Computer Systems - Detailed Notes

Introduction to Computer Systems

• A computer is an electronic device that accepts data (input), processes it, and generates results (output).

• A computer system includes the computer itself along with additional hardware and software.

• Key components: Central Processing Unit (CPU), memory, input/output devices, and storage devices.

• These components work together to deliver the desired output.

• Computer systems vary in form and size, from high-end servers to personal desktops, laptops, tablets, and smartphones.

• Figure 1.1 shows the block diagram of a computer system, illustrating data flow between components.

Central Processing Unit (CPU)

• The CPU is the electronic circuitry that performs the actual processing; often referred to as the "brain" of the computer.

• Also known as the processor.

• Physically, a CPU is placed on one or more microchips, which are integrated circuits (ICs) made of semiconductor materials.

• Alan Turing Quote: "A computer would deserve to be called intelligent if it could deceive a human into believing that it was human."

CPU Operation

• The CPU receives instructions and data through programs.

• It fetches the program and data from memory, performs arithmetic and logic operations according to the instructions, and stores the result back in memory.

• During processing, the CPU stores data and instructions in its local memory called registers.

  • Registers are part of the CPU chip, limited in size and number.

  • Different registers store data, instructions, or intermediate results.

• Main CPU components: Arithmetic Logic Unit (ALU) and Control Unit (CU).

  • ALU performs arithmetic and logic operations.

  • CU controls sequential instruction execution, interprets instructions, and manages data flow through memory, ALU, and I/O devices.

• The CPU is also known as a microprocessor (covered in section 1.5).

Input Devices

• Devices that send control signals to a computer.

• Convert input data into a digital form that the computer can accept.

• Examples: keyboard, mouse, scanner, touch screen (Figure 1.2).

• Braille keyboards are available for visually impaired users.

• Voice input is possible (e.g., Google voice search).

• Input data is temporarily stored in the main memory (RAM).

• For permanent storage, data and instructions are stored in secondary memory.

Output Devices

• Devices that receive data from a computer for display or physical production.

• Convert digital information into human-understandable form.

• Examples: monitor, projector, headphone, speaker, printer (Figure 1.3).

• Braille display monitors are useful for visually challenged users.

• Common printer types: inkjet, laserjet, and dot matrix.

• 3D printers create physical replicas of digital 3D designs.

  • Used in manufacturing for prototypes and explored in the medical field for developing body organs.

Evolution of Computers

• Computing devices have evolved from simple calculators to powerful data processors in a relatively short time.

• Figure 1.4 shows a timeline of key inventions in computing technology.

Key Inventions

• Abacus: Mechanical device for simple arithmetic calculations (500 BC).

• Pascaline: Mechanical calculator invented by Blaise Pascal for addition and subtraction (1642).

• Analytical Engine: Invented by Charles Babbage, a mechanical computing device for inputting, processing, storing, and displaying output; considered the basis of modern computers (1834).

• Tabulating Machine: Designed by Herman Hollerith for summarising data stored on punched cards; a step towards programming (1890).

• Turing Machine: The theoretical general-purpose programmable machine capable of solving any problem by executing a program stored on punched cards (1937).

• EDVAC/ENIAC: Computers developed based on John Von Neumann's concept of stored program, capable of storing data and programs in memory (1945).

• Transistor: Developed at Bell Labs using semiconductor materials, replacing vacuum tubes (1947).

• Integrated Circuit (IC): A silicon chip containing an entire electronic circuit in a small area, drastically reducing computer size (1970).

Von Neumann Architecture

• Figure 1.5 illustrates the Von Neumann architecture.

• Consists of a CPU for processing, memory for storing data and programs, I/O devices, and communication channels.

• ENIAC is the first binary programmable computer based on Von Neumann architecture.

Advancements in Integration

• Large Scale Integration (LSI): Integrated a complete CPU on a single chip (microprocessor) in the 1970s.

• Moore’s Law: Predicted exponential growth in the number of transistors on a microchip.

• Very Large Scale Integration (VLSI): Integrated around 3 million components on a small chip in the 1980s, increasing processing power.

• Super Large Scale Integration (SLSI): Fabricates a high density of transistors (approx. components) on a single IC.

• Figure 1.6 illustrates the exponential growth in the number of transistors used in integrated circuits (ICs) over time.

Personal Computing

• IBM introduced its first personal computer (PC) for home use in 1981.

• Apple introduced Macintosh machines in 1984.

• The introduction of Graphical User Interface (GUI)-based operating systems by Microsoft increased PC popularity.

• The growth of the World Wide Web (WWW) in the 1990s accelerated computer usage.

• Laptops provided portable personal computing.

• Smartphones, tablets, and personal digital assistants leveraged processor miniaturisation, faster memory, and high-speed data connectivity.

• The next wave includes wearable gadgets (such as smartwatches and lenses) and smart appliances in the Internet of Things (IoT) that utilise Artificial Intelligence (AI).

Computer Memory

• Computer systems need memory to store data and instructions for processing.

• Main (primary) memory is typically what is referred to when discussing computer memory.

• Secondary memory (storage device) is used for permanent storage.

Units of Memory

• Computer systems use binary numbers to store and process data.

• The binary digits 0 and 1 are called bits.

• Bits are grouped to form words.

• Nibble: A 4-bit word (e.g., 1001, 1010).

• Byte: An 8-bit word (e.g., 01000110, 01111100).

• Bytes are grouped into larger units.

• Table 1.1 shows measurement units for digital data.

Table 1.1: Measurement Units for Digital Data

Types of Memory

• Two types of memory: primary and secondary.

Primary Memory

• Essential component: Programs and data are loaded before processing.

• The CPU interacts directly with primary memory for both read and write operations.

• Two types: Random Access Memory (RAM) and Read-Only Memory (ROM).

  • RAM: Volatile (data is retained as long as power is supplied).

    • Used for temporary data storage while the computer is working.

    • When the computer starts or an application launches, programs and data are loaded into RAM.

    • Faster than secondary memory.

  • ROM: Non-volatile (contents are not lost when power is off).

    • Used for permanent storage of rarely changed content (e.g., startup program/boot loader for loading the OS).

Cache Memory

• High-speed memory between the CPU and primary memory.

• Stores copies of frequently accessed data from primary memory, reducing access time.

• The CPU checks the cache first; if data is found, it is read from the cache; otherwise, primary memory is accessed.

Secondary Memory

• Limited storage capacity in primary memory; RAM is volatile, ROM is read-only.

• Secondary memory permanently stores data and instructions for future use.

• Non-volatile and has a larger storage capacity than primary memory.

• Slower and cheaper than main memory.

• Cannot be accessed directly by the CPU; contents must be loaded into main memory for CPU access.

• Examples: Hard Disk Drive (HDD), CD/DVD, Memory Card (Figure 1.7).

• Solid State Drives (SSDs) support faster data transfer speeds than Hard Disk Drives (HDDs).

• Portable flash drives (pendrives) have simplified data transfer.

Data Transfer Between Memory and CPU

• Data transfers occur between the CPU and primary memory, and between primary and secondary memory.

Buses

• Data is transferred using physical wires called buses (e.g., USB port to hard disk, hard disk to main memory).

• Three types of buses:

  • Data bus: Transfers data between components.

  • Address bus: Transfers addresses between the CPU and main memory.

  • Control bus: Communicates control signals between components.

• These buses collectively form the system bus (Figure 1.8).

Data Flow

• Data from input devices or the hard disk needs to be placed in main memory before processing by the CPU.

• The CPU uses the address bus to specify the memory location for reading or writing data.

• The CPU specifies read or write control signals via the control bus during instruction execution.

• The data bus is bidirectional because the CPU may read from or write to main memory.

• The control and address buses are unidirectional.

• Memory controller manages data flow into and out of main memory.

Microprocessors

• Early CPUs occupied a large room; now, they can be placed on a single microchip.

• A processor (CPU) implemented on a single microchip is called a microprocessor. Nowadays, nearly all CPUs are microprocessors.

• Microprocessors are small-sized electronic components that perform data processing, arithmetic, and logical operations.

• Modern microprocessors are built over integrated circuits with millions of components like resistors, transistors, and diodes.

• Microprocessors have evolved, increasing processing capability while decreasing physical size and cost.

• Current processors can process millions of instructions per millisecond.

• Table 1.2 lists different microprocessor generations, time periods, and underlying technology.

Table 1.2: Generations of Microprocessor

Microprocessor Specifications

• Classified based on chip type, word size, memory size, clock speed, etc.

Word Size

• The maximum number of bits a microprocessor can process at a time.

• Earlier, 8 bits was the maximum.

• Presently, minimum is 16 bits, and maximum is 64 bits.

Memory Size

• RAM size varies depending on word size.

• Initially, RAM was small (4MB) due to 4/8 bit word size.

• With 64-bit word size, RAM can be up to 16 Exabytes (EB).

Clock Speed

• The internal clock generates pulses (signals) at regular intervals.

• Indicates the speed at which the computer can execute instructions.

• Measured in Hertz (Hz) and Kilohertz (kHz) earlier; now measured in Gigahertz (GHz).

Cores

• A basic computation unit of the CPU.

• Earlier processors had only one core; now, multicore processors execute multiple tasks.

• CPUs with two, four, and eight cores are called dual-core, quad-core, and octa-core processors, respectively.

Microcontrollers

• A small computing device with a CPU, fixed RAM, ROM, and peripherals embedded on a single chip (Figure 1.9).

• Examples: keyboard, mouse, washing machine, digital camera, pendrive, remote controller, microwave.

• Designed for specific tasks, reducing size and cost.

• Embedded in another device or system to perform specific functionality.

• Example: a washing machine microcontroller controls the washing cycle automatically, saving time and effort.

Data and Information

• Computers primarily process data.

• A computer system considers everything as data (instructions, pictures, videos, etc.).

• Data can be raw and unorganized facts processed to get meaningful information.

• Understanding data types is crucial for understanding computer functioning.

Data and Its Types

• Input devices provide raw data in the form of facts, concepts, instructions, etc.

• Internally, everything is stored in binary form (0 and 1).

• Externally, data can be input as text (A-Z, a-z, 0-9, special symbols) or in other languages.

• The input data may be from different sources in different formats (e.g., images as RGB pixels, videos as frames).

Structured Data

• Data following a strict record structure.

• Easy to comprehend; stored in a data file.

• Organized in row/column format.

• May be sorted in ascending or descending order.

• Examples: monthly attendance records (Table 1.3), sales transactions, online ticketing, ATM transactions.

Table 1.3: Structured Data Example - Monthly Attendance Records

Unstructured Data

• Data not organized in a pre-defined record format.

• Examples: audio and video files, graphics, text documents, social media posts, satellite images (Figure 1.10).

Semi-structured Data

• Data with no well-defined structure but with internal tags or markings to separate data elements.

• Examples: email documents, HTML pages, CSV files (Figure 1.11).

• Each data value is preceded by a tag (Name, Month, Class, Attendance) for interpretation.

Data Capturing, Storage, and Retrieval

• To process data, it must be input or captured, stored, and then retrieved.

Data Capturing

• Involves gathering data from different sources in digital form.

• May vary from simple instruments like keyboards and barcode readers (Figure 1.12) to social media comments and remote sensors.

• Heterogeneity among data sources can make capturing complex.

Data Storage

• Storing captured data for later processing.

• Data is produced at a high rate, making storage challenging.

• Decreasing cost of digital storage devices helps.

• Numerous storage devices are available (Figure 1.7).

• Data servers in large organizations store vast amounts of data for processing efficiently.

• The cost of setting up and maintaining data servers can be high.

Data Retrieval

• Fetching data from storage devices for processing.

• Challenges increase with database size.

• Minimizing data access time is crucial for faster processing.

Data Deletion and Recovery

• Deletion is a significant threat to digital data.

• Storage devices malfunction, or users/hackers delete data.

• Deleting data means changing details at the bit level, which is time-consuming.

• When data is deleted, its address entry is marked as free without actual deletion.

• Data recovery retrieves deleted, corrupted, or lost data from secondary storage.

• Recovery is possible if the memory space has not been overwritten.

Security Concerns

• Unauthorized deletion can be avoided by limiting access and using passwords.

• Files can be encrypted for protection.

• Unwanted data recovery can be mitigated by using proper tools to delete or shred data before disposing of old or faulty storage devices.

Software

• Software comprises the set of instructions needed to operate computer hardware.

• It is a component that cannot be physically touched.

• Software and hardware together complete any task.

• Each software is written for a computational purpose.

• Examples: operating systems (Ubuntu, Windows), word processing tools (LibreOffice, Microsoft Word), video players (VLC Player), photo editors (GIMP).

• A soft copy is a document or image stored on disk; a hard copy is a printed version.

Need of Software

• To make computer hardware useful and operational.

• Software knows how to make different hardware components work together and communicate with the end-user.

• Software acts as an interface between users and hardware.

Categories of Software

• Classified by mode of interaction with hardware and functions performed:

  • System software

  • Programming tools

  • Application software

System Software

• Provides basic functionality to operate a computer by directly interacting with hardware.

• Manages different hardware components.

• Provides services to the end user or other software.

• Examples: operating systems, system utilities, device drivers.

Operating System

• System software that operates the computer.

• Manages application programs and provides access and security.

• Examples: Windows, Linux, Macintosh, Ubuntu, Fedora, Android, iOS.

System Utilities

• Software for maintenance and configuration.

• Some utilities are shipped with the OS (disk defragmentation, formatting).

• Others improve performance (anti-virus, disk cleaner).

Device Drivers

• Ensures proper functioning of a device.

• Operating system manages overall computer working.

• Device drivers handle specific hardware-level operations.

• Acts as an interface between devices and the OS, hiding operational details.

Programming Tools

• Needed to write instructions for the computer.

• Computer languages bridge the gap between human-readable code and machine language.

• Translators convert high-level code to machine language.

• Code editors (e.g., IDLE in Python) are used to write instructions.

Classification of Programming Languages

• Low-level languages (machine dependent): machine language and assembly language.

  • Machine language uses 1s and 0s.

  • Assembly language uses English-like words but is computer-specific.

• High-level languages (machine independent): simpler to write; require translators.

  • Instructions use English-like sentences (e.g., C++, Java, Python).

Language Translators

• Convert assembly or high-level language to machine language.

• Source code is converted into object (machine) code (Figure 1.14).

• Types of translators: assembler, compiler, interpreter.

  • Assembler: converts assembly code to machine code for a specific microprocessor.

  • Compiler: converts entire source code into machine code.

  • Interpreter: translates one line at a time.

Program Development Tools

• Text editors allow creating source code files.

• Integrated Development Environment (IDE) contains a text editor, building tools, and debugger.

• Examples: Python IDLE, Netbeans, Eclipse, Atom, Lazarus.

• Debuggers detect and correct errors in source code.

Application Software

• Works on top of system software to cater to user requirements.

• General purpose and customized application software.

General Purpose Software

• Developed for generic applications to cater to a broad audience.

• Examples: spreadsheet tools (Calc), Adobe Photoshop, GIMP, web browsers (Mozilla), iTunes.

Customized Software

• Tailor-made application software to meet specific organizational needs.

• Examples: websites, school management software, accounting software.

Proprietary or Free and Open Source Software

• Free and Open Source Software (FOSS): source code is freely available for development (e.g., Ubuntu, Python, Libreoffice, Mozilla Firefox).

• Freeware: freely available but source code may not be available (Skype, Adobe Reader).

• Proprietary software: must be purchased from the vendor (Microsoft Windows, Tally, Quickheal).

Operating System

• A resource manager managing hardware (CPU, RAM, Disk, Network, I/O devices), software, system security, and user access.

• Most important system software.

• Examples: Windows, Linux, Android, Macintosh.

Objectives of an Operating System

• Provide services for building and running application programs.

  • Loads programs into memory and allocates CPU for execution.

  • Decides the order of execution amongst multiple programs.

• Provide a user interface for interaction.

  • Takes commands from the user for processing.

OS User Interface

• Different types of user interfaces (Figure 1.15):

Command-based Interface

• Requires users to enter commands to perform tasks (creating, editing, deleting files).

• Requires remembering commands.

• Keyboard is the primary input device.

• Often less interactive and allows running one program at a time.

• Examples: MS-DOS, Unix.

Graphical User Interface (GUI)

• Lets users run programs or give instructions using icons, menus, and visual options.

• Icons represent files and programs; windows represent running programs.

• Mouse and keyboard are common input devices.

• Examples: Microsoft Windows, Ubuntu, Fedora, Macintosh.

Touch-based Interface

• Smartphones, tablets, and PCs allow interaction using touch input.

• Examples: Android, iOS; Windows 8.1 and 10 support touch-based interfaces.

Voice-based Interface

• Users use voice commands to make a computer work.

• Examples: iOS (Siri), Android (Google Now), Microsoft Windows 10 (Cortana).

Gesture-based Interface

• Smartphones and laptops allow gesture based interaction using waving, tilting, and eye motion.
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Functions of Operating System

• Provides essential services for managing the computer system.

Process Management

• Manages tasks (processes) in execution.

• Allocates required resources.

• Exchanges information among processes.

• Allocates CPU among processes in minimum time.

Memory Management

• Allocates and frees memory to running processes.

• Dynamically manages memory without affecting other processes.

• Tracks memory locations as free or occupied.

File Management

• Creates, updates, deletes, and protects files in secondary memory.

• Protects files from unauthorized access.

Device Management

• Manages heterogeneous I/O devices.

• Interacts with device drivers and software.

• Provides options for device configuration.

• Restricts device access to authorized users and software.

Summary

• A computing device processes input data as per given instructions to generate the desired output.

• A computer system has four physical components: CPU, Primary Memory, Input Device, and Output Devices.

• These are referred to as the hardware of the computer.

• A computer system has two primary memory types: RAM (volatile) and ROM (non-volatile).

• The system bus transfers data, addresses, and control signals between components of the computer system.

• A microprocessor is a small electronic component that performs basic arithmetic and logical operations on data.

• A microcontroller is a small computing device that controls a CPU, fixed RAM, ROM, and other peripherals embedded on a single chip.

• Software is a set of instructions written to achieve the desired tasks, categorised mainly as system software, programming tools, and application software.

• Hardware cannot function on its own and needs software to be operational.

• An operating system is an interface between the user and the computer, supervising the working of the computer system.