Computer System Organisation

Hardware and software. Hardware is the electro-mechanical (physical) parts; software is the collection of programs executed on the computer.

System software interacts directly with the hardware and provides an environment where users can work with friendly interfaces. Examples: Linux, GCC compiler, Python interpreter, device drivers.

Application software helps users perform routine tasks and uses mechanisms provided by the operating system. Examples: accounting software, word processors, browsers, games.

Input unit, output unit, CPU (control unit, ALU, registers, cache), primary memory and secondary memory (together forming the memory unit).

Registers and the arithmetic logic unit. The hard disk, keyboard and RAM are outside the CPU.

Registers are tiny, high-speed memory locations inside the CPU. They hold data, memory addresses, intermediate results and the instructions currently being executed. They are very expensive to build, so their number and capacity are limited.

• Instruction fetch: the control unit brings the next instruction from memory to the CPU.
• Instruction decode: the decoding logic identifies the action to be performed.
• Operand fetch: if data is needed, it is read from memory into a register (skipped if no data is needed).
• Instruction execution: the ALU performs the operation under the control unit's direction. The control unit then fetches the next instruction.

It performs all operations, controls system activities, processes information quickly and coordinates tasks.

Hertz, usually gigahertz (GHz) for modern processors. It tells how many clock cycles the processor performs per second.

Desktop/laptop: Intel Core i5/i7, AMD Ryzen, Apple M-series. Smartphones/tablets: Apple A-series, Snapdragon, MediaTek.

Heterogeneous computing means using dedicated processors for specific tasks alongside the general-purpose CPU.

• GPU: has hundreds or thousands of simple processors for parallel calculations; used for graphics and training AI models.
• TPU: built to handle the mathematical operations of neural networks, more efficient for AI than a CPU or standard GPU.
• NPU: an accelerator in smartphones and laptops that runs AI tasks efficiently on the device.

A motherboard is a large Printed Circuit Board (PCB) to which all major components are connected. It also provides expansion slots. Essential parts: CPU socket, memory (RAM) slots, buses, system clock, BIOS chip, and connectors for peripherals.

Buses are electronic pathways that carry communication between the CPU, memory and peripherals. The system clock generates precise timing signals that synchronise all operations.

Ports are physical connectors on the exterior of a computer that connect peripherals for data transfer, power supply or signal transmission.

Expansion slots are sockets on the motherboard for installing expansion cards that add or enhance functions in desktop or server computers.

• PCIe (Peripheral Component Interconnect Express): current standard for high-speed communication with expansion cards.
• M.2: small modern slot used mainly for high-speed SSDs or wireless network cards.
• AGP (Accelerated Graphics Port): older legacy slot for graphics cards, replaced by PCIe.

Volatile memory needs continuous power; all content is lost when power is removed. Example: RAM (also cache and registers).

Non-volatile memory keeps its data when power is off. Example: ROM, hard disk.

1 nibble = 4 bits; 1 byte = 8 bits; 1 KB = 1024 bytes; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB. Beyond that: PB, EB, ZB, YB, each 1024 times the previous.

No. Data and instructions to be processed by the CPU must be placed in RAM, because the CPU accesses primary memory directly. Secondary storage is too slow and is not directly accessible, so the loader copies the program into RAM before execution.

BIOS is a small program stored in ROM. When the computer is switched on, it performs the Power-On Self-Test (POST) to check that memory, keyboard and display work properly. If all is well, it starts loading the operating system into RAM.

Both initialise hardware and load the operating system. UEFI is the modern replacement: it offers faster booting, better security (Secure Boot), support for larger storage devices, and a more flexible, user-friendly environment. Traditional BIOS lacks these advantages.

PROM: manufactured blank, programmed only once. EPROM: erased with ultraviolet light and rewritten. EEPROM: erased electrically and reprogrammed many times.

The CPU is much faster than RAM and would otherwise wait for data. Cache is a small, very fast memory that stores frequently accessed data, instructions and intermediate results. The CPU first checks the cache; if the data is found (a cache hit) it is read immediately, which is far quicker than accessing RAM. This reduces the delay between the CPU and main memory.

L1: closest to the CPU, extremely fast but small. L2: larger than L1, slightly slower, still faster than main memory. L3: shared among cores in multi-core processors, larger and slower than L2.

Fastest to slowest: Registers → Cache → RAM → Hard disk.

RAM contents are lost when power is off, so secondary memory provides permanent storage. It has much larger capacity at lower cost and is portable (pen drives, external disks).

Primary memory is directly accessed by the CPU, is faster, but has limited capacity and (for RAM) is volatile. Secondary memory is slower, larger and non-volatile.

NVMe is a communication protocol designed for high-speed SSDs that use the PCIe interface. Older SSDs used SATA, created for slower hard disks, which prevented them from reaching full speed. NVMe lets the computer and SSD communicate directly through PCIe lanes and handle thousands of parallel requests, giving faster start-up, application loading and heavy tasks like gaming and video editing.

Cloud storage is a modern form of secondary storage where data is stored on remote servers and accessed over the Internet. It is scalable, supports backup and recovery, and uses encryption and access control. Examples: Google Drive, OneDrive, Dropbox, Apple iCloud.

• Keyboard: enters text and commands.
• Mouse: points at and selects items on the screen.
• Scanner: converts physical documents into digital form.
• Microphone: captures sound.

Others: touchscreen, joystick, webcam, biometric sensor, barcode reader, OMR, MICR.

OMR reads marked answer sheets; MICR processes bank cheques.

LCD monitors; TFT monitors (LCD with improved image quality); LED monitors (energy-efficient flat panels); OLED monitors (better contrast, thinner designs, used in high-end computers, laptops and smartphones); and projectors, which project images onto a large screen.

Dot matrix printers (billing counters, invoices, multipart forms), laser printers, inkjet printers and thermal printers (homes, offices, shops, hospitals, industries).

Plotters produce large, high-precision drawings (drum and flatbed types) for engineering, architecture and map making. 3D printers create physical objects from digital models using plastic, resin or metal. Speakers are audio output devices.

Operating system, language processors, linkers and loaders, and utility software.

An OS is a set of programs that controls the hardware and provides services to application software, programmers and users. It is the first program loaded into memory at start-up with the help of BIOS. Examples: DOS, Windows, Unix, Linux, macOS, Android, iOS, HarmonyOS.

• Process management: runs multiple processes by switching the CPU between them quickly; scheduling decides which process gets the CPU and for how long.
• Memory management: allocates and deallocates RAM and prevents one process from interfering with another's memory.
• File management: creates and maintains the file system; creates, reads, writes, renames, copies, moves and deletes files; manages permissions.
• Device management: controls I/O devices through device drivers and decides which process may use a device.

Special software through which the OS communicates with a hardware device. It converts OS commands into device-specific commands.

Advantage of a compiler: the compiled stand-alone machine code runs faster. Advantage of an interpreter: errors are reported line by line, so debugging is easier and execution starts without waiting for full compilation.

An assembler translates assembly language (mnemonics like ADD, MOV) into machine code. Instruction codes and register names vary for each CPU type, so the assembler must be customised for a specific CPU architecture.

Source code is first compiled to bytecode, which a Virtual Machine (VM) runs, using a JIT compiler to convert frequently used parts to native code. This makes the same bytecode portable: it runs on any machine that has the VM.

Disadvantage: an extra layer (the VM) is required, and execution is generally slower than directly compiled native code, at least initially.

The linker runs after the compiler/assembler. It merges separately compiled object files with required library code to produce one complete executable program.

The loader is part of the OS. It allocates space in RAM, copies the executable's instructions and data from disk into memory, and transfers control to the program's starting instruction.

• File management tools: create, copy, delete, rename, move, search and organise files (Windows File Explorer, macOS Finder).
• Antivirus software: detects, prevents and removes malware such as viruses, worms, trojans, spyware and ransomware.
• Backup tools: periodically duplicate data so it can be recovered after disk failure, corruption or deletion.
• Disk cleaner: finds temporary files, cache and junk, and long-unused files, and offers to delete them to free space.
• Compression tools: zip files to occupy less space and unzip them later (WinZip, WinRAR, 7-Zip).

General purpose: common everyday tasks such as word processing, presentations, spreadsheets, databases. Customised: developed for specific needs, such as inventory, payroll, school management, hotel reservation or weather forecasting software.

Problem specification → Algorithm design → Coding → (Translation) → Debugging → Execution and testing → (Documentation).

The input (data available), the process (operations needed to transform the data) and the expected output.

An algorithm is a structured, step-by-step description of how to solve a problem, usually written in natural language. A flowchart is a visual, diagrammatic representation of the algorithm. Together they act as the blueprint of code.

• Syntax error: rules of the language are violated; detected automatically by the compiler/interpreter.
• Logical error: improper program logic; the program runs but gives wrong output. Needs careful analysis by the programmer.
• Run-time error: occurs during execution, causing unexpected termination, e.g. division by zero or opening a non-existent file.

Testing runs the program with varied input data to verify it consistently gives accurate results and meets the original requirements. Documentation describes the program: internal (comments, technical details) and external (user manuals and instructions).

Start and Stop, Input (Read/Accept), Process, Assignment, Output (Print/Write), Conditional (If / Else / End of If) and Repetition (Repeat / While).

So: (a) reading a variable → parallelogram; (b) assignment a = 3 → rectangle; (c) testing whether a > 0 → rhombus.

Algorithm:

Step 1: Start
Step 2: Input R
Step 3: AREA = 3.14 × R × R
Step 4: Print AREA
Step 5: Stop

Flowchart: Start (ellipse) → Input R (parallelogram) → AREA = 3.14 × R × R (rectangle) → Print AREA (parallelogram) → Stop (ellipse), joined by arrows.

Algorithm:

Step 1: Start
Step 2: Input A, B, C
Step 3: If (A < B) Then go to Step 4, else go to Step 7
Step 4: If (A < C) Then SMALL = A, else SMALL = C
Step 5: Go to Step 9
Step 7: If (B < C) Then SMALL = B, else SMALL = C
Step 9: Print SMALL
Step 10: Stop

Flowchart: Start → Input A, B, C → decision A < B?. On True, a second decision A < C? gives SMALL = A (True) or SMALL = C (False). On False, a decision B < C? gives SMALL = B (True) or SMALL = C (False). All paths join at Print SMALL → Stop.

Step 1: Start
Step 2: Input N1, N2
Step 3: If (N1 > N2) Then
Step 4:   Print N1
Step 5: Else
Step 6:   Print N2
Step 7: End of If
Step 8: Stop

If the condition is true, the statements between If and Else run and the Else part is skipped; if false, the Else part runs instead.