Introduction to Information Technology

Packet

A packet is a small, fixed- or variable-sized unit of data into which a message is divided for transmission over a network. Each packet has a header (source and destination address, sequence number, error-control information), the payload (a part of the actual data) and sometimes a trailer (checksum).

Working of packet switching

1. Division. The sender's message is broken into small packets, and each is given a header with the destination address and a sequence number.
2. Transmission. No dedicated path is set up. Each packet is sent independently into the network and enters the first router/switch.
3. Store and forward / routing. Each intermediate node stores the packet briefly, checks it for errors, looks at the destination address and forwards it along the best available link at that moment. Packets of the same message may therefore take different routes (datagram approach), or follow one pre-set virtual circuit.
4. Reassembly. At the destination, the packets (which may arrive out of order) are arranged using the sequence numbers and joined to rebuild the original message. Lost or damaged packets are requested again.

Advantages: efficient use of links because many users share the same channel, no call setup delay, packets can avoid failed or congested paths, and it is reliable. Disadvantages: variable delay (jitter), and the overhead of headers and reassembly. The Internet uses packet switching.

TCP/IP

TCP/IP (Transmission Control Protocol / Internet Protocol) is the suite of communication protocols on which the Internet works. It defines how data is divided into packets, addressed, routed and reassembled.

• TCP (transport layer) breaks the message into segments, numbers them, provides reliable delivery with acknowledgements and retransmission, and reassembles them in order at the destination.
• IP (network layer) gives each packet source and destination IP addresses and routes it across networks.

Its layers are Application, Transport, Internet and Network Access.

Packet switching

In packet switching, a message is split into small units called packets. Each packet carries a header with the source and destination addresses, a sequence number and error-checking information. No dedicated path is reserved in advance.

Working

1. The sender's message is divided into packets of fixed or variable size.
2. Each packet is sent into the network independently. Routers read the destination address and forward the packet to the next best hop according to the current traffic and routing tables.
3. Different packets of the same message may take different routes and may arrive out of order.
4. At the destination, the packets are put back in order by their sequence numbers and the original message is rebuilt. Lost or damaged packets are requested again.

Advantages: efficient use of bandwidth because many users share the same links, no call set-up delay, and alternative routes if a link fails. Disadvantage: variable delay (jitter) and the overhead of headers and reassembly.

Packet switching can be datagram (each packet routed independently, as in IP) or virtual circuit (a logical path set up first, then all packets follow it).

Computer bus

A bus is a set of parallel wires or lines that forms a common communication path along which data, addresses and control signals travel between the CPU, memory and input/output devices. Its width (number of lines) and speed decide how much data can be moved at a time.

Functions of the three buses

1. Data bus. It carries the actual data and instructions between the CPU, memory and I/O devices. It is bidirectional, because data can be read from or written to memory. Its width (8, 16, 32 or 64 bits) decides how many bits can be transferred in one operation, and so affects the speed of the computer.

2. Address bus. It carries the address of the memory location or I/O port that the CPU wants to read from or write to. It is unidirectional (from the CPU to memory/devices). Its width decides the maximum memory the CPU can address; an address bus of $n$ lines can address $2^n$ locations (for example, 32 lines give $2^{32} = 4$ GB).

3. Control bus. It carries control and timing signals from the CPU to coordinate the other units and status signals back from them, such as memory read/write, I/O read/write, interrupt request, clock, reset and bus request/grant. It is bidirectional as a group of lines.

Fourth generation of computers (1971 to present)

The fourth generation is based on the microprocessor, in which thousands to millions of transistors are integrated on a single chip using Large Scale Integration (LSI) and Very Large Scale Integration (VLSI). The Intel 4004 (1971) was the first microprocessor.

Key features

• Microprocessor technology. The whole CPU is built on a single chip, which made computers small, cheap and reliable.
• Small size and low cost. This led to the personal computer (PC), for example the IBM PC and Apple Macintosh, and to home and office use.
• High speed and large memory. Semiconductor memory (RAM and ROM chips) replaced magnetic cores, and speeds reached millions of instructions per second and later more.
• Low power consumption and little heat. Air cooling or a small fan is enough, and breakdowns are rare.
• High-level languages and software. Languages such as C, C++ and Pascal became common, and operating systems with a GUI (Windows, Mac OS), word processors, spreadsheets and databases appeared.
• Networking. LAN, WAN and the Internet developed, so computers could share data and resources.
• Storage and I/O. Floppy disks, hard disks, optical disks, the mouse and the graphical display were introduced.
• Portability. Laptops and handheld machines appeared.

Examples: IBM PC, Apple II, Intel 8085/8086-based machines, CRAY-2 supercomputer.

Security mechanism

A security mechanism is a method, tool or procedure designed to detect, prevent or recover from a security attack. It implements security services such as confidentiality, integrity, authentication, authorisation, availability and non-repudiation, protecting information and systems from unauthorised access, misuse, modification and loss.

Technologies used for implementing security mechanisms

1. Encryption (cryptography). Data is converted into unreadable cipher text using algorithms and keys (symmetric: AES, DES; asymmetric: RSA), so only authorised parties can read it. It gives confidentiality.
2. Authentication. Verifies the identity of a user through passwords, PINs, smart cards, OTP, and biometrics (fingerprint, face, iris), often combined as multi-factor authentication.
3. Access control and authorisation. Permissions and roles decide who can read, write or execute which resource (access control lists, role-based access).
4. Firewall. A hardware or software barrier that filters traffic between a trusted network and an untrusted one by rules.
5. Digital signature and certificates. Provide authentication, integrity and non-repudiation of messages and documents, using a Certificate Authority and Public Key Infrastructure (PKI).
6. Antivirus and anti-malware software. Detect, block and remove viruses, worms, trojans, ransomware and spyware.
7. Intrusion detection and prevention systems (IDS/IPS). Monitor network and system activity for suspicious behaviour and alert or block it.
8. VPN and secure protocols. Virtual Private Networks, SSL/TLS, HTTPS, SSH and IPsec protect data passing over public networks.
9. Hashing. Hash functions (SHA-256) create a fixed-size digest to check integrity and to store passwords safely.
10. Backup and recovery. Regular copies of data and disaster-recovery plans maintain availability after loss or attack.
11. Audit and logging. Records of activities help trace and investigate incidents.
12. Physical security. Locks, guards, CCTV and biometric entry for the equipment.

Security mechanism

A security mechanism is a method, tool or procedure designed to detect, prevent or recover from a security attack. It is used to implement security services such as confidentiality, integrity, authentication, authorization, availability and non-repudiation, and so to protect data, computers and networks from unauthorized access, modification or destruction.

Technologies used to implement security mechanisms

1. Encryption (cryptography): Plain text is converted into unreadable cipher text using an algorithm and a key (symmetric or asymmetric), so that only authorized parties can read it. It provides confidentiality.
2. Digital signature: The sender signs a message with his or her private key. The receiver verifies it using the sender's public key. It provides authentication, integrity and non-repudiation.
3. Authentication and access control: Passwords, PINs, smart cards, biometrics (fingerprint, iris, face) and two-factor authentication verify identity. Access control lists and role-based permissions decide what an authenticated user may do.
4. Firewall: Hardware or software placed between a trusted internal network and an untrusted network (the Internet). It filters incoming and outgoing traffic according to security rules.
5. Antivirus and anti-malware software: Scans files and memory for viruses, worms, trojans and spyware and removes or quarantines them.
6. Intrusion detection and prevention systems (IDS/IPS): Monitor network traffic for suspicious activity and raise alerts or block it.
7. Virtual private network (VPN) and secure protocols: SSL/TLS, HTTPS, IPsec and SSH create encrypted tunnels over public networks.
8. Backup and recovery, and audit logs: Regular backups restore data after loss or attack. Logs record user activity for later investigation.

Purpose and function of a firewall

A firewall is a hardware device, a software program or a combination of both that sits between a trusted private network (such as a company LAN) and an untrusted network (such as the Internet). Its purpose is to protect the internal network from unauthorised access, attacks and malicious traffic. It does this by inspecting every incoming and outgoing packet and allowing or blocking it according to a set of security rules. Its main functions are:

• Filtering traffic according to rules (IP address, port, protocol, content).
• Blocking unauthorised access and hacking attempts from outside.
• Logging and monitoring network activity, and raising alerts on suspicious traffic.
• Hiding internal addresses (through NAT / proxying) and enforcing the organisation's security policy.

Three types of firewall

1. Packet-filtering firewall. It works at the network layer. Each packet is examined on its own and compared with an access control list (source and destination IP address, port number, protocol). Packets that match an "allow" rule pass; others are dropped. It is fast and cheap, but it does not understand the application data or the state of the connection, so it is easier to fool (for example with spoofed addresses).

2. Application-level (proxy) firewall. It works at the application layer. The firewall acts as an intermediary: the client connects to the proxy, and the proxy makes a separate connection to the real server. Because it understands protocols such as HTTP, FTP and SMTP, it can inspect the actual content and authenticate users. It is the most secure of the three, but it is slower and needs a proxy for each application.

3. Stateful inspection firewall. It works at the network and transport layers and keeps a state table of active connections. A packet is allowed only if it belongs to a legitimate, established connection (or starts a permitted new one). It gives better security than simple packet filtering with better speed than a proxy.

Characteristics of a computer

1. Speed. A computer performs millions or billions of instructions per second; modern processors work in nanoseconds.
2. Accuracy. Results are exact as long as the input and program are correct; errors are normally due to wrong data or programs ("garbage in, garbage out"), not the machine.
3. Diligence. It does not get tired, bored or lose concentration, so it can do the same work for hours with the same speed and accuracy.
4. Versatility. The same computer can be used for many tasks, such as typing, calculation, design, communication, games and control of machines.
5. Storage capacity. It can store huge amounts of data and programs in memory and secondary storage and retrieve them quickly.
6. Automation. Once a program is loaded, it carries out the instructions by itself without human intervention.
7. Reliability and repeatability. It gives the same result for the same input, and can repeat a job any number of times.
8. Remembrance and connectivity. Data can be kept for years without being forgotten, and computers can be networked to share data and resources.

Significant limitations of a computer

• No intelligence of its own. It does only what it is told; it cannot think, reason or make decisions outside the instructions of its program.
• Dependent on instructions. Without a program and correct input it is useless; wrong instructions give wrong results.
• No feelings or common sense. It cannot judge, feel, or use experience and intuition like humans.
• No self-learning (in the ordinary sense). It cannot improve unless it is programmed to do so.
• Dependence on power and hardware. It stops working without electricity, and is subject to hardware failure, viruses and security threats.

Switching technique

In a large network, it is not practical to connect every pair of devices by a direct link. A switching technique is the method by which data is moved from the source to the destination through intermediate nodes (switches) of the network, so that links are shared among many users.

Three kinds of switching

1. Circuit switching
2. Message switching
3. Packet switching

Circuit switching

A dedicated physical path (circuit) is established between sender and receiver before data transfer starts, and it stays reserved for the whole conversation. It has three phases: (i) circuit setup, (ii) data transfer, (iii) circuit release. The traditional telephone network uses it.

• Advantages: constant data rate, no delay in the middle of the transfer, data arrives in order, and no need to add per-packet addressing.
• Disadvantages: setup time before transfer, the channel is wasted when no data is being sent (idle time cannot be used by others), and if the path fails the connection is lost.

Message switching

There is no dedicated path. The whole message is sent from the source to the first switch, stored there completely, and then forwarded to the next node when a link is free (store-and-forward). Each message carries the destination address. Telegram and e-mail systems work in this manner.

• Advantages: no call setup, links are shared, so better link utilisation; messages can be queued when the network is busy.
• Disadvantages: each node needs large storage to hold whole messages, delay at each node is long and variable, so it is unsuitable for real-time use such as voice.

Database Management System (DBMS)

A DBMS is a software system that is used to create, store, organise, retrieve, update and control access to a database (an organised collection of related data). Examples are MySQL, Oracle, SQL Server and MS Access. It sits between the users or application programs and the data files.

Importance

• Reduces data redundancy and keeps data consistent.
• Allows several users to share data at the same time (concurrency control).
• Provides security through user accounts and access rights.
• Maintains integrity of data through constraints, and gives backup and recovery.
• Provides data independence and easy querying (for example with SQL), so information is retrieved quickly.

Data redundancy

Data redundancy means the unnecessary duplication of the same data in several places (files or tables). For example, a student's address stored in both the library file and the account file. It wastes storage and increases the work of updating.

Data inconsistency

Data inconsistency occurs when different copies of the same data do not agree with each other, usually because redundant copies were not all updated. For example, a student's address is changed in the library file but still shows the old value in the account file. Redundancy is therefore the cause and inconsistency is its result.

Parts of the CPU

The Central Processing Unit (CPU) is the brain of the computer. It interprets and executes instructions and controls the other parts. It has three main parts.

1. Arithmetic Logic Unit (ALU). It carries out all arithmetic operations (addition, subtraction, multiplication, division) and logical operations (AND, OR, NOT, comparisons such as greater than, equal to). It takes data from registers, performs the operation and returns the result to a register.

2. Control Unit (CU). It directs and coordinates all operations of the computer. It fetches instructions from memory, decodes them, and sends control signals to the ALU, memory and input/output devices so that the instruction is executed in the correct order and time. It does not process data itself.

3. Registers. These are very small, very fast storage locations inside the CPU that hold data, instructions and addresses temporarily during processing. Important registers are the Program Counter (address of the next instruction), Instruction Register (current instruction), Memory Address Register, Memory Data Register and the Accumulator (results of ALU operations).

Many textbooks also mention the cache memory and internal buses as part of the modern CPU, which speed up access to data and connect these parts.

Instruction cycle

The instruction cycle (fetch-decode-execute cycle) is the sequence of steps that the CPU repeats to process every instruction of a stored program, from the time it is taken from memory until it is carried out.

Steps

1. Fetch. The Program Counter (PC) holds the address of the next instruction. This address is put on the address bus, the instruction is read from main memory into the Instruction Register (IR), and the PC is incremented to point to the following instruction.
2. Decode. The control unit decodes the instruction in the IR to find out the operation to be performed (opcode) and the operands or their addresses.
3. Fetch operands (if required). If the instruction needs data from memory, the effective address is calculated and the operand is read into a register.
4. Execute. The ALU or other unit performs the operation (arithmetic, logic, data transfer, jump, etc.) according to the control signals.
5. Store (write back). The result is written to a register or to memory, and the cycle begins again with the next instruction.

The cycle is repeated until a halt instruction is reached.

Convert $(44.467)_{10}$ to base 16

Integer part: divide 44 by 16 repeatedly.

$44 = 16 \times 2 + 12 \quad (\text{remainder } 12 = C)$ $2 = 16 \times 0 + 2 \quad (\text{remainder } 2)$

Reading the remainders upward gives $(44)_{10} = (2C)_{16}$.

Fractional part: multiply 0.467 by 16 repeatedly and note the integer parts.

Reading the integer parts downward gives $(0.467)_{10} \approx (0.778D4F)_{16}$. The expansion does not terminate, so it is stopped after six places.

$(44.467)_{10} \approx (2C.778D4F)_{16}$

Software

Software is a set of programs, instructions and data that tells the computer hardware what to do and how to do it. It is the intangible part of the computer, which cannot be touched, unlike hardware.

Two main categories

1. System software. It manages and controls the computer hardware and provides a platform on which application software runs. It works mostly in the background and is needed to run the computer at all.

   • Operating systems (Windows, Linux, Android) manage memory, processes, files and devices.
   • Utility programs (antivirus, disk cleanup, backup), device drivers, and language translators (compiler, interpreter, assembler) are also system software.

2. Application software. It is designed to perform specific tasks for the user, such as writing, calculating, designing or communicating. It depends on system software to run.

   • General-purpose: word processors (MS Word), spreadsheets (MS Excel), web browsers, presentation software.
   • Special-purpose/customised: payroll, library management, banking and hospital software.

Internet

The Internet is a worldwide network of interconnected computer networks that use the TCP/IP protocol suite to communicate and share information. It provides services such as e-mail, file transfer, the web and online chat.

Protocol

A protocol is a set of rules and conventions that governs how data is formatted, transmitted, received and acknowledged between devices in a network, so that different devices can understand each other. Examples are HTTP, FTP, SMTP and TCP/IP.

WWW (World Wide Web)

The WWW is a system of interlinked hypertext documents (web pages) stored on web servers all over the Internet, which are accessed using a web browser and a URL through the HTTP protocol. It is only one of the services that run over the Internet.

Function of the TCP/IP protocol

TCP/IP is the protocol suite that makes the Internet work.

• TCP (Transmission Control Protocol) divides the data into packets at the sender, numbers them, ensures reliable delivery (acknowledgement, retransmission of lost packets, error checking), and reassembles the packets in correct order at the receiver.
• IP (Internet Protocol) gives each device a unique IP address and is responsible for addressing and routing each packet independently from the source to the destination across networks.

Together, TCP/IP allows reliable end-to-end communication between different types of computers and networks.

Impact of IT on organisations

• Higher efficiency and productivity. Automation of routine work (accounting, payroll, inventory) saves time and reduces errors and cost.
• Better communication and collaboration. E-mail, video conferencing and intranets connect staff and branches in different places instantly.
• Improved decision making. Management information systems, databases and data analysis give timely and accurate information to managers.
• Better customer service and new markets. Online banking, e-commerce and call centres give 24-hour service and reach customers globally.
• Changes in structure and jobs. Organisations become flatter, work from home is possible, and new jobs appear, but some clerical jobs disappear and staff need continuous training.
• Challenges. High cost of setup, security risks (hacking, data theft), and dependence on technology.

Impact of IT on individuals

• Education and learning. Online courses, e-books and search engines make knowledge available anywhere.
• Communication and social life. Social media, messaging and video calls keep people in contact, although they can reduce face-to-face contact.
• Convenience. Online shopping, mobile banking, bill payment and e-government services save time and travel.
• Employment and skills. New opportunities such as freelancing and IT jobs appear; digital skills become necessary.
• Entertainment and health. Streaming, games, telemedicine and fitness apps improve life, but excess use may cause addiction, eye strain and loss of privacy.

Sequential access

In sequential access, data is read or written one record after another in a fixed order, starting from the beginning. To reach a particular record, all the records before it must be passed over. Access time therefore depends on the position of the data and is long for data at the end. It is suitable when most records are processed in order (for example, backup and payroll processing).

Example of a sequential access device: magnetic tape (tape drive).

Direct (random) access

In direct access, any record can be reached directly by its address or location without reading the preceding data, so the access time is almost the same for every record. It is suitable for applications that need fast, individual retrieval (for example, databases, files and an operating system).

Example of a direct access device: hard disk (also CD/DVD, SSD or pen drive). A disk head moves directly to the required track and sector.

Difference in brief: sequential access is slow for random retrieval but cheap and good for backup, whereas direct access is fast and flexible but usually more expensive.

(a) IoT (Internet of Things)

The Internet of Things is a network of physical objects ("things") such as sensors, home appliances, vehicles, wearable devices and machines that are embedded with electronics, software and network connectivity so that they can collect data and exchange it over the Internet, often without human intervention.

• Working: sensors collect data, which is sent through a network (Wi-Fi, Bluetooth, mobile network) to a cloud or server, processed, and then results or commands are sent back to the devices or users.
• Applications: smart homes (lights, thermostats, security cameras), smart agriculture, healthcare monitoring, smart cities, industrial automation, vehicle tracking.
• Issues: security and privacy of data, standardisation, and cost.

(b) Data warehousing and data mining

A data warehouse is a large, central, subject-oriented, integrated, time-variant and non-volatile repository of data collected from several operational databases, organised to support analysis and decision making rather than daily transactions. Data is extracted, transformed and loaded (ETL) into it and kept as historical data, for example sales data of a company over many years.

Data mining is the process of discovering useful patterns, relationships, trends and knowledge from large amounts of data using statistics, machine learning and database techniques. Typical tasks are classification, clustering, association rules (such as "customers who buy bread also buy butter") and prediction. It is often applied to the data stored in a data warehouse, for example in marketing, fraud detection and banking.

Main functions of a digital computer

A digital computer processes data represented as discrete (binary) values. Its main functions are:

1. Input: accepting data and instructions from the user through input devices.
2. Storage: keeping data, instructions and results in memory (primary and secondary).
3. Processing: performing arithmetic and logical operations on the data according to the program.
4. Output: presenting the processed results (information) through output devices.
5. Control: directing and coordinating all the above operations.

Block diagram

            +-------------------------------+
            |              CPU              |
 Input      |   +---------+   +---------+   |      Output
 unit ----> |   |  ALU    |<->| Control |   | ---> unit
            |   +---------+   |  Unit   |   |
            |        ^        +---------+   |
            +--------|----------------------+
                     v
            +-------------------+
            |  Memory unit      |<--> Secondary storage
            +-------------------+

Explanation

• Input unit: converts data from the human form (keyboard, mouse, scanner) into binary form and passes it to the computer.
• Central Processing Unit (CPU): the processor. The ALU performs arithmetic and logical operations. The Control Unit fetches and decodes instructions and sends control signals to the other units. Registers inside the CPU hold data temporarily.
• Memory (storage) unit: main memory (RAM/ROM) holds the programs and data currently in use, and secondary storage (hard disk) keeps them permanently.
• Output unit: converts the binary results into a human-readable form (monitor, printer, speaker).

Data and instructions flow from the input unit to memory, then to the CPU for processing, and the results go back to memory and out through the output unit.

Network device

A network device is a hardware component used to connect computers and other devices in a network and to carry, direct or regulate the flow of data between them.

Features

1. Repeater. It works at the physical layer. It receives a weak signal, regenerates (amplifies and cleans) it and retransmits it, so that the signal can travel a longer distance. It connects two segments of the same network and does not filter data.

2. Hub. A multiport repeater, working at the physical layer. A frame received on one port is broadcast to all the other ports, so all devices share the same collision domain and bandwidth. It is cheap but inefficient and not secure.

3. Switch. It works at the data link layer. It learns the MAC addresses of connected devices and forwards a frame only to the port of the destination device. This reduces collisions and traffic and gives each port dedicated bandwidth. It is the central device in a modern LAN.

4. Bridge. It works at the data link layer and connects two LAN segments. It reads the MAC address, filters traffic and forwards only the frames that need to cross to the other segment, so it divides collision domains. It usually has two ports (a switch is a multiport bridge).

5. Router. It works at the network layer. It uses IP addresses and routing tables to select the best path and forward packets between different networks (for example, a LAN and the Internet). It also provides functions like NAT, DHCP and basic firewalling.

6. Gateway. It connects networks that use different protocols or architectures, and may work at all seven layers. It translates protocols and data formats between them (for example, between a LAN and a mainframe network, or an e-mail gateway). It is also often the entry and exit point of a network.

Operating system

An operating system (OS) is system software that acts as an interface between the user (and application programs) and the computer hardware. It manages the hardware resources and provides services to programs. Examples are Windows, Linux, macOS and Android.

Functions of an operating system

1. Process management. The OS creates, schedules, suspends and terminates processes. It decides which process gets the CPU and for how long (CPU scheduling), provides synchronisation and communication between processes, and handles deadlocks.

2. Memory management. It keeps track of which parts of main memory are in use and by whom, allocates and de-allocates memory to processes, and uses techniques such as paging, segmentation and virtual memory so that programs larger than physical memory can run.

3. File management. It organises files and directories, creates, deletes, reads, writes and renames files, controls access permissions, and maps files onto secondary storage.

4. Device (I/O) management. It controls input and output devices through device drivers, uses buffering, caching and spooling, and allocates devices to processes.

5. Secondary storage management. It manages free space, allocates disk space and schedules disk requests.

6. Security and protection. It protects data and resources from unauthorised access using user accounts, passwords, and permissions.

7. User interface. It provides a command line interface (CLI) or graphical user interface (GUI) to interact with the system.

8. Error detection and resource allocation. It detects and handles hardware and software errors, and accounts for resource usage.

9. Networking and job accounting. It supports communication over a network and keeps records of resource use by users.

Fifth generation of computers (present and future, from about 1980s onward)

The fifth generation is based on ULSI (Ultra Large Scale Integration) microprocessors and aims at computers with artificial intelligence.

Key features

• Artificial intelligence. Computers are designed to reason, learn, make decisions and solve problems, using expert systems, robotics and natural language processing.
• ULSI technology. Millions of transistors are placed on a single chip, giving very high speed, small size and low power consumption.
• Parallel processing. Many processors work at the same time, which increases the processing power.
• Natural language and voice interaction. Speech recognition and voice input/output, handwriting recognition, and user-friendly interfaces.
• Large storage and high speed. Very large memory and storage, such as optical disks, SSDs and cloud storage.
• Networking. Highly developed network and Internet connectivity, and portable devices such as laptops, tablets and smartphones.
• Programming languages. Use of high-level languages and AI languages such as PROLOG and LISP.
• Other developments. Quantum computing, nanotechnology and bio-computing are being researched.

Scheduling

Scheduling is the method by which the operating system decides which of the processes in the ready queue gets the CPU next, and for how long, so as to keep the CPU busy and to improve throughput, response time and fairness. The part of the OS that does this is the CPU scheduler (dispatcher).

Non-pre-emptive scheduling

Once the CPU has been allocated to a process, the process keeps it until it finishes or voluntarily gives it up by waiting for I/O. The OS cannot take the CPU away in the middle.

• Examples: First Come First Served (FCFS), Shortest Job First (non-pre-emptive), Priority (non-pre-emptive).
• Advantages: simple, less overhead, no context switching in the middle of execution.
• Disadvantages: a long process can make others wait (convoy effect), poor response time, not suitable for time-sharing or real-time systems.

Pre-emptive scheduling

The OS can take the CPU away from a running process before it completes, for example when a higher-priority process arrives or the time slice expires. The interrupted process goes back to the ready queue.

• Examples: Round Robin, Shortest Remaining Time First, pre-emptive Priority scheduling.
• Advantages: better response time and fairness, suitable for time-sharing and real-time systems.
• Disadvantages: more overhead from context switching, more complex, and there may be problems with shared data (race conditions) and starvation of low-priority processes.

Convert $(3748.658)_{10}$ to binary, octal and hexadecimal

Integer part, 3748, to binary: repeated division by 2 (remainders from the last to the first).

$3748 = 2048 + 1024 + 512 + 128 + 32 + 4 = 2^{11}+2^{10}+2^{9}+2^{7}+2^{5}+2^{2}$

$(3748)_{10} = (111010100100)_2$

Fractional part, 0.658, to binary: multiply by 2.

So $(0.658)_{10} \approx (0.101010000111)_2$ (12 places).

Binary: $(3748.658)_{10} \approx (111010100100.101010000111)_2$

Octal (group the bits in threes from the point: 111 010 100 100 . 101 010 000 111):

$(3748.658)_{10} \approx (7244.5207)_8$

Check: $7\times512 + 2\times64 + 4\times8 + 4 = 3584+128+32+4 = 3748$.

Hexadecimal (group in fours: 1110 1010 0100 . 1010 1000 0111):

$(3748.658)_{10} \approx (EA4.A87)_{16}$

Check: $14\times256 + 10\times16 + 4 = 3584+160+4 = 3748$.

Data model

A data model is a collection of concepts and tools used to describe the structure of a database: the data, the relationships among data, the constraints on them and the semantics. It provides the means to achieve data abstraction. Examples are the hierarchical, network, relational and entity-relationship (E-R) models. The relational model, which stores data in tables of rows and columns, is the most widely used.

Schema

A schema is the overall logical design or description of the database, that is, its structure: the tables, their attributes, data types, keys and relationships. It is defined when the database is designed and changes rarely. It is like a blueprint. For example, STUDENT(Roll, Name, Address, Phone) is the schema of a student table.

Instance

An instance is the actual collection of data stored in the database at a particular moment of time. It changes every time data is inserted, updated or deleted. For example, the rows currently in the STUDENT table are an instance of that schema.

So the schema is the structure that stays almost constant, while the instance is the content that changes continually.

Computer security

Computer security is the protection of computer systems, hardware, software, data and networks from theft, damage, unauthorised access, misuse and disruption. It aims to ensure confidentiality (only authorised persons can read data), integrity (data is not altered improperly) and availability (the system and data are available when needed). Threats include viruses, worms, hacking, phishing and denial-of-service attacks. Common measures are passwords and authentication, encryption, antivirus software, backups, access control and firewalls.

Functions of a firewall

A firewall stands between a private network and an external network such as the Internet and controls the traffic between them. Its functions are:

• Packet filtering: allows or blocks packets according to rules based on IP address, port and protocol.
• Access control: blocks unauthorised users and services from entering the internal network, and can also restrict what internal users may reach.
• Monitoring and logging: records traffic and attempted attacks and raises alerts.
• Hiding the internal network: with NAT or proxy services, internal IP addresses are not exposed to outsiders.
• Preventing attacks: helps to stop hacking attempts, malware and denial-of-service traffic, and may also support VPN and content filtering.

Impacts of IT on individuals in everyday life

• Communication. E-mail, instant messaging, social media and video calls let people stay in touch with family and friends anywhere at low cost.
• Education. Online classes, e-learning platforms, digital libraries and search engines make learning easier, and give access to information at any time.
• Shopping and banking. E-commerce, mobile banking, digital wallets (eSewa, Khalti) and online bill payment save time, travel and effort.
• Entertainment. Streaming of music and video, online games and e-books are available on demand.
• Health. Online appointments, telemedicine, health records and fitness-tracking devices improve care.
• Work and employment. Computers increase productivity, allow working from home and freelancing, and create jobs in IT. Some routine jobs, however, are lost because of automation.
• Government services. Online forms, tax filing, results and certificates are easier to obtain.
• Negative effects. Loss of privacy, cyber crime and online fraud, addiction to the Internet and games, health problems (eye strain, back pain), less face-to-face interaction, and the digital divide between those who have access and those who have not.

(a) Modulation and demodulation

Modulation is the process of changing a characteristic of a high-frequency carrier signal (amplitude, frequency or phase) in accordance with the information signal, so that the information can be transmitted efficiently over a communication channel such as a telephone line, radio or cable. At the sender, a modem converts the digital signal of the computer into an analog signal in this way.

The main types are:

• Amplitude modulation (AM / ASK): the amplitude of the carrier varies.
• Frequency modulation (FM / FSK): the frequency of the carrier varies.
• Phase modulation (PM / PSK): the phase of the carrier varies.

Demodulation is the reverse process at the receiver: the original information signal is recovered (extracted) from the modulated carrier, i.e. analog to digital. The device which does both is called a modem (modulator-demodulator).

(b) Cryptography

Cryptography is the science and technique of securing information by converting it into an unreadable form so that only the intended receiver can read it.

• Plaintext: the original readable message. Ciphertext: the scrambled message. Encryption: converting plaintext to ciphertext using an algorithm and a key. Decryption: converting ciphertext back to plaintext.
• Symmetric-key cryptography: the same secret key is used for encryption and decryption (for example DES, AES). It is fast, but the key must be shared securely.
• Asymmetric (public-key) cryptography: a pair of keys is used: a public key to encrypt and a private key to decrypt (for example RSA). It solves the key-sharing problem and is used for digital signatures.

Cryptography provides confidentiality, integrity, authentication and non-repudiation, and is used in e-commerce, online banking, and secure web (HTTPS).

Security mechanism

A security mechanism is a method, process or device designed to detect, prevent or recover from a security attack, so that the security of a computer system or network is maintained. Examples are encryption, digital signatures, access control, authentication, firewalls, antivirus software and backup.

Two approaches

1. Preventive approach: measures that stop attacks before they happen, such as passwords/authentication, encryption, firewalls and access control.
2. Detective and corrective approach: measures that identify attacks and recover from them, such as intrusion detection, antivirus scanning, audit logs and data backup and recovery.

Types of software with examples

Software is mainly of two types.

1. System software. Programs that control and manage the computer hardware and give a platform for other software.

   • Operating system: Windows, Linux, macOS, Android.
   • Language translators: compiler, interpreter and assembler (for example GCC, Python interpreter).
   • Utility software: antivirus, disk defragmenter, file compression (WinRAR), backup tools.
   • Device drivers: printer or graphics card driver.

2. Application software. Programs written for the user to perform specific tasks.

   • General-purpose (packaged) software: word processor (MS Word), spreadsheet (MS Excel), presentation (PowerPoint), database (MS Access), web browsers (Chrome).
   • Customised (tailored) software: payroll system, hospital or library management system, banking software built for one organisation.
   • Also: multimedia software (VLC, Photoshop), educational and entertainment software (games).

Data warehousing and data mining (2 marks)

A data warehouse is a large, centralised, subject-oriented, integrated, time-variant and non-volatile repository of historical data collected from several operational sources. It is designed for querying and analysis (decision support) rather than for day-to-day transactions.

Data mining is the process of discovering useful patterns, trends, associations and anomalies in large data sets using statistics, machine learning and database techniques. For example, a supermarket may find that customers who buy bread often buy butter.

Two types of language translators (3 marks)

1. Assembler. It translates assembly language (mnemonics such as MOV, ADD) into machine code. The translation is nearly one-to-one, so the output is closely tied to a particular processor.
2. Compiler. It translates the whole of a high-level program (C, C++, Java) into machine code or object code in one go, reporting all errors after analysing the full source. The resulting executable runs fast and does not need the compiler again.

(An interpreter is the third type. It translates and executes the program one statement at a time, so it is slower in execution but easier for debugging, for example in Python.)

Significance of IoT in a smart city (5 marks)

The Internet of Things (IoT) connects sensors, devices and systems through the Internet so that they can collect and exchange data without human intervention. A smart city uses this data to improve services and quality of life. Its significance is as follows.

• Traffic management. Sensors and cameras at junctions monitor vehicle flow, and signals adjust automatically to reduce congestion, as in Kathmandu's traffic-control pilots.
• Waste management. Smart bins report when they are full, so collection trucks follow optimised routes and save fuel.
• Energy and street lighting. Street lights dim or switch off when there is no movement, and smart meters give accurate consumption data.
• Water and environment monitoring. Sensors detect pipe leaks and measure air quality (for example PM2.5), giving early warnings about pollution.
• Public safety. Connected CCTV, emergency alerts and disaster sensors (floods, earthquakes) allow faster response.
• Smart parking and transport. Apps show free parking spaces and real-time bus positions, which saves time and fuel.

Overall, IoT makes a city more efficient, safer, cleaner and cheaper to run.

Computer

A computer is an electronic device that accepts data as input, stores it, processes it under the control of a stored program according to instructions, and produces information as output. It works at high speed and with high accuracy.

Block diagram of a digital computer

              +---------------------------------------------+
              |                    CPU                      |
 +--------+   |  +-----------+          +----------------+  |   +---------+
 | Input  |-->|  |  Control  |<-------->|  Arithmetic &  |  |-->| Output  |
 | Unit   |   |  |   Unit    |          |  Logic Unit    |  |   | Unit    |
 +--------+   |  +-----------+          +----------------+  |   +---------+
              |        ^   Registers          ^             |
              +--------|-----------------------|------------+
                       v                       v
                +---------------------------------+
                |        Main (Primary) Memory    |
                +---------------------------------+
                                ^
                                v
                +---------------------------------+
                |     Secondary (Auxiliary) Memory|
                +---------------------------------+

Explanation of the units

1. Input unit. It accepts data and instructions from the user and converts them into binary form that the computer understands. Examples are the keyboard, mouse, scanner and microphone.
2. Central Processing Unit (CPU). It is the brain of the computer and has three parts.
   • Arithmetic Logic Unit (ALU). It performs arithmetic operations (addition, subtraction, multiplication, division) and logical operations (AND, OR, NOT, comparisons).
   • Control Unit (CU). It fetches instructions from memory, decodes them, and directs and coordinates all other units by sending control signals. It does not process data itself.
   • Registers. These are small, very fast storage locations inside the CPU, such as the Program Counter, Instruction Register and Accumulator, used to hold data and instructions temporarily.
3. Memory unit.
   • Primary memory (RAM and ROM) holds the programs and data currently in use. RAM is volatile; ROM is non-volatile.
   • Secondary memory (hard disk, SSD, optical disc) stores programs and data permanently and in large quantity.
4. Output unit. It converts the processed binary results into a form that people can understand, such as the monitor display or printed paper. Examples are the monitor, printer and speaker.

Working. Data and instructions enter through the input unit and are stored in memory. The control unit fetches each instruction, the ALU executes it, the results are stored back in memory, and finally the output unit presents them to the user.

Transmission media

Transmission media are the physical paths or channels through which data signals travel from a sender to a receiver in a network. They are of two kinds.

• Guided (wired) media carry the signal along a physical conductor: twisted pair cable, coaxial cable and optical fibre.
• Unguided (wireless) media carry the signal through the air or space: radio waves, microwaves and infrared.

Twisted pair cable

A twisted pair cable consists of pairs of insulated copper wires twisted around each other. The twisting reduces electromagnetic interference and crosstalk from neighbouring pairs and external sources.

Features

• It is cheap, light and flexible, so it is easy to install and widely used in telephone lines and LANs (Ethernet).
• Two types exist: Unshielded Twisted Pair (UTP) and Shielded Twisted Pair (STP), which has a metallic shield and gives better noise protection at higher cost.
• It is classified in categories (Cat 5, Cat 5e, Cat 6), with speeds from 10 Mbps up to 1 Gbps or more over short distances.
• The usual maximum segment length is about 100 m, after which the signal weakens (attenuation) and needs a repeater.
• It uses the RJ-45 connector for networks and the RJ-11 for telephones.
• It is more affected by noise, and has less bandwidth and security than coaxial or fibre cable.

Binary subtraction: $10101_2 - 10010_2$

Align the bits (positions 4 to 0) and subtract column by column from the right.

    1 0 1 0 1
  - 1 0 0 1 0
  -----------
    0 0 0 1 1

• Position 0: $1 - 0 = 1$
• Position 1: $0 - 1$ is not possible, so borrow 1 from position 2. Then $10_2 - 1 = 1$, and position 2 becomes $0$.
• Position 2: $0 - 0 = 0$
• Position 3: $0 - 0 = 0$
• Position 4: $1 - 1 = 0$

Result:

$10101_2 - 10010_2 = 00011_2 = 11_2$

Verification in decimal

$10101_2 = 16+4+1 = 21$

$10010_2 = 16+2 = 18$

$21 - 18 = 3, \qquad 11_2 = 2 + 1 = 3$

Both results agree, so the answer $11_2$ (decimal 3) is correct.

Database system

A database system is a computerised system for storing, organising and retrieving an organised collection of related data (the database), managed by software called the Database Management System (DBMS), together with the application programs and the people who use it. It lets many users share data conveniently and securely, with controlled redundancy.

Components of a database system

1. Hardware. The physical devices on which the database runs: the computer, storage disks, servers and network equipment.
2. Software. The DBMS itself (MySQL, Oracle, SQL Server) plus the operating system and application programs that use the data. The DBMS defines, stores, retrieves and controls the data.
3. Data. The actual stored facts. It is the most important component, and includes the user data and the metadata (data about data, kept in the data dictionary).
4. Users. The people who interact with the system:
   • Database administrator (DBA), who manages the database, security and backups;
   • Application programmers, who write programs that access the data;
   • End users, who query or update data through forms and reports.
5. Procedures. The rules and instructions for how to design, use, back up and maintain the database.

Bus

A bus is a set of parallel wires or lines that carries data, addresses or control signals between the components of a computer (CPU, memory and I/O devices). It is a shared communication pathway.

Types of bus (system bus)

1. Data bus. It carries the actual data between the CPU, memory and I/O devices. It is bidirectional, and its width (8, 16, 32, 64 bits) decides how much data moves at a time.
2. Address bus. It carries the memory or I/O address that the CPU wants to read or write. It is unidirectional (from CPU to memory), and its width fixes the addressable memory: $n$ lines can address $2^n$ locations.
3. Control bus. It carries control and timing signals such as read, write, interrupt, clock and reset, which coordinate the operations of all units. It is mostly bidirectional.

Need for memory hierarchy

No single memory is at once very fast, large and cheap. Fast memory (registers, cache) is costly and small; large memory (hard disk) is cheap but slow. A memory hierarchy arranges memory in levels:

Registers -> Cache -> Main memory (RAM) -> Secondary storage -> Tertiary/backup
 (fastest, smallest, costly)                    (slowest, largest, cheapest)

We need it for these reasons.

• To get speed close to the fastest level and capacity close to the largest level at a reasonable total cost.
• To reduce the speed gap between the fast CPU and the slow storage, so the CPU waits less.
• It works because of the principle of locality: programs reuse the same data and instructions again and again, so frequently used items stay in the faster levels.
• It provides permanent storage of large data in the lower levels while the upper levels serve the CPU.

Information Technology (IT)

Information Technology is the use of computers, software, networks, telecommunication and other electronic devices to create, store, process, retrieve, transmit and secure information. In simple terms, IT combines computing and communication technology to handle information.

Application areas of IT

• Education. E-learning, online classes, digital libraries, smart classrooms, online admission and result systems.
• Banking and finance. ATMs, mobile and internet banking, digital wallets (eSewa, Khalti), online payment and stock trading.
• Business and commerce. E-commerce, inventory and payroll systems, ERP, CRM and online marketing.
• Healthcare. Patient records, medical imaging (CT, MRI), telemedicine and hospital management software.
• Government. E-governance, online tax filing, online citizen services, national ID and e-voting systems.
• Communication. E-mail, video conferencing, social media and instant messaging.
• Entertainment and media. Video streaming, online games, digital music and special effects in films.
• Science and engineering. Simulation, computer-aided design (CAD), weather forecasting and space research.
• Transport and tourism. Online ticket reservation, GPS navigation, air-traffic control and hotel booking.
• Agriculture and industry. Automation, robotics, precision farming and weather and market information.

(a) E-commerce and E-governance

E-commerce is the buying and selling of goods and services, and the transfer of money and data, over the Internet or other electronic networks. Main types are B2B (business to business), B2C (business to consumer), C2C (consumer to consumer) and C2B. Examples are Daraz, Amazon and Sastodeal in Nepal. Its benefits are 24-hour availability, wide market reach, lower costs, easy price comparison and convenient online payment. Its challenges are security and fraud, lack of trust, delivery problems and the need for Internet access.

E-governance is the use of ICT by government to deliver services and information to citizens, businesses and its own departments in a fast, transparent and efficient way. Its types are G2C (government to citizen), G2B, G2G and G2E. Examples are online passport and licence applications, online tax filing, the National ID system and the e-procurement (Public Procurement Monitoring Office) system in Nepal. Its benefits are transparency, less corruption, saving of time and cost, and wider access to services.

(b) Big data

Big data refers to extremely large and complex data sets that are too big for traditional database tools to store and process. They come from social media, sensors, transactions, logs and mobile devices. It is described by the main V's.

• Volume. The enormous amount of data (terabytes to petabytes).
• Velocity. The speed at which data is generated and must be processed.
• Variety. Structured, semi-structured and unstructured data (text, images, video).
• Veracity. The quality and reliability of data.
• Value. The useful insight obtained from the data.

Tools include Hadoop, Spark and NoSQL databases. Applications include customer behaviour analysis, fraud detection, healthcare, weather prediction and recommendation systems.

Reasons for using hexadecimal and octal number systems

Computers work internally in binary, but long binary strings are hard for humans to read, write and remember. Octal (base 8) and hexadecimal (base 16) are used as compact shorthand because their bases are powers of 2.

• Easy conversion. One octal digit is exactly 3 binary bits ($8 = 2^3$), and one hexadecimal digit is exactly 4 bits ($16 = 2^4$). Binary can be converted by simply grouping bits, for example $1011\,0110_2 = B6_{16}$.
• Compactness. A long binary number becomes three or four times shorter, which reduces errors in writing and reading.
• Convenient for bytes and words. A byte is exactly two hexadecimal digits; hex suits 8, 16, 32 and 64-bit word sizes.
• Wide use. Hexadecimal is used for memory addresses, machine code, colour codes (such as #FF5733), MAC addresses and debugging. Octal is used for Unix file permissions (such as 755) and in older computers with 12, 24 and 36-bit words.

Binary addition of 23 and 12

Convert to binary:

$23_{10} = 10111_2, \qquad 12_{10} = 01100_2$

Add column by column from the right:

  carry:  1 1 1
          1 0 1 1 1
        + 0 1 1 0 0
        -----------
        1 0 0 0 1 1

• Bit 0: $1+0 = 1$
• Bit 1: $1+0 = 1$
• Bit 2: $1+1 = 10$, write 0, carry 1
• Bit 3: $0+1+1 = 10$, write 0, carry 1
• Bit 4: $1+0+1 = 10$, write 0, carry 1
• Bit 5: the carry 1 is written

Result: $100011_2$.

Verification in decimal

$100011_2 = 32 + 2 + 1 = 35$

$23 + 12 = 35$

The two results match, so the binary addition is correct.

Network topology

Network topology is the physical or logical arrangement of the nodes (computers and devices) and the links connecting them in a network. It decides how devices are connected, how data flows, and the cost, performance and reliability of the network. Common topologies are bus, star, ring, mesh and tree.

1. Bus topology

All nodes are connected to a single common cable called the backbone or bus, with a terminator at each end. Nodes attach through drop lines or T-connectors.

 [PC1]   [PC2]   [PC3]   [PC4]
   |       |       |       |
 --+-------+-------+-------+--  (backbone cable)

Features:

• Cheap and simple; it needs little cable and is easy to extend.
• Data is broadcast to all nodes, and only the addressed node accepts it.
• A break in the backbone stops the whole network.
• Performance falls as traffic and the number of nodes increase (collisions); fault finding is difficult.

2. Star topology

Every node is connected to a central device, a hub or switch, by its own cable.

        [PC1]
          |
 [PC4]---[HUB]---[PC2]
          |
        [PC3]

Features:

• Easy to install, manage and extend; adding or removing a node does not affect others.
• Failure of one cable or node affects only that node, and faults are easy to isolate.
• Good performance, especially with a switch.
• Failure of the central hub stops the entire network, and more cable is needed, so cost is higher.

3. Ring topology

Each node is connected to exactly two neighbours, forming a closed loop. Data travels in one direction (or two in a dual ring) from node to node, each acting as a repeater. Access is often controlled by a token.

   [PC1]---[PC2]
     |       |
   [PC4]---[PC3]

Features:

• Orderly access by token passing, so there are no collisions, and performance is predictable even under heavy load.
• Every node regenerates the signal, so it works over longer distances.
• A failure of one node or cable can break the whole ring (unless a dual ring is used).
• Adding or removing a node disturbs the network, and troubleshooting is harder.

Three parts of the Central Processing Unit (CPU)

The CPU is the brain of the computer, which fetches, decodes and executes instructions. It has three main parts.

1. Arithmetic Logic Unit (ALU)

It performs all arithmetic operations (addition, subtraction, multiplication, division) and logical operations (AND, OR, NOT, XOR and comparisons such as greater than, less than and equal to). It takes operands from registers and returns the result to a register, such as the accumulator, and sets status flags (zero, carry, sign, overflow).

2. Control Unit (CU)

It directs and coordinates the operation of the whole computer. It fetches an instruction from memory, decodes it, and then generates timing and control signals that tell the ALU, memory and input/output devices what to do and when. It controls the flow of data but does not process the data itself.

3. Registers

Registers are small, very high-speed storage locations inside the CPU that hold the data, instructions and addresses being processed. Important registers are the Program Counter (PC), Instruction Register (IR), Memory Address Register (MAR), Memory Data Register (MDR) and Accumulator (AC).

(Some textbooks list the three parts as ALU, control unit and memory unit or registers; the functions are the same.)

Deadlock

A deadlock is a situation in an operating system in which a set of processes are all blocked permanently, because each process holds some resource and waits for another resource held by another process in the set. None of them can proceed.

Example: Process P1 holds resource R1 and requests R2, while process P2 holds R2 and requests R1. Both wait forever.

Reasons (necessary conditions) for deadlock

A deadlock occurs only when all four Coffman conditions hold at the same time.

1. Mutual exclusion. At least one resource can be used by only one process at a time (non-shareable), such as a printer.
2. Hold and wait. A process holding at least one resource is waiting to acquire additional resources held by others.
3. No pre-emption. A resource cannot be forcibly taken from a process; it is released only voluntarily.
4. Circular wait. A closed chain of processes exists, where each process waits for a resource held by the next one in the chain.

Practical causes include limited resources, poor resource-allocation order and many processes competing for the same resources. Deadlock is handled by prevention (breaking one of the conditions), avoidance (Banker's algorithm), detection and recovery, or ignoring it.

Binary encoding

Binary encoding is the representation of all data (numbers, text, images, sound, instructions) using only two symbols, 0 and 1 (bits). Each piece of data is converted to a unique pattern of bits by a coding scheme: for example, the letter A is 01000001 in ASCII, and the decimal number 5 is 101.

Common schemes are pure binary for numbers, BCD, ASCII and Unicode for characters, and pixel values for images.

Why computers use binary encoding

• Two-state electronic devices. Transistors and switches have two stable states, on and off (high and low voltage), which map directly to 1 and 0.
• Reliability. Distinguishing just two voltage levels is easy, so noise and small voltage changes rarely cause errors.
• Simple and cheap circuits. Logic gates and memory cells for two states are simple, small and cheap to build.
• Boolean algebra. Logical operations (AND, OR, NOT) match binary values, so arithmetic and decisions can be built from logic circuits.
• Easy storage. Magnetic (north/south), optical (pit/land) and flash (charged/uncharged) media naturally store two states.
• Universality. Any kind of information can be encoded in bits, so one machine can process all types of data.

Logical address

A logical address (virtual address) is the address generated by the CPU while a program is running. It is the address seen by the program and is relative to the program's own address space, which starts at 0. The set of all logical addresses is the logical address space. The user can see and use it.

Physical address

A physical address is the actual location in the main memory (RAM) unit, seen by the memory hardware through the memory address register. The set of all physical addresses is the physical address space. The user cannot access it directly.

Mapping. The Memory Management Unit (MMU) translates the logical address into the physical address at run time. In the simple relocation-register scheme:

$\text{Physical address} = \text{Logical address} + \text{Base (relocation) register}$

For example, if the base register is 14000 and the logical address is 346, the physical address is $14000 + 346 = 14346$.

Differences

Characteristics of a database system

1. Self-describing nature. A database contains not only the data but also a complete description of its structure (tables, data types, constraints), stored as metadata in the data dictionary or system catalog.
2. Data abstraction and independence. Users see a conceptual view without the details of storage. The structure can be changed without rewriting the programs (program-data independence), including physical and logical data independence.
3. Reduced data redundancy. Data is stored once and shared, which saves space and avoids inconsistency.
4. Data consistency and integrity. Integrity constraints (primary key, foreign key, check) keep the data accurate and valid.
5. Support for multiple views. Different users can see different customised parts of the same database.
6. Data sharing and multi-user transaction processing. Many users can access the data at the same time, and concurrency control keeps it correct (ACID properties).
7. Security. Access rights and authorisation limit who can read or change which data.
8. Backup and recovery. The system can restore the data after a failure.
9. Query language. Standard languages such as SQL allow easy retrieval and update.
10. Persistent storage. Data is kept permanently and can be retrieved efficiently using indexes.

Security policy

A security policy is a formal, written document of an organisation that states the rules, guidelines and procedures for protecting its information, computer systems, networks and other assets. It defines what is allowed and not allowed, who is responsible, and what action is taken against a violation. It forms the basis for all security mechanisms and practices.

Objectives. To protect the confidentiality, integrity and availability (the CIA triad) of information, to comply with law, and to guide employees' behaviour.

Typical contents

• Purpose and scope. What assets, systems and people are covered.
• Roles and responsibilities. Of management, administrators and users.
• Acceptable use policy. Proper use of computers, e-mail and the Internet.
• Access control and password policy. Who gets access, password length and change rules, and multi-factor authentication.
• Data classification and protection. Handling of confidential, internal and public data, and encryption.
• Network and physical security. Firewalls, antivirus, locked server rooms.
• Backup and disaster recovery. Backup schedules and recovery plans.
• Incident response. How to report and handle breaches.
• Penalties and review. Disciplinary action for violations and periodic updating of the policy.

An effective policy is clear, realistic, enforceable, approved by management and communicated to everyone, with regular training.

Impacts of IT on organizations

Information Technology has changed the way organisations are structured, work and compete. The main impacts are as follows.

Positive impacts

• Higher efficiency and productivity. Automation of routine tasks (accounting, payroll, inventory) saves time and effort and reduces errors.
• Better decision-making. Management information systems, data analysis and decision-support systems give timely and accurate information.
• Improved communication and collaboration. E-mail, video conferencing and messaging connect employees, branches, customers and suppliers across the world.
• Cost reduction. Less paperwork, fewer staff for routine work, online services and remote work lower operating costs.
• Better customer service. Websites, online support, CRM and mobile apps allow 24-hour service and personalised attention.
• Wider markets and new business models. E-commerce, online banking and digital marketing reach customers globally.
• Competitive advantage and innovation. Organisations can offer new products and respond faster to change.
• Changes in structure. Organisations become flatter, with fewer levels of middle management, and use virtual teams and outsourcing.

Negative impacts

• Job displacement and the need to retrain staff, and resistance to change.
• High cost of setting up and maintaining technology.
• Security and privacy risks, such as hacking, data theft and viruses.
• Dependence on technology, so system failure can stop the business.
• Information overload and stress on employees.

(a) Importance of networking

A computer network connects two or more computers and devices so they can share data and resources. Its importance is as follows.

• Resource sharing. Printers, scanners, storage and software can be shared by many users, which saves cost.
• Information sharing and communication. Users exchange files, e-mail, messages and video calls quickly.
• Centralised data and management. Data is kept on servers, which makes backup, updating and security control easier.
• Cost saving. Shared hardware and licences, and lower travel and paper costs.
• Reliability. Data can be stored on several machines, so a failure does not mean total loss.
• Remote access and collaboration. Users can work from different places on the same project.
• Access to the Internet and e-services. E-commerce, e-learning, banking and entertainment depend on networks.
• Scalability. New devices and users can be added easily.

(b) Digital signature

A digital signature is an electronic code, created with a cryptographic technique, that proves the authenticity and integrity of a digital message or document and that the sender cannot deny sending it. It is the electronic equivalent of a handwritten signature.

How it works (public-key cryptography):

1. The sender computes a hash (message digest) of the message.
2. The sender encrypts the hash with their private key; this is the digital signature, which is sent with the message.
3. The receiver decrypts the signature with the sender's public key to obtain the hash, and computes the hash of the received message independently.
4. If both hashes are equal, the message is genuine and unchanged.

It provides: authentication (the sender's identity), integrity (no alteration) and non-repudiation (the sender cannot deny). Digital certificates issued by a Certificate Authority bind a public key to its owner. It is used in e-governance, online banking, e-filing of tax and software distribution, and is legally recognised by Nepal's Electronic Transactions Act.

Components of a general-purpose computer

A general-purpose computer (Von Neumann stored-program model) has five main components:

1. Input unit. Accepts data and instructions (keyboard, mouse, scanner).
2. Central Processing Unit (CPU). Made up of the ALU (arithmetic and logic operations), the Control Unit (fetches, decodes and controls execution) and registers (fast internal storage).
3. Memory unit. Main memory (RAM, ROM) stores the running programs and data; secondary memory stores them permanently.
4. Output unit. Presents results (monitor, printer, speaker).
5. Buses/interconnection. Data, address and control buses link the units.

Block diagram

 +---------+      +-------------------------------------------+      +----------+
 |  Input  | ---> |                   CPU                     | ---> |  Output  |
 |  Unit   |      |  +---------+   +------------------------+ |      |  Unit    |
 +---------+      |  | Control |<->|  ALU                   | |      +----------+
                  |  | Unit    |   +------------------------+ |
                  |  +---------+   Registers: PC, IR, AC,     |
                  |                MAR, MDR                   |
                  +--------------------^----------------------+
                                       | (address, data, control buses)
                          +------------v-------------+
                          |   Main memory (RAM/ROM)  |
                          +------------^-------------+
                                       |
                          +------------v-------------+
                          |   Secondary storage      |
                          +--------------------------+

Function of the PC, IR and AC registers

• Program Counter (PC). It holds the address of the next instruction to be fetched from memory. After each fetch it is automatically incremented to point to the following instruction, and a jump or branch instruction loads a new address into it. This keeps the sequence of execution.
• Instruction Register (IR). It holds the current instruction that has just been fetched from memory. The control unit decodes the instruction in the IR (opcode and operand) and generates the control signals to execute it.
• Accumulator (AC). It is a general-purpose register that holds one operand and the result of ALU operations. For example, in ADD X, the contents of memory location X are added to the AC and the sum is stored back in the AC. It also acts as the register for data transfer between the CPU and memory or I/O.

Instruction cycle. The PC supplies the address, the instruction is read into the IR, the PC is incremented, the instruction is decoded and executed using the ALU and AC, and the cycle repeats.

Software

Software is a set of programs, instructions and related data that tells the computer hardware what to do and how to do it. Unlike hardware, it is intangible. Without software, the hardware cannot do useful work.

Categories of software

Software is broadly divided into system software, application software and (sometimes added) utility and programming software.

1. System software

It manages and controls the hardware and provides a platform for running other software. Its types and functions are as follows.

• Operating system (OS) (Windows, Linux, Android). It manages the processor, memory, files and devices, provides a user interface and runs programs. It acts as the interface between the user and the hardware.
• Device drivers. They let the OS communicate with hardware such as printers and graphics cards.
• Language translators (assembler, compiler, interpreter). They convert programs written in assembly or high-level languages into machine code.
• Utility software (antivirus, disk cleanup, backup, file compression). It helps to maintain, analyse and optimise the system.
• Firmware/BIOS. It starts up the computer and performs the power-on self-test.

2. Application software

It is designed to perform specific tasks for the user.

• General-purpose (packaged) software. Word processors (MS Word), spreadsheets (Excel), presentation software, database software, web browsers and media players.
• Specific-purpose (tailor-made/custom) software. Developed for a particular organisation or purpose, such as payroll, banking, hospital management, inventory or railway reservation systems.
• Application software also includes open-source and proprietary types, according to licence.

3. Programming software

Tools used by developers to write, test and debug programs: editors, compilers, debuggers and IDEs (Visual Studio, Code::Blocks).

Summary. System software runs the computer itself; application software helps the user to do work; programming software helps programmers to create new software.

Database Management System (DBMS)

A database is an organised collection of related data. A Database Management System is a software system that allows users to define, create, store, manipulate, retrieve, control and share a database. It is the interface between the database, the users and the application programs. Examples are MySQL, Oracle, Microsoft SQL Server, PostgreSQL and MS Access.

Main functions: data definition (DDL), data manipulation (DML, queries), security and access control, concurrency control, backup and recovery, and integrity enforcement.

Advantages of DBMS

1. Controlled data redundancy. Data is stored in one place and shared, so duplication and wasted storage are reduced.
2. Data consistency. Because there is a single copy, an update appears everywhere, and conflicting copies are avoided.
3. Data sharing. Many authorised users and applications can use the same data at the same time.
4. Data integrity. Constraints (primary key, foreign key, check, not null) keep the data accurate and valid.
5. Data security. Authentication, authorisation and access privileges protect data from unauthorised access.
6. Data independence. The structure of the data can be changed without changing the application programs.
7. Backup and recovery. The DBMS restores the database after a crash or hardware failure.
8. Concurrent access control. Transactions and locking let multiple users work without interfering with each other.
9. Efficient data access. Indexing, query optimisation and a query language (SQL) give fast retrieval of data.
10. Standards and reduced development time. Common standards, tools and report generators make application development faster and cheaper.
11. Multiple views. Different users can see different parts of the data.

Disadvantages (briefly): high cost of software and hardware, complexity, the need for trained staff, and a heavier impact if the system fails.

Computer

A computer is an electronic device that accepts data as input, processes it according to a stored set of instructions (a program), stores it, and produces meaningful information as output. It can perform arithmetic and logical operations quickly and accurately.

Characteristics of a computer

1. Speed. A computer can process millions or billions of instructions per second; work that takes people hours is done in seconds.
2. Accuracy. The results are highly accurate. Errors normally arise from wrong input or a faulty program, not from the machine (GIGO, garbage in, garbage out).
3. Diligence. It can work for long hours without tiredness, boredom or loss of concentration, giving the same speed and accuracy every time.
4. Versatility. It can perform many different kinds of tasks, such as calculation, document preparation, communication, design and entertainment, simply by changing the program.
5. Storage capacity. It can store huge amounts of data and programs in its memory and retrieve them quickly when required.
6. Automation. Once a program is stored, the computer executes the instructions automatically without human intervention.
7. Reliability. Modern computers are dependable and seldom fail.
8. Remembrance (memory). Information can be kept for as long as required without being forgotten.
9. Lack of intelligence (no IQ). A computer has no intelligence of its own; it works only as instructed and cannot decide on its own.
10. Connectivity. Computers can be networked to share data and resources.

Adding $+7$ and $-17$ using 2's complement (8-bit representation)

Step 1: Represent $+7$.

$+7 = 00000111$

Step 2: Represent $-17$ in 2's complement.

• $+17 = 00010001$
• 1's complement (invert all bits): $11101110$
• Add 1: $11101110 + 1 = 11101111$

So $-17 = 11101111$.

Step 3: Add the two numbers.

$$\begin{array}{r} 00000111 \\ +\;11101111 \\ \hline 11110110 \end{array}$$

There is no carry out of the sign bit, and the sign bit is 1, so the result is negative and is stored in 2's complement form.

Step 4: Find the magnitude of the result.

• Invert $11110110$: $00001001$
• Add 1: $00001010 = 10$

Hence the result is $-10$, which agrees with $7 + (-17) = -10$.

Information Technology (IT)

Information Technology is the use of computers, software, networks, telecommunication and other electronic devices to create, store, process, retrieve, transmit and exchange information. It combines computing with communication technology.

Applications of IT in education

• E-learning and online classes: Platforms such as Google Classroom, Zoom and Moodle let students learn from anywhere, which was especially useful when physical classes were closed.
• Digital library and e-books: Students can access journals, books and research papers online at any time.
• Multimedia teaching: Projectors, smart boards, animations, simulations and videos make difficult concepts easier to understand.
• Administration and management: Admission, attendance, fee collection, timetables and result processing are done with school management software, which saves time and reduces errors.
• Online examination and evaluation: Online tests, automatic marking and publishing of results online give fast and fair assessment.
• Research and communication: The Internet, email and social media help students and teachers search for information, collaborate and share notes.
• Distance and open education: Learners who cannot attend regular classes can study through virtual universities and online courses.
• Special education: Screen readers, speech recognition and assistive tools help students with disabilities.

Memory hierarchy

The memories of a computer are arranged in a hierarchy. Moving down the hierarchy, access time increases, capacity increases and cost per bit decreases. Moving up, memory is faster, smaller and more expensive.

Explanation

1. Registers: Small storage units inside the CPU that hold the data and instructions currently being executed. They are the fastest memory.
2. Cache memory: A small, very fast memory between the CPU and RAM. It stores frequently used data and instructions so that the CPU does not wait for RAM.
3. Main memory (primary memory): RAM and ROM. RAM is volatile and holds the programs and data in use. ROM is non-volatile and holds the boot program.
4. Secondary memory: Non-volatile, high-capacity permanent storage such as hard disk and solid state drive, used to store programs and files.
5. Tertiary or offline memory: Removable media such as CD/DVD, tape and pen drives, used for backup and archives.

Utility software

Utility software is system software that helps to analyze, configure, optimize and maintain the computer. It does not run the hardware directly as the operating system does, but it supports the operating system and keeps the computer working efficiently.

Purposes of utility software

• Maintenance and optimization: Disk defragmenters and disk cleanup tools arrange files and remove junk files to free space and improve speed.
• Protection from malware: Antivirus programs detect and remove viruses, spyware and other malicious software.
• Data backup and recovery: Backup utilities copy data to another location and restore it after accidental loss or failure.
• File compression: Tools such as WinZip and WinRAR reduce file size to save space and make transfer easier.
• File management: File managers and search tools help to copy, move, rename, organize and find files.
• Security: Encryption utilities and firewalls protect data and control network access.
• Disk management: Partitioning, formatting and disk checking tools prepare and repair storage devices.
• System monitoring: Task manager and diagnostic tools show resource usage and help to find faults.

Computer network

A computer network is a collection of two or more computers and other devices (printers, servers, switches) connected to each other through a transmission medium so that they can share data, resources and information. Examples are LAN, MAN, WAN and the Internet.

Importance of networking

• Resource sharing: Printers, scanners, storage and software can be shared by many users, which lowers cost.
• Data sharing and communication: Users can quickly exchange files, email, messages and video calls.
• Centralized data and easy backup: Data kept on a server can be managed, secured and backed up from one place.
• Cost saving: Sharing hardware and software licences reduces the expense of buying them for each user.
• Reliability: If one computer fails, data and services can be provided from another one.
• Remote access and collaboration: People in different places can work together on the same project and access data from anywhere.
• Access to the Internet and services: Networks provide online banking, e-commerce, e-learning and entertainment.
• Better security and control: Administrators can set user rights and policies centrally.

(a) Smart city

A smart city is an urban area that uses information and communication technology, sensors, the Internet of Things and data analytics to manage its resources and services efficiently and to improve the quality of life of its citizens.

Main features:

• Smart transport: traffic lights that respond to traffic, GPS-based public transport, and online ticketing.
• Smart energy and utilities: smart grids and smart meters, and monitoring of water supply and waste collection.
• Smart governance: online public services, e-payment and CCTV-based public safety.
• Smart environment: air quality monitoring and pollution control.
• Smart healthcare and education: telemedicine and e-learning.

Its benefits are less congestion and pollution, better safety, efficient use of resources and quick services. Its challenges are high cost, privacy and security of data, and the need for skilled people and good infrastructure.

(b) Big data

Big data means extremely large and complex collections of data that cannot be stored, processed or analyzed with traditional database tools. The data comes from social media, sensors, transactions, mobile devices, and so on.

Characteristics (the Vs of big data):

• Volume: a huge amount of data.
• Velocity: data is generated and processed at high speed.
• Variety: structured, semi-structured and unstructured data (text, images, video).
• Veracity: the quality and reliability of the data.
• Value: useful insight that can be extracted.

Technologies such as Hadoop, Spark and NoSQL databases are used. Applications include business analysis, healthcare, fraud detection, recommendation systems and weather forecasting.

Operating system

An operating system (OS) is system software that manages the hardware and software resources of a computer and provides an interface between the user and the hardware. Operating systems are classified according to how they handle users and jobs.

1. Batch operating system

Similar jobs are collected into batches and executed one after another without user interaction. The user submits the job and receives the output later. CPU idle time is reduced, but there is no interaction, and debugging is difficult and turnaround time is long. It was used in early mainframes.

2. Multiprogramming operating system

Several programs are kept in main memory at the same time. When one program waits for input or output, the CPU switches to another program, so the CPU utilization is high.

3. Time-sharing (multitasking) operating system

The CPU time is divided into small slices (quanta) and given to each user or task in turn. Switching is so fast that each user feels they are using the computer alone. It gives quick response and allows many tasks to run, as in UNIX and Windows.

4. Multiuser operating system

Many users can access the same computer at the same time through terminals. The system manages their resources and security. Examples are UNIX and Linux servers.

5. Multiprocessing (parallel) operating system

Two or more CPUs work together in a single system and share memory, bus and peripherals. It gives higher throughput and reliability, because if one processor fails, the others continue.

6. Distributed operating system

Many independent computers connected by a network work together and appear to the user as a single system. Resources and load are shared. It is reliable and scalable, but it is complex and needs good network security.

7. Network operating system

It runs on a server and lets clients share files, printers and other resources over a network, with users and security managed centrally. Examples are Windows Server and Novell NetWare.

8. Real-time operating system (RTOS)

It responds to inputs within a fixed time limit. A hard real-time system must never miss a deadline (missile control, pacemakers, air traffic control). A soft real-time system can tolerate a small delay (multimedia, online booking).

9. Mobile and embedded operating system

These are designed for devices with limited resources, such as Android and iOS on phones and embedded OSs in washing machines and routers.

Single-user and multi-user systems

Single-user systems (MS-DOS, Windows 98) serve one user at a time, while multi-user systems allow several users, as described above.

Transmission medium

A transmission medium is the physical path through which data travels from the sender to the receiver in a network. It is of two types: guided (wired) and unguided (wireless).

A. Guided media

In guided media, the signal travels along a physical conductor.

1. Twisted pair cable

Two insulated copper wires are twisted around each other to reduce interference and crosstalk.

 ~~~~ twisted copper pair ~~~~
  \\\\//\\\\//\\\\//\\\\//

• UTP (Unshielded Twisted Pair): Cheap and easy to install, used in LANs (Cat5, Cat6, RJ-45 connector) and telephone lines. It is more affected by noise.
• STP (Shielded Twisted Pair): Has a metal shield around the pairs, so it has less interference but costs more.
• Typical range is up to about 100 m, with speeds from 10 Mbps up to 10 Gbps.

2. Coaxial cable

 [ copper core ] [ insulation ] [ braided metal shield ] [ outer jacket ]

A central copper conductor is surrounded by an insulator, a braided metal shield and an outer plastic jacket. It has higher bandwidth and better noise immunity than twisted pair. It is used in cable TV and older Ethernet (10Base2, 10Base5). Thicker types can reach several hundred metres, and the cable is more expensive and bulky.

3. Optical fibre cable

 [ core (glass) ] [ cladding ] [ buffer/coating ] [ jacket ]
   light ===> total internal reflection ===>

Data is sent as light pulses through a thin glass or plastic core. The cladding has a lower refractive index, so light is kept in the core by total internal reflection.

• Single-mode fibre: a thin core, laser light, long distances (tens of kilometres).
• Multimode fibre: a thicker core, LED light, short distances.
• Advantages: very high bandwidth and speed, low attenuation, no electromagnetic interference, high security.
• Disadvantages: expensive, and installation and repair need skill.

B. Unguided media

In unguided media, signals travel through air or space as electromagnetic waves without a physical conductor.

1. Radio waves (3 kHz to 1 GHz)

They are omnidirectional, pass through walls and travel long distances. Used in AM/FM radio, TV broadcast, cordless phones and Wi-Fi.

2. Microwaves (1 GHz to 300 GHz)

 [Tower A] )))) line of sight (((( [Tower B]

They are unidirectional and need line of sight between antennas, which are placed on towers about 40 to 50 km apart. Used in mobile phone networks and point-to-point links. They are affected by weather and obstacles.

3. Satellite communication

          [Satellite]
          /         \
  [Earth station] [Earth station]

A ground station sends (uplink) a signal to a satellite in orbit, which amplifies it and sends it back (downlink) to another station. It covers very large areas (GPS, TV broadcast, VSAT) but has a high cost and propagation delay.

4. Infrared (300 GHz to 400 THz)

Short-range, line-of-sight communication that cannot pass through walls. Used in TV remote controls and short device-to-device transfer.

5. Bluetooth is a short-range radio technology (about 10 m) for connecting phones, headsets and other devices.

Comparison

Three-level architecture of a database system (ANSI-SPARC architecture)

The three-level architecture separates the user's view of the database from the way the data is physically stored. It has three levels: the external level, the conceptual level and the internal level.

 External level   : View 1   View 2   ...  View n      (users)
                       \       |       /
                  external/conceptual mapping
 Conceptual level : Community view of the whole database (logical schema)
                          |
                  conceptual/internal mapping
 Internal level   : Physical storage of data (files, indexes, blocks)

1. External level (view level)

This is the highest level and describes the part of the database that is relevant to a particular user or group of users. Each user has a different view, called an external schema or sub-schema. It hides the rest of the data and shows only the necessary data. For example, a student can see only his or her results, while an accountant sees only fee records. It gives security and makes the database easy to use.

2. Conceptual level (logical level)

This is the middle level and describes the whole database for the entire organization. It defines all entities, attributes, relationships, constraints and security information. It is independent of how the data is physically stored. There is only one conceptual schema, and it is designed by the database administrator (DBA). It is typically represented by an ER model or a relational schema.

3. Internal level (physical level)

This is the lowest level and describes how the data is actually stored on the storage device: file organization, record formats, indexes, access paths, data compression and encryption. The internal schema is concerned with efficiency and storage space.

Mappings

• External/conceptual mapping: converts a user's view to the conceptual schema.
• Conceptual/internal mapping: converts the conceptual schema to the physical storage structure.

Data independence

• Logical data independence: the conceptual schema can be changed (adding a new attribute) without changing external views or application programs.
• Physical data independence: the internal schema can be changed (new storage device or index) without changing the conceptual schema.

Advantages of this architecture: it supports multiple views of the same data, hides complexity from users, provides data independence, and makes the database easier to maintain.

Components of a computer

A computer system consists of hardware, software and the user (peopleware). The main components are as follows.

1. Input unit

It accepts data and instructions from the user and converts them into a form the computer can understand. Examples are keyboard, mouse, scanner and microphone.

2. Central Processing Unit (CPU)

The CPU is the brain of the computer and processes the data. It has three parts:

• Arithmetic Logic Unit (ALU): performs arithmetic operations (add, subtract) and logical operations (compare, AND, OR).
• Control Unit (CU): directs and coordinates the activities of all other units by fetching, decoding and executing instructions.
• Registers: small, fast storage locations inside the CPU that hold the data being processed.

3. Memory unit

It stores data, instructions and results. It is of two kinds: primary memory (RAM and ROM) and secondary memory (hard disk, SSD).

4. Output unit

It presents the processed result to the user in a readable form. Examples are monitor, printer and speaker.

5. Software

It is the set of programs that tells the hardware what to do. System software (operating system, utilities) and application software (word processor, games) are the two main types.

6. User (peopleware)

The people who operate the computer and write programs.

 Input --> [ Memory <--> CPU (ALU + CU) ] --> Output
                 ^
            Secondary storage

Difference between mainframe computer and supercomputer

Primary memory

Primary memory (main memory) is the memory directly accessed by the CPU. It stores the programs and data that are currently being used, and it is fast but limited in size.

Types of primary memory

1. RAM (Random Access Memory)

RAM is volatile, so its contents are lost when power is switched off. Data can be read and written. It is of two types:

• SRAM (Static RAM): uses flip-flops, is fast and expensive, and does not need refreshing. It is used for cache memory.
• DRAM (Dynamic RAM): uses capacitors, is slower and cheaper, and needs periodic refreshing. It is used as main memory.

2. ROM (Read Only Memory)

ROM is non-volatile, so its contents remain when power is off. It stores permanent programs such as the BIOS and boot loader. Its types are:

• PROM: programmable once by the user.
• EPROM: erasable with ultraviolet light and reprogrammable.
• EEPROM: erasable electrically, and used in flash memory.
• Mask ROM: programmed by the manufacturer during fabrication.

3. Cache memory

A small, very fast memory between the CPU and RAM that stores frequently used data and instructions to increase processing speed.

(a) Convert $(366)_8$ to hexadecimal

First convert each octal digit to 3 binary bits:

$3 = 011,\quad 6 = 110,\quad 6 = 110$

$(366)_8 = (011110110)_2$

Regroup into 4 bits from the right:

$0\,1111\,0110 = 0000\,1111\,0110$

$1111 = F,\quad 0110 = 6$

$(366)_8 = (F6)_{16}$

Check: $3\times 64 + 6\times 8 + 6 = 246$ and $F6_{16} = 15\times 16 + 6 = 246$.

(b) Subtract $(1001111)_2$ from $(1110111)_2$

$(1110111)_2 - (1001111)_2$

Using 2's complement (7 bits):

• 1's complement of $1001111$ is $0110000$.
• Add 1: $0110000 + 1 = 0110001$.
• Add to the minuend: $1110111 + 0110001 = 1\,0101000$.
• Discard the carry out: $0101000$.

$(1110111)_2 - (1001111)_2 = (101000)_2$

Check: $119 - 79 = 40 = (101000)_2$.

Programming languages

A programming language is a set of rules and instructions used to write programs that a computer can execute. The main types are as follows.

1. Machine language (first generation)

It consists of binary instructions (0s and 1s) that the CPU executes directly. It is the fastest language, needing no translator, but it is hard to write, error-prone and machine dependent.

2. Assembly language (second generation)

It uses mnemonics such as ADD, MOV and SUB instead of binary. It needs an assembler to translate it to machine code. It is easier than machine language but still machine dependent and low level. It is used for device drivers and embedded systems.

3. High-level language (third generation)

These use English-like statements and are independent of the machine. They need a compiler or interpreter. Examples are C, C++, Java, Python and Pascal. They are easy to learn, write and debug, and portable. They may be:

• Procedural (C, Pascal), where a program is a sequence of procedures.
• Object-oriented (C++, Java), where a program is organized in classes and objects.

4. Very high-level or fourth-generation language (4GL)

These are non-procedural languages that state what is to be done rather than how. Examples are SQL, report generators and query languages. They are used for database and application development.

5. Fifth-generation language (5GL)

These are used in artificial intelligence and are based on constraints and logic, so that the computer solves the problem. Examples are Prolog and Mercury.

Language translators

• Assembler: assembly language to machine code.
• Compiler: translates the whole program at once.
• Interpreter: translates and executes the program line by line.

Cryptography

Cryptography is the science and art of securing information by converting plain text into unreadable cipher text (encryption) and converting it back (decryption) using algorithms and keys. It provides confidentiality, integrity, authentication and non-repudiation.

Private key (symmetric) cryptography

The same secret key is used by both the sender and the receiver for encryption and decryption.

 Plain text --[Encrypt with key K]--> Cipher text --[Decrypt with key K]--> Plain text

• It is fast and suitable for large amounts of data.
• Its main problem is the safe distribution of the key, and with n users, n(n-1)/2 keys are needed.
• Examples: DES, 3DES, AES, RC4.

Public key (asymmetric) cryptography

Two mathematically related keys are used: a public key, which is known to everyone, and a private key, which is kept secret by its owner.

 Plain text --[Encrypt with receiver's PUBLIC key]--> Cipher text --[Decrypt with receiver's PRIVATE key]--> Plain text

• Anyone can encrypt a message to the receiver with the public key, but only the receiver can decrypt it with the private key.
• If the sender encrypts with his or her private key, anyone can verify it with the public key. This is the basis of digital signatures.
• It removes the key distribution problem, but it is slower than symmetric encryption.
• Examples: RSA, ElGamal, ECC, Diffie-Hellman (key exchange).

In practice, public key cryptography is used to exchange a session key, and symmetric cryptography then encrypts the actual data.

Computer bus

A computer bus is a set of parallel wires or lines that carries data, addresses and control signals between the CPU, memory and input/output devices. There are three types:

• Data bus: carries data in both directions. Its width (8, 16, 32, 64 bits) determines how many bits are transferred at a time.
• Address bus: carries the address of the memory location or I/O device, in one direction from the CPU. Its width decides the maximum memory that can be addressed ($2^n$ locations for n lines).
• Control bus: carries control signals such as read, write, clock and interrupt.

Working of a laser printer

A laser printer is a non-impact page printer that uses a laser beam and static electricity to print on paper. The steps are:

1. Cleaning: The photosensitive drum is cleaned of previous toner and its charge is removed.
2. Charging: A charge roller gives the drum a uniform positive (or negative) charge.
3. Writing (exposure): The printer controller converts the page data to a bitmap. A laser beam, reflected by a rotating mirror, scans the drum. Where the laser hits, the charge is discharged, forming an invisible electrostatic image of the page.
4. Developing: Toner powder, with a charge, sticks only to the discharged image areas of the drum.
5. Transferring: The paper passes under the drum and gets a charge that pulls the toner from the drum to the paper.
6. Fusing: The paper passes between heated rollers (the fuser), which melt the toner and press it permanently into the paper.
7. Cleaning and discharge: The drum is cleaned again and made ready for the next page.

Laser printers give high-quality, fast and quiet printing.

Cloud computing

Cloud computing is the delivery of computing services such as servers, storage, databases, networking, software and analytics over the Internet ("the cloud") on a pay-as-you-use basis, instead of owning and maintaining physical hardware. Users access resources on demand from any place with an Internet connection.

Service models

• IaaS (Infrastructure as a Service): virtual machines, storage and networks (AWS EC2).
• PaaS (Platform as a Service): a platform for developing and deploying applications (Google App Engine).
• SaaS (Software as a Service): ready-to-use software (Gmail, Google Docs, Office 365).

Deployment models: public, private, hybrid and community cloud.

Advantages of cloud computing

• Cost saving: no need to buy expensive hardware and software, and payment is only for what is used.
• Scalability and flexibility: resources can be increased or decreased quickly as needed.
• Accessibility: data and applications are available anywhere, at any time, on any device.
• Automatic updates and maintenance: the provider manages hardware and software updates.
• Backup and disaster recovery: data is stored in multiple locations and is easy to recover.
• Collaboration: many users can work on the same file simultaneously.
• High availability and reliability.

(a) Societal impacts of IT

Information technology has changed the way people live, work and communicate.

Positive impacts

• Communication: email, social media and video calls connect people instantly across the world.
• Education: e-learning, online resources and digital libraries are available to all.
• Business and economy: e-commerce, online banking and digital payment make transactions fast, and new jobs and industries are created.
• Health: telemedicine, electronic health records and better diagnosis.
• Governance: e-government services improve transparency and access to public services.

Negative impacts

• Unemployment due to automation of jobs.
• Privacy and security issues: data theft, hacking, identity theft and cyber crime.
• Health problems: eye strain, back pain and addiction to the Internet and games.
• Digital divide between those who have access and those who do not.
• Social problems: cyberbullying, spread of false information, and reduced face-to-face interaction.
• E-waste and environmental damage.

(b) Internet of Things (IoT)

The Internet of Things is a network of physical objects ("things") that have sensors, software and network connectivity built in, so that they can collect and exchange data over the Internet without human interaction.

A typical IoT system has sensors/devices that collect data, connectivity (Wi-Fi, Bluetooth, 5G) that sends it, a cloud or data-processing system that analyzes it, and a user interface or actuators that act on it.

Applications: smart homes (smart lights, thermostats, security cameras), smart cities, wearable health devices, agriculture (soil sensors and irrigation), industrial automation and connected vehicles.

Issues: security and privacy, device compatibility and large amounts of data.