and disadvantage of each.

(d) Describe the difference between a compiler and an interpreter. State one advantage and one disadvantage of each.

Solution:

(a) System software manages and controls the hardware and provides a platform for other software to run. Examples: operating systems (Windows, Linux, macOS), device drivers, utility programs (disk defragmenter, antivirus).

Application software is designed to perform specific tasks for the user. Examples: word processors (Microsoft Word), web browsers (Chrome, Firefox), spreadsheet software (Excel).

(b) Main functions of an operating system:

1. Memory management: allocates memory to running programs and ensures programs do not interfere with each other’s memory.
2. Processor management: schedules CPU time among running processes, deciding which process runs and for how long.
3. Device management: controls and communicates with hardware devices through device drivers.
4. File management: organises files in a directory structure, handles file creation, deletion, and access permissions.
5. User interface: provides a way for users to interact with the computer (command line or graphical user interface).
6. Security: manages user accounts, passwords, and access permissions to protect data.

(c) Open source software: software whose source code is freely available for users to view, modify, and distribute. Example: Linux operating system. Advantage: free to use and customisable; disadvantage: may lack official technical support and can have compatibility issues.

Proprietary software: software whose source code is owned by a company and is not freely available. Users purchase a licence to use it. Example: Microsoft Windows. Advantage: typically has official support and guarantees of quality; disadvantage: users cannot modify it and must pay licence fees.

(d) A compiler translates the entire source code into machine code in one go before execution, producing an executable file. Advantage: the compiled program runs faster because translation is already done. Disadvantage: the entire program must be recompiled after any change, and error messages may be harder to relate to specific lines of code.

An interpreter translates and executes the source code one line at a time. Advantage: easier for debugging because errors are reported immediately when the offending line is reached. Disadvantage: the program runs more slowly because translation happens during execution.

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UT-3: Data Representation

Question:

(a) Convert the binary number $11010110_2$ to decimal, hexadecimal, and octal.

(b) Explain why computers use binary to represent data. What is the advantage of using hexadecimal as a shorthand notation for binary?

(c) A computer stores text using 8-bit ASCII. The ASCII code for ‘A’ is $65$ and for ‘a’ is $97$. Convert the word “Cat” to its binary representation using ASCII codes.

(d) Explain the difference between lossy and lossless compression. Give one example of a file format that uses each type.

Solution:

(a) Binary to decimal: $11010110_2 = 128 + 64 + 0 + 16 + 8 + 0 + 2 + 0 = 218$.

Decimal to hexadecimal: $218 / 16 = 13$ remainder $10$. $13 = \text{D}$, $10 = \text{A}$. So $218_{10} = \text{DA}_{16}$.

Decimal to octal: $218 / 8 = 27$ remainder $2$. $27 / 8 = 3$ remainder $3$. $3 / 8 = 0$ remainder $3$. So $218_{10} = 332_8$.

(b) Computers use binary because electronic circuits have two states: on (high voltage, representing 1) and off (low voltage, representing 0). Transistors in logic gates naturally operate in these two states, making binary the natural representation for digital systems.

Hexadecimal is a useful shorthand because one hexadecimal digit represents exactly four binary digits (bits). This makes large binary numbers much easier to read and write. For example, the 16-bit binary number $1101111010101101$ is much easier to read as $\text{DEAD}_{16}$.

(c) ASCII codes: $\text{C} = 67$, $\text{a} = 97$, $\text{t} = 116$.

• $\text{C} = 67_{10} = 64 + 2 + 1 = 01000011_2$
• $\text{a} = 97_{10} = 64 + 32 + 1 = 01100001_2$
• $\text{t} = 116_{10} = 64 + 32 + 16 + 4 = 01110100_2$

“Cat” in binary: $01000011\, 01100001\, 01110100$

(d) Lossy compression: permanently removes some data to reduce file size. The original file cannot be perfectly reconstructed from the compressed version. Some quality is lost. Example: JPEG (images), MP3 (audio).

Lossless compression: reduces file size without losing any data. The original file can be perfectly reconstructed from the compressed version. Example: PNG (images), FLAC (audio), ZIP (general files).

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Integration Tests

IT-1: Hardware, Software, and Data Combined

Question:

(a) A school is planning to upgrade its computer lab. Describe three hardware specifications they should consider and explain how each affects performance for typical school applications (web browsing, word processing, and light programming).

(b) A student writes a program in Python and runs it on their computer. Describe the sequence of events from typing the source code to the program producing output. Include the roles of the editor, operating system, interpreter, RAM, and CPU.

(c) A hospital stores patient records digitally. Each record contains the patient’s name (20 characters), date of birth (10 characters), blood type (3 characters), and a 1-byte boolean flag indicating whether the patient has allergies. Calculate the storage required for 5000 patient records in: (i) bytes, (ii) kilobytes, assuming each character uses 1 byte.

(d) Evaluate the claim that “cloud storage will completely replace local storage.” Present arguments for and against.

Solution:

(a) Three hardware specifications:

1. RAM: Determines how many applications can run simultaneously without slowdown. For school use, 8 GB is sufficient for web browsing and word processing, while 16 GB is better for programming with IDEs.
2. Processor (CPU): Affects the speed at which programs execute. A modern multi-core processor (e.g., Intel i5 or AMD Ryzen 5) provides adequate performance for school tasks.
3. Storage type (SSD vs HDD): SSDs have much faster read/write speeds than HDDs, meaning faster boot times, quicker application loading, and better overall responsiveness. SSDs are strongly recommended.

(b) The student types the Python source code in a text editor (which runs as an application managed by the operating system). When the student runs the program, the operating system loads the Python interpreter from storage into RAM and schedules it for execution on the CPU. The interpreter reads the source code line by line, translating each line into machine code and executing it on the CPU. The operating system manages memory allocation, ensuring the program and its data are stored in RAM while running. When the program produces output, the operating system sends it to the display (via the graphics subsystem).

(c) Each record: $20 + 10 + 3 + 1 = 34$ bytes.

(i) 5000 records: $5000 \times 34 = 170{,}000$ bytes. (ii) $170{,}000 / 1024 = 166.02\,\text{KB} \approx 166\,\text{KB}$.

(d) Arguments for: cloud storage offers accessibility from any device, automatic backup, scalable capacity, and reduced hardware costs for the user. Many applications already operate primarily in the cloud.

Arguments against: local storage is faster (no network latency), works without internet access, provides complete data ownership and control, and avoids ongoing subscription costs. Sensitive data (medical records, financial information) may be subject to privacy regulations that favour local storage. Cloud services can suffer outages and security breaches. In practice, a hybrid approach (local storage for speed and sensitive data, cloud for backup and accessibility) is the most practical solution.

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IT-2: System Architecture and Performance

Question:

(a) Describe the Von Neumann architecture. Name the key components and explain the fetch-decode-execute cycle.

(b) Explain the purpose of cache memory. Where is it located and how does it improve performance?

(c) A computer system uses a 32-bit address bus and a 64-bit data bus. Explain what this means and calculate the maximum addressable memory.

(d) Solid state drives (SSDs) use NAND flash memory while hard disk drives (HDDs) use magnetic platters. Compare and contrast these two storage technologies in terms of speed, durability, cost per GB, and typical use cases.

Solution:

(a) The Von Neumann architecture is a computer design model in which:

• Data and instructions are both stored in the same memory (the stored-program concept).
• A single set of buses connects the memory to the CPU, input devices, and output devices.
• The CPU contains an ALU, control unit, and registers.

The fetch-decode-execute cycle:

1. Fetch: The address of the next instruction (held in the PC) is copied to the MAR. The instruction is fetched from memory (at the address in the MAR) and placed in the MDR. The PC is incremented.
2. Decode: The instruction in the MDR is decoded by the control unit to determine what operation to perform and which operands are needed.
3. Execute: The instruction is executed (e.g., the ALU performs a calculation, or data is moved to/from memory).

(b) Cache memory is a small, very fast type of memory located between the CPU and RAM (often on or very near the CPU chip). It stores frequently accessed data and instructions so that the CPU can retrieve them faster than accessing RAM. Since cache is much faster than RAM but much more expensive, it is kept small (typically a few MB). When the CPU needs data, it checks cache first; if the data is found (a “cache hit”), it is retrieved quickly. If not (a “cache miss”), the data must be fetched from slower RAM. This significantly reduces the average time the CPU spends waiting for data.

(c) A 32-bit address bus means the CPU can address $2^{32}$ unique memory locations. A 64-bit data bus means 64 bits (8 bytes) of data can be transferred simultaneously between the CPU and memory in a single bus cycle.

Maximum addressable memory $= 2^{32}$ bytes $= 4{,}294{,}967{,}296\,\text{bytes} = 4\,\text{GB}$.

(d)

Feature	SSD (NAND Flash)	HDD (Magnetic)
Speed	Fast read/write (hundreds of MB/s)	Slower read/write (tens to hundreds of MB/s)
Durability	No moving parts, resistant to physical shock	Moving parts, vulnerable to physical damage
Cost per GB	More expensive	Less expensive
Capacity	Typically 256 GB to 4 TB	Typically 500 GB to 20 TB
Use cases	Boot drive, applications, laptops, fast workloads	Bulk storage, backups, archival data

SSDs are preferred for the operating system and frequently used applications due to their speed. HDDs remain useful for large-capacity storage where speed is less critical.

Summary

The key principles covered in this topic are linked in the sub-pages above. Focus on understanding the definitions, applying the formulas or frameworks, and evaluating strengths and limitations of each approach.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.

Common Pitfalls

• Confusing RAM (volatile, temporary) with ROM (non-volatile, permanent) or with storage (hard disk).
• Forgetting that hexadecimal digits A-F represent the decimal values 10-15.
• Confusing the address bus (carries memory addresses, determines max memory) with the data bus (carries data, determines transfer width).
• Stating that the CPU “saves” data to hard disk directly — the CPU works with RAM; storage access goes through the operating system.
• Confusing compilers and interpreters: compilers translate all code before execution; interpreters translate and execute one line at a time.