Computer System

System Bus

A system bus is a set of parallel wires or lines that connects the CPU, main memory and input/output devices and carries data, addresses and control signals between them. It lets the components of a computer communicate with each other.

Types of bus

1. Data bus

• It carries the actual data between the CPU, memory and I/O devices.
• It is bidirectional, since data can move both to and from the CPU.
• Its width (8, 16, 32 or 64 bits) decides how many bits move at a time, so it affects the speed of the computer.

2. Address bus

• It carries the address of the memory location or I/O device that the CPU wants to read from or write to.
• It is unidirectional, because addresses go only from the CPU to memory or devices.
• Its width decides the maximum memory that can be addressed: an $n$-line address bus can address $2^n$ locations.

3. Control bus

• It carries control and timing signals from the control unit, such as read, write, interrupt and clock signals.
• It coordinates and controls the activities of all devices attached to the system.
• It carries signals in both directions (commands out, status signals in).

Block Diagram of a Computer System

   +-----------+         +--------------------------------+         +-------------+
   |   INPUT   |  data   |      CENTRAL PROCESSING UNIT   |  result |   OUTPUT    |
   |   UNIT    |-------->|  +---------+     +-----------+ |-------->|    UNIT     |
   | keyboard, |         |  | CONTROL |<--->|   ALU     | |         | monitor,    |
   | mouse ... |         |  |  UNIT   |     | (arith &  | |         | printer ... |
   +-----------+         |  +----+----+     |  logic)   | |         +-------------+
                         |       |          +-----------+ |
                         +-------|------------------------+
                                 v  ^
                         +-----------------+      +-----------------+
                         |  PRIMARY MEMORY |<---->| SECONDARY MEMORY|
                         |  (RAM / ROM)    |      | (HDD, SSD ...)  |
                         +-----------------+      +-----------------+

Data and instructions enter through the input unit and are stored in memory. The CPU fetches and executes them, and the results are sent to the output unit.

Control Unit (CU). The control unit is the manager of the computer. It does not process data itself but directs and coordinates all other units. Its functions are:

• fetching instructions from memory and decoding them;
• sending control signals to the ALU, memory, input and output devices;
• controlling the sequence of operations and data flow between units;
• synchronising the operations with the system clock.

Arithmetic and Logic Unit (ALU). The ALU is the part of the CPU where all calculations and decisions are carried out. It has two sections:

• Arithmetic section: performs addition, subtraction, multiplication and division.
• Logic section: performs comparisons (greater than, less than, equal to) and logical operations such as AND, OR and NOT.

The ALU receives data from memory/registers, processes it as directed by the control unit, and returns the result to a register or memory. It uses registers such as the accumulator to hold intermediate results.

Primary memory. It is the main memory directly accessible by the CPU, holding the programs and data currently in use.

• RAM (Random Access Memory): read/write and volatile; its contents are lost when power is off.
• ROM (Read Only Memory): non-volatile; stores permanent start-up instructions (BIOS).

It is faster than secondary memory but smaller and costlier per byte.

(c) Hard disk

A hard disk stores data permanently (secondary storage). Keyboard is an input device, while monitor and printer are output devices.

Primary Memory

Primary memory (main memory) is the memory that the CPU can access directly. It holds the data and programs that are currently being processed. It is fast and made of semiconductor chips, but has a smaller capacity than secondary storage. RAM and ROM are its two main types.

Difference between RAM and ROM

Computer Architecture

Computer architecture is the conceptual design and fundamental operational structure of a computer system. It describes how the main hardware units — input unit, central processing unit, memory unit and output unit — are organised and interconnected, and how data and control signals flow among them to execute a program.

Block Diagram

        +----------------+                        +----------------+
        |                |                        |                |
        |  INPUT UNIT    |                        |  OUTPUT UNIT   |
        | (Keyboard,     |                        | (Monitor,      |
        |  Mouse,        |                        |  Printer,      |
        |  Scanner...)   |                        |  Speaker...)   |
        +--------+-------+                        +--------+-------+
                 |                                         ^
                 |  data / instructions                    |  results
                 v                                         |
        +--------+-----------------------------------------+-------+
        |                    SYSTEM  UNIT  (CPU)                   |
        |                                                          |
        |   +----------------+   +----------------+   +---------+  |
        |   |  CONTROL UNIT  |<->|  ALU           |<->| REGISTERS| |
        |   |  (CU)          |   | (Arithmetic &  |   | (Accumul-| |
        |   |                |   |  Logic Unit)   |   |  ator,PC,| |
        |   +-------+--------+   +--------+-------+   |  IR,MAR, | |
        |           |                     |           |  MBR...) | |
        +-----------|---------------------|-----------+----------+
                    |                     |
                    v                     v
        +-----------+---------------------+----------------------+
        |                     MEMORY  UNIT                         |
        |   Primary Memory (RAM, ROM)  |  Secondary Memory (HDD,   |
        |                              |  SSD, Pen drive, DVD)     |
        +----------------------------------------------------------+

Functions of the Components

1. Input Unit

• Accepts data and instructions from the user in human-readable form (keyboard, mouse, scanner, microphone, joystick).

• Converts them into binary form (0s and 1s) that the computer can process.
• Supplies the converted data to the CPU/memory for processing.

2. Central Processing Unit (CPU) The CPU is the brain of the computer; it processes data and controls the whole system. It has three parts:

• Control Unit (CU): Fetches instructions from memory one by one, decodes them, and generates control signals that direct all other units to perform the required operations. It does not process data itself; it only controls and coordinates.
• Arithmetic and Logic Unit (ALU): Performs all arithmetic operations (addition, subtraction, multiplication, division) and logical operations (comparison, AND, OR, NOT). The result is stored in the accumulator.
• Registers: Small, very fast storage locations inside the CPU that hold data and instructions temporarily during processing — e.g. Accumulator (holds result), Program Counter (address of next instruction), Instruction Register (current instruction), MAR and MBR (memory address and data).

3. Memory Unit

• Primary memory (RAM, ROM): Stores data, instructions and intermediate results directly accessible to the CPU. RAM is volatile and holds the currently running programs; ROM is non-volatile and holds permanent start-up instructions (BIOS).
• Secondary memory (hard disk, SSD, pen drive, DVD): Stores data and programs permanently in large quantities; data must be loaded into primary memory before use.

4. Output Unit

• Receives processed results from the CPU/memory.
• Converts binary results back into human-readable form (text, image, sound) and presents them through devices such as monitor, printer, plotter and speaker.

Data Flow

Data entered through the input unit is stored in memory; the control unit fetches and decodes instructions and directs the ALU to process the data using registers; the result is sent back to memory and finally delivered to the user through the output unit.

(a) DRAM

DRAM is a type of RAM and loses its contents when power is switched off, so it is volatile. Hard disk and ROM keep data without power, and a mouse is not a memory.

(b) Pixel

A pixel (picture element) is the smallest dot that can be displayed on a monitor screen. Bit and byte are units of data, not of display.

(c) 1024 MB

By the binary system, 1 GB = 1024 MB. The other options are not equal to a gigabyte.

Impact vs Non-impact Printers

(a) USB

USB (Universal Serial Bus) is a port used to connect devices such as pen drives and keyboards. A register is a CPU storage cell, High Definition is a quality standard and DVD is a storage medium.

(a) DROM

The types of ROM are MROM, PROM, EPROM and EEPROM. "DROM" is not a ROM type.

(c) Third Generation

Third-generation computers (about 1964 to 1971) replaced transistors with Integrated Circuits (ICs), which made computers smaller, faster and cheaper.

(b) Reduced the size and cost of the computer

An IC packs many transistors and components on one small silicon chip. This reduces the size, cost and heat of the computer and improves its speed and reliability.

(d) DVD

A DVD is read and written using a laser beam, so it is optical storage. Cache and RAM are semiconductor memories, and ROM is also a semiconductor memory.

(b) System Unit

The CPU is mounted on the motherboard inside the system unit (casing) of the computer.

(d) Bit

A bit (binary digit), 0 or 1, is the smallest unit of information. A nibble is 4 bits and a byte is 8 bits.

RAM vs ROM

Memory

Memory is the unit of a computer that stores data, instructions and results for processing. It is measured in bytes.

Classification

1. Primary memory (main memory): directly accessed by the CPU; includes RAM, ROM and cache memory.
2. Secondary memory (auxiliary memory): permanent, large-capacity storage such as hard disk, CD/DVD, pen drive and magnetic tape.

Two types of primary memory

1. RAM (Random Access Memory)

RAM is the working memory of the computer. It can be read and written, and it holds the programs and data currently in use. It is volatile, so the data is lost when the power is off. Its types are SRAM (fast, used in cache) and DRAM (needs refreshing, used as main memory).

2. ROM (Read Only Memory)

ROM stores permanent instructions, such as the BIOS and bootstrap program, which the computer needs at start-up. It is non-volatile, so its contents remain without power. Its normal use is reading only. Types are PROM, EPROM and EEPROM.

Microprocessor

A microprocessor is an integrated circuit (single chip) that contains the arithmetic and logic unit, control unit and registers of a computer, and performs the processing of data. It is the CPU of a microcomputer. Examples are Intel Core and AMD Ryzen processors.

Functions

• Fetching instructions: it fetches instructions and data from memory.
• Decoding: it decodes each instruction to find what operation is to be done.
• Executing: it carries out arithmetic operations (addition, subtraction) and logical operations (comparison, AND, OR, NOT).
• Controlling: it controls and coordinates the timing and working of memory and the input and output devices.
• Storing results: it temporarily holds data and results in its registers and sends output results to memory or output devices.
• Handling interrupts: it responds to requests from devices and manages data transfer between them.

Super Computer

A super computer is the fastest and most powerful type of computer. It has thousands of processors working in parallel and can perform trillions of calculations per second (measured in FLOPS). It is very large and costly. Examples are Fugaku and Summit.

Uses in real life

• Weather forecasting: it processes huge amounts of data to predict weather, storms and floods.
• Scientific research: it simulates nuclear reactions, space and the behaviour of molecules.
• Medicine: it is used for drug design, studying DNA and modelling diseases.
• Space and defence: it helps in launching and tracking satellites and in military simulations.
• Engineering and animation: it models aircraft or earthquakes and renders high-quality animated films.
• Artificial intelligence: it trains very large AI models.

Block Diagram of a Computer System

            +--------------------------------------+
            |     CENTRAL PROCESSING UNIT (CPU)    |
            |                                      |
 INPUT      |  +--------------+   +--------------+ |      OUTPUT
 UNIT  ---->|  | Control Unit |<->|     ALU      | |----> UNIT
(keyboard,  |  +------+-------+   +------+-------+ | (monitor,
 mouse)     |         |   Registers      |         |  printer)
            +---------+------------------+---------+
                      |                  |
                      v                  v
            +--------------------------------------+
            |   MEMORY UNIT (primary / secondary)  |
            +--------------------------------------+

• Input unit: takes data and instructions from the user and converts them into a form the computer understands.
• CPU: processes the data. It consists of the control unit, the ALU and registers.
• Memory unit: stores data, instructions and results before, during and after processing.
• Output unit: converts the results into a form the user can understand and presents them.

Control Unit (CU)

The control unit is the part of the CPU that directs and coordinates the working of all parts of the computer. It does not process data itself but tells other units what to do.

Functions:

• It fetches instructions from memory one by one and decodes them.
• It sends control signals to the ALU, memory and I/O devices so that each operation is carried out at the right time.
• It controls the flow of data between the CPU, memory and I/O devices.
• It controls the sequence of instruction execution using the program counter and instruction register.
• It manages timing with the clock so that all operations are synchronised.

Computer Architecture

Computer architecture is the design and organisation of a computer's hardware: how the processor, memory and input/output units are structured, connected and made to work together to execute instructions.

Block diagram

   +-----------+         +--------------------------------+         +-------------+
   |   INPUT   |  data   |      CENTRAL PROCESSING UNIT   |  result |   OUTPUT    |
   |   UNIT    |-------->|  +---------+     +-----------+ |-------->|    UNIT     |
   | keyboard, |         |  | CONTROL |<--->|   ALU     | |         | monitor,    |
   | mouse ... |         |  |  UNIT   |     | (arith &  | |         | printer ... |
   +-----------+         |  +----+----+     |  logic)   | |         +-------------+
                         |       |          +-----------+ |
                         +-------|------------------------+
                                 v  ^
                         +-----------------+      +-----------------+
                         |  PRIMARY MEMORY |<---->| SECONDARY MEMORY|
                         |  (RAM / ROM)    |      | (HDD, SSD ...)  |
                         +-----------------+      +-----------------+

Microprocessor. It is the whole CPU built on a single integrated circuit (chip). It contains the control unit, ALU and registers, and fetches, decodes and executes instructions. Its speed is measured in GHz. Examples: Intel Core i5, AMD Ryzen.

Control Unit (CU). The control unit is the manager of the computer. It does not process data itself but directs and coordinates all other units. Its functions are:

• fetching instructions from memory and decoding them;
• sending control signals to the ALU, memory, input and output devices;
• controlling the sequence of operations and data flow between units;
• synchronising the operations with the system clock.

Arithmetic and Logic Unit (ALU). The ALU is the part of the CPU where all calculations and decisions are carried out. It has two sections:

• Arithmetic section: performs addition, subtraction, multiplication and division.
• Logic section: performs comparisons (greater than, less than, equal to) and logical operations such as AND, OR and NOT.

The ALU receives data from memory/registers, processes it as directed by the control unit, and returns the result to a register or memory. It uses registers such as the accumulator to hold intermediate results.

Components of a Computer System

A computer system has three components: hardware, software and liveware (the user). The hardware parts are shown below.

   +-----------+         +--------------------------------+         +-------------+
   |   INPUT   |  data   |      CENTRAL PROCESSING UNIT   |  result |   OUTPUT    |
   |   UNIT    |-------->|  +---------+     +-----------+ |-------->|    UNIT     |
   | keyboard, |         |  | CONTROL |<--->|   ALU     | |         | monitor,    |
   | mouse ... |         |  |  UNIT   |     | (arith &  | |         | printer ... |
   +-----------+         |  +----+----+     |  logic)   | |         +-------------+
                         |       |          +-----------+ |
                         +-------|------------------------+
                                 v  ^
                         +-----------------+      +-----------------+
                         |  PRIMARY MEMORY |<---->| SECONDARY MEMORY|
                         |  (RAM / ROM)    |      | (HDD, SSD ...)  |
                         +-----------------+      +-----------------+

• Input unit: takes data and instructions into the computer (keyboard, mouse, scanner).
• CPU: the brain of the computer. The control unit directs and coordinates all operations, and the ALU performs arithmetic (+, -, ×, ÷) and logical (comparison, AND, OR, NOT) operations.
• Memory: primary memory (RAM, ROM) holds the data and programs in use; secondary memory (hard disk, SSD) stores them permanently.
• Output unit: presents results (monitor, printer, speaker).
• Software: the programs (system software and application software) that tell the hardware what to do.
• Liveware: the people who operate and use the computer, such as users and programmers.

(a) Cache Memory

Cache is a small, very high-speed memory placed between the CPU and RAM that stores frequently used data. It is faster than RAM, ROM and DVD.

(d) mouse

A mouse is an input device because it sends data (pointer position and clicks) into the computer. Speaker, printer and monitor are output devices as they give results out of the computer.

(c) Memory bus

The three buses of a computer system are the address bus, data bus and control bus. "Memory bus" is not a standard bus type.

Secondary Memory

Secondary memory (auxiliary memory) is the non-volatile, permanent storage of a computer. It stores data and programs even when the power is switched off. It has a much larger capacity than main memory but is slower to access. The main types are:

1. Magnetic Memory

Data is stored as magnetised spots on a magnetic surface. It is cheap, has a large capacity and is reliable.

• Hard Disk Drive (HDD): The main storage device of a computer. It consists of rapidly rotating magnetic platters and read/write heads. Capacity: hundreds of GB to several TB. It is used to store the operating system, software and user files.
• Magnetic Tape: A long plastic ribbon coated with magnetic material, kept in a cassette. It is a sequential-access device (data is read one after another), so it is slow but very cheap. It is mainly used for backup and archiving of large amounts of data.
• Floppy Disk: An old removable magnetic disk of 3.5 inch size with 1.44 MB capacity. It is now obsolete.

2. Optical Memory

Data is written and read with the help of a laser beam, which burns tiny pits on the surface of the disk. These are cheap, portable and light in weight.

• CD (Compact Disc): Capacity about 700 MB. Types: CD-ROM (read only), CD-R (write once) and CD-RW (rewritable). Used for music, software and small data files.
• DVD (Digital Versatile Disc): Capacity 4.7 GB to 17 GB (single to double layer). Used for movies and large software.
• Blu-ray Disc: Uses a blue-violet laser, so it stores much more data — about 25 GB to 50 GB. Used for high-definition video and large backups.

3. Flash / Solid-State Memory

Data is stored in semiconductor chips with no moving parts. It is fast, silent, shock-proof and consumes less power, but is costlier per GB.

• SSD (Solid State Drive): Used in place of HDD in modern laptops. Very fast access speed and no mechanical parts.
• Pen Drive / Flash Drive: A small USB removable device of a few GB to hundreds of GB, used to carry data from one computer to another.
• Memory Card: A small card used in cameras, mobiles and tablets for storing photos, videos and music.

4. Other Types

• Punched cards and paper tape: Very old media that stored data as holes in paper. They are no longer used.

In short: magnetic memory (HDD, tape) gives cheap bulk storage, optical memory (CD, DVD, Blu-ray) gives portable read-mostly storage, and flash memory (SSD, pen drive, memory card) gives fast, portable, non-volatile storage.

Computer System

A computer system is a combination of hardware, software and the user (liveware) that work together to accept data, process it according to instructions, store it and produce information as output.

   +-----------+         +--------------------------------+         +-------------+
   |   INPUT   |  data   |      CENTRAL PROCESSING UNIT   |  result |   OUTPUT    |
   |   UNIT    |-------->|  +---------+     +-----------+ |-------->|    UNIT     |
   | keyboard, |         |  | CONTROL |<--->|   ALU     | |         | monitor,    |
   | mouse ... |         |  |  UNIT   |     | (arith &  | |         | printer ... |
   +-----------+         |  +----+----+     |  logic)   | |         +-------------+
                         |       |          +-----------+ |
                         +-------|------------------------+
                                 v  ^
                         +-----------------+      +-----------------+
                         |  PRIMARY MEMORY |<---->| SECONDARY MEMORY|
                         |  (RAM / ROM)    |      | (HDD, SSD ...)  |
                         +-----------------+      +-----------------+

Input unit. It accepts data and instructions from the user and converts them into a form the computer understands (binary). Examples: keyboard, mouse, scanner, microphone, touch screen, webcam.

Output unit. It receives processed results from the CPU and converts them into a human-readable form. Examples: monitor (soft copy), printer and plotter (hard copy), speaker (audio).

Flow: Input unit -> CPU/memory -> Output unit.

(a) Speaker

A speaker gives sound out of the computer, so it is an output device. Keyboard, mouse and trackball are input devices.

(d) Fourth

In the NEB syllabus, the fourth generation (microprocessor based, from 1971 onwards) is the one in which the Internet, networking and GUI became widespread.

(b) Jon Von Neumann

John von Neumann proposed that both the program and the data be kept in the computer memory, known as the stored program concept.

(a) Peripheral Component Interconnect

PCI is a standard bus/slot on the motherboard used to attach expansion cards such as sound or network cards.

(d) All of above

Keyboard enters typed data, MICR (Magnetic Ink Character Reader) reads magnetic ink characters on cheques, and BCR (Bar Code Reader) reads bar codes. All three are input devices.

(c) Magnetism

A diskette (floppy disk) has a magnetic coating, and data is stored as magnetised spots on its surface.

(a) Operating System

An operating system is software, i.e. a set of programs. CPU, keyboard and hard disk are physical parts and so are hardware.

(c) Plotter

A plotter produces large drawings on paper, so it is an output device. Mouse, MICR and OCR are input devices.

(b) Monitor

Softcopy output is temporary and seen on a screen, as with a monitor. Plotter and printer give hardcopy on paper, and a scanner is an input device.

(c) Plotter

Hardcopy output is a permanent copy on paper, which a plotter produces. Monitor and projector give softcopy output, and a scanner is an input device.

(a) Random Access Memory

RAM is the primary, volatile memory in which any location can be read or written directly in any order.

(d) All of the above

The ALU (Arithmetic and Logic Unit) performs arithmetic operations (addition, subtraction, multiplication, division) and logical operations (comparison such as AND, OR, NOT).

(a) motherboard

The motherboard is the main circuit board of the computer. The microprocessor, memory and other components are mounted on it or connected through it.

(a) Control unit

The control unit directs and coordinates the flow of signals and data between the CPU, memory and input/output devices.

(c) Diligence

Diligence means a computer can work for long hours without getting tired, bored or losing concentration, unlike a human.

(b) Charles Babbage

Charles Babbage designed the Difference Engine and the Analytical Engine, and is called the father of computers.

(d) I.Q.

Versatility, accuracy and diligence are characteristics of a computer. A computer has no intelligence of its own (zero I.Q.) and works only on given instructions.

(c) 8192

1 MB = 1024 KB, and 1 KB = 1024 bytes = $1024 \times 8 = 8192$ bits. So 8192 is the figure that matches the options, which count bits per KB. Counted fully, 1 MB is $1024 \times 1024 \times 8 = 8{,}388{,}608$ bits, but no option gives this.

(d) 1024

In computer storage each unit is $2^{10} = 1024$ times the previous one, so 1 MB = 1024 KB. The other options (8190, 8191, 8192) are not the number of kilobytes in a megabyte.

(b) Four

A word is the number of bits the processor handles in one operation, so a 32-bit processor has a 32-bit word, which is $32 \div 8 = 4$ bytes. Option (b), "four", is the one that matches this (the unit should be bytes, not bits).

(a) Bar Code Reader

A bar code reader uses a light beam to scan the pattern of black and white lines and converts it into digital data. Mouse, touch pad and keyboard cannot read bar patterns.

(d) MICR

MICR (Magnetic Ink Character Recognition) reads the characters printed in magnetic ink on cheques, such as the account and cheque numbers, so banks use it to process cheques quickly. The other devices are not used for this.

(d) all of the above

A system bus consists of three types: the control bus, the data bus and the address bus. Since all three are bus types, "all of the above" is correct.

(b) Multiple bits at once

A parallel port has many data lines, so it sends several bits (commonly 8 bits, one byte) simultaneously. A serial port sends one bit at a time.

(a) Miss

When the CPU looks for data at a memory level (such as the cache) and does not find it, it is a miss, and the data must be fetched from a slower level. If the data is found it is a hit.

(a) RAM

Data entered through an input device is held temporarily in RAM (main memory) so the CPU can process it. RAM is volatile, so the data is lost when power is off unless it is saved to a disk.

(a) bits

A bit (binary digit) is 0 or 1 and is the smallest unit of data in a computer. A nibble is 4 bits and a byte is 8 bits, so they are not the basic unit.

Primary vs Secondary Memory

Memory

Memory is the part of a computer that stores data, instructions and results, either temporarily or permanently, so that the CPU can use them. It is measured in bytes (KB, MB, GB).

RAM (Random Access Memory)

RAM is the main working memory of a computer. The CPU can read from and write to any location directly, and the access time is almost the same for every location, hence "random access".

• It is volatile: its contents are lost when power is switched off.
• It temporarily holds the operating system parts, programs and data currently in use.
• More RAM lets a computer run more programs smoothly.
• It is of two main types: SRAM (static RAM), which is faster and costlier and uses flip-flops, mainly for cache; and DRAM (dynamic RAM), which needs periodic refreshing and is used as main memory.

Computer Memory

Computer memory is the part of a computer that stores data, instructions and results, temporarily or permanently, for use by the CPU. It is measured in bytes (KB, MB, GB, TB). It is classified into primary and secondary memory.

Primary memory

Primary memory is directly accessed by the CPU and is fast.

1. RAM (Random Access Memory): read/write, volatile working memory that holds the programs and data currently being processed. It is of two types, SRAM (faster, used for cache) and DRAM (used as main memory, needs refreshing).

2. ROM (Read Only Memory): non-volatile memory that permanently holds start-up instructions such as the BIOS. Its contents are normally only read. Types include PROM, EPROM and EEPROM.

Secondary memory

Secondary memory is the non-volatile, large-capacity storage used to keep programs and data permanently. It is slower than primary memory and the CPU reaches it through the primary memory.

1. Magnetic disk (hard disk): data is stored as magnetised spots on rotating platters read by heads. It has large capacity (hundreds of GB to TB) and is used for the operating system and files.

2. Optical disk (CD/DVD): data is stored as pits and lands and read by a laser beam. It is portable and cheap, used for software, music and video distribution. A CD holds about 700 MB and a DVD about 4.7 GB.

Primary Memory

Primary memory (main memory) is the memory directly accessed by the CPU. It stores the data and programs currently in use. It is fast but of limited size. RAM, ROM and cache are primary memories.

ROM (Read Only Memory)

ROM is a non-volatile memory whose contents are normally only read, not changed. The data is stored by the manufacturer and is kept even when the power is off.

• It stores permanent instructions, such as the BIOS and bootstrap program, which start the computer and check its hardware.
• It is cheaper to make and more reliable than RAM, since it has no moving parts.

Types of ROM:

• PROM (Programmable ROM): can be programmed once by the user, after which it cannot be changed.
• EPROM (Erasable PROM): can be erased by ultraviolet light and reprogrammed.
• EEPROM (Electrically EPROM): can be erased and rewritten electrically; flash memory is a development of it.

Cache Memory

Cache memory is a very small, very fast memory placed between the CPU and main memory (RAM). It keeps copies of frequently used data and instructions so the CPU can get them quickly without waiting for slower RAM. This reduces the average access time and improves performance. If the required data is in the cache it is a hit; if not, it is a miss.

Types of cache memory

• Level 1 (L1) cache: built inside the CPU chip. It is the fastest and smallest (a few KB up to some tens of KB per core).
• Level 2 (L2) cache: larger and a little slower than L1. It sits inside the CPU or very close to it, and holds data not found in L1.
• Level 3 (L3) cache: larger again, shared by the cores of a multi-core processor, and slower than L2 but still faster than RAM.

CPU

The CPU (Central Processing Unit) is the brain of the computer. It interprets and executes instructions of a program and processes data. It has three main parts: the arithmetic and logic unit (ALU), the control unit (CU) and registers.

Role in the computer system

• Executes instructions: it fetches each instruction from memory, decodes it and executes it, repeating this cycle.
• Performs calculations and decisions: the ALU does arithmetic and logical operations such as addition and comparison.
• Controls the system: the control unit directs and coordinates the working of all hardware, including input, output and memory.
• Manages data flow: it moves data between memory, input devices and output devices.
• Stores data temporarily: the registers hold data, addresses and results during processing.

The speed of the CPU, measured in GHz, largely decides the performance of the computer.

Port

A port is a socket or connecting point on a computer where an external device (such as a keyboard, monitor or printer) is connected through a cable, so data can pass between the device and the computer. Examples are USB, HDMI, VGA, serial and parallel ports.

HDMI Port

HDMI (High-Definition Multimedia Interface) is a port that carries both digital video and audio through a single cable.

• It connects a computer or laptop to a monitor, projector or television.
• It supports high-definition video such as Full HD and 4K, along with multichannel digital sound.
• Because one cable carries both picture and sound, it reduces cable clutter.
• It transfers uncompressed digital signals, so picture quality is good.

1. USB port (Universal Serial Bus)

A USB port is a serial port used to connect many devices such as keyboards, mice, printers, pen drives and cameras. It supports hot swapping (plugging in without restarting), is plug-and-play and can supply power to small devices. Newer versions (USB 3.x) give much higher speeds.

2. Serial port

A serial port sends data one bit at a time over a single line. It is simple, works over long cable distances and was used for modems and older mice. It is slower than a parallel port, and it has mostly been replaced by USB.

Other types include parallel, VGA and HDMI ports.

Application Areas of Computers

1. Education: used for e-learning, online classes, research, digital libraries and result processing.
2. Banking and finance: used for ATMs, online banking, record keeping and mobile payments.
3. Business and offices: used for accounts, billing, inventory, e-commerce and preparing documents.
4. Health and medicine: used in patient records, scanning (CT, MRI), diagnosis and surgery assistance.
5. Communication: used for email, video calls, social media and the Internet.
6. Entertainment: used for games, movies, music, animation and streaming.
7. Government: used for tax collection, e-governance, citizen records, voting and national ID systems.
8. Science and engineering: used for design (CAD), simulation, weather forecasting and space research.
9. Transport: used for reservation of tickets, air traffic control and traffic signals.

An input device is a hardware device used to enter data and instructions into a computer.

1. Keyboard: It is the main input device with keys for letters, numbers and symbols. When a key is pressed it sends the corresponding code to the computer. It is used to type text and commands.

2. Mouse: It is a pointing device that moves a pointer on the screen. It has buttons for clicking and a wheel for scrolling. It is used to select and open items in a graphical interface.

3. Scanner: It reads printed text, pictures or documents and converts them into digital form that can be stored and edited. It is used to digitise documents and photos.

4. Microphone: It converts sound into electrical signals and then into digital data. It is used for voice recording, voice calls and voice commands.

Other examples are webcam, light pen, joystick and touch screen.

Input device: Keyboard

A keyboard is the most common input device. It has alphabet, number, function and special keys. When a key is pressed, the keyboard sends its code to the computer. It is used to enter text, numbers and commands.

Output device: Monitor

A monitor is the main output device that displays the results of processing as text, pictures and video on a screen. It is of types such as LCD and LED. The sharpness of the display depends on its resolution (pixels). It is also called a visual display unit (VDU).

Block Diagram of a Computer System

A computer system has an input unit, a central processing unit (control unit + ALU), memory and an output unit.

   +-----------+         +--------------------------------+         +-------------+
   |   INPUT   |  data   |      CENTRAL PROCESSING UNIT   |  result |   OUTPUT    |
   |   UNIT    |-------->|  +---------+     +-----------+ |-------->|    UNIT     |
   | keyboard, |         |  | CONTROL |<--->|   ALU     | |         | monitor,    |
   | mouse ... |         |  |  UNIT   |     | (arith &  | |         | printer ... |
   +-----------+         |  +----+----+     |  logic)   | |         +-------------+
                         |       |          +-----------+ |
                         +-------|------------------------+
                                 v  ^
                         +-----------------+      +-----------------+
                         |  PRIMARY MEMORY |<---->| SECONDARY MEMORY|
                         |  (RAM / ROM)    |      | (HDD, SSD ...)  |
                         +-----------------+      +-----------------+

Arithmetic and Logic Unit (ALU). The ALU is the part of the CPU where all calculations and decisions are carried out. It has two sections:

• Arithmetic section: performs addition, subtraction, multiplication and division.
• Logic section: performs comparisons (greater than, less than, equal to) and logical operations such as AND, OR and NOT.

The ALU receives data from memory/registers, processes it as directed by the control unit, and returns the result to a register or memory. It uses registers such as the accumulator to hold intermediate results.

(a) pen drive

A pen drive stores data in flash memory, which is non-volatile semiconductor memory with no moving parts. CD and DVD are optical media and the hard disk is magnetic.

Impact Printers

Impact printers produce text and images by physically striking an inked ribbon against the paper with a print head or hammers, like a typewriter. Characters are formed by the pressure of the strike.

Features:

• They are noisy and the print quality is usually low.
• They can print on multipart (carbon) paper, so several copies are made at once.
• They are cheap to run and durable, so they are used for invoices, bills and receipts.

Types:

• Dot-matrix printer: a head with a matrix of pins (9 or 24) strikes the ribbon, forming characters from dots. It can print graphics.
• Daisy-wheel printer: characters are on petals of a wheel that spins and a hammer strikes the chosen character. It gives good text quality but cannot print graphics.
• Line printer: prints a whole line at a time, using a drum or chain; it is very fast and used with large volumes of output.