Describe the principal operations of hardware devices

3.1 Computers and Their Components

Objective

Describe the principal operations of all hardware devices, including registers, the Von Neumann model, memory hierarchy, logic‑gate fundamentals, embedded systems and monitoring‑control loops. Explain related concepts such as buffers, RAM/ROM sub‑types, peripheral ports and the role of the control unit.

1. Von Neumann Architecture & Memory Hierarchy

• Von Neumann model: a single shared memory stores both data and program instructions; the CPU fetches instructions sequentially unless a branch occurs.
• Memory hierarchy (from fastest/most expensive to slowest/cheapest):

  1. CPU registers (few bytes)
  2. Cache (L1, L2, L3) – SRAM, very low latency
  3. Main memory – DRAM
  4. Secondary storage – HDD, SSD, optical discs, magnetic tape

• Each level acts as a buffer for the next, reducing average access time.

2. CPU – Core Components and Operations

2.1 Registers (key for the instruction cycle)

2.2 Arithmetic‑Logic Unit (ALU)

• Performs arithmetic (add, subtract, multiply, divide) and logical (AND, OR, NOT, XOR) operations on data supplied by registers.
• Outputs result to a destination register and sets status flags.

2.3 Control Unit (CU)

• Generates the sequence of control signals that orchestrate the fetch‑decode‑execute‑write‑back cycle.
• Handles interrupts (hardware or software) by saving the current PC, jumping to an interrupt service routine, and restoring state on return.
• Implements simple pipeline stages (fetch, decode, execute, memory, write‑back) in modern CPUs to increase instruction throughput.

2.4 Instruction Cycle (using Register‑Transfer Notation)

IF  MAR ← PC;  PC ← PC + 1;  READ → MDR
ID  IR ← MDR;  Decode(IR)
EX  ALU ← Operands;  (e.g., ACC ← ACC + R1)
WB  MDR → Destination;  Update Flags

Some textbooks combine the last two steps into a single “Store” phase.

3. Memory Devices

• Read – Transfer data from a memory location to the CPU (via MDR).
• Write – Transfer data from the CPU to a memory location.
• Refresh (DRAM only) – Periodically rewrite stored charge to prevent loss.

3.1 RAM & ROM Sub‑types

4. Logic Gates and Logic Circuits (required by 3.2)

4.1 Basic Gate Symbols & Truth Tables

4.2 Combinational Example – Half‑Adder

• Inputs: A, B
• Sum = A XOR B
• Carry = A AND B

5. Input Devices

• Signal detection – Capture mechanical, optical or electrical events.
• Analog‑to‑Digital conversion (A/D) – Sample and quantise the analogue signal.
• Encoding – Convert the digitised value into a standard code (e.g., scan codes for keyboards).

6. Output Devices

• Digital‑to‑Analog conversion (D/A) – Required for audio and many video signals.
• Rendering / Formatting – Turn binary data into pixels, ink marks or sound waves.

7. Storage Devices (Secondary Memory)

• Sequential access – Fixed order reading/writing (magnetic tape).
• Random access – Directly address any location (HDD, SSD, optical disc).
• Erase/Write cycle (Flash) – A block must be erased before programming; limited cycles affect endurance.

8. Bus, Control Unit & Buffers

• Data bus – Carries binary data between CPU, memory and I/O.
• Address bus – Transports the memory address of the source or destination.
• Control bus – Carries read/write, interrupt, clock and other timing signals.
• Buffers – Temporary storage that decouples devices with different speeds (e.g., printer buffer, video frame buffer).

8.1 Peripheral Connection Ports

• USB – universal serial bus, hot‑plug, power delivery.
• HDMI – uncompressed video + audio.
• VGA – legacy analogue video.
• Ethernet (RJ‑45) – LAN/WAN networking.
• Audio jacks (3.5 mm) – headphones, microphones, line‑out.
• Serial (RS‑232) / Parallel – legacy industrial or printer interfaces.

9. Embedded Systems (required by 3.1)

• Definition: A computer system dedicated to a specific control task, often with real‑time constraints, and usually integrated into a larger device.
• Typical components: microcontroller (CPU + on‑chip RAM/ROM), I/O ports, sensors and actuators.
• Principal operations:

  1. Read sensor data (A/D conversion if needed).
  2. Execute a small, often looped, program stored in non‑volatile memory.
  3. Control actuators (digital output, PWM, etc.).

• Examples: digital watches, microwave ovens, automotive engine‑control units, IoT temperature logger.

10. Monitoring and Control Systems

• Sensor (input) – Converts a physical quantity (temperature, light, pressure) into a digital signal.
• Processor (CPU) – Reads the sensor, runs decision‑making software, possibly using interrupts.
• Actuator (output) – Receives a control signal and performs a physical action (motor, valve, LED).

Typical closed‑loop example: thermostat → temperature sensor → CPU decides → heating element (actuator).

11. Summary of Principal Operations