Computer Science Class 10 Chapter 1: Computer Systems

A bit (binary digit) is the smallest unit of data in a computer and is either 0 or 1.

A byte is a group of 8 bits. One byte can make 2⁸ = 256 different patterns.

A number system is a way of writing numbers using a fixed set of digits or symbols. The number of symbols is its base.

Integer part: 1x32 + 0x16 + 0x8 + 1x4 + 0x2 + 1x1 = 37.

Fraction part: 1x1/2 + 0x1/4 + 1x1/8 + 1x1/16 = 0.5 + 0.125 + 0.0625 = 0.6875.

Answer: (37.6875)₁₀

Divide by 2: 179 -> 89 r1, 44 r1, 22 r0, 11 r0, 5 r1, 2 r1, 1 r0, 0 r1. Read upwards: (10110011)₂.

Group the bits in 3s: 010 110 011 = 2 6 3, so (263)₈.

3 = 0011, A = 1010, F = 1111. Join them: 0011 1010 1111. Remove the leading zeros.

(1110101111)₂

Long binary numbers are hard to read and easy to copy wrongly. One octal digit stands for exactly 3 bits and one hexadecimal digit for exactly 4 bits, so a number becomes 3 or 4 times shorter and can be converted by grouping bits. Example: 1111 1111 = FF.

1's complement of 00011101: invert every bit = 11100010.

2's complement of 01010001: invert = 10101110; add 1 = 10101111.

2's complement of 0110: invert = 1001, add 1 = 1010.

1101 + 1010 = 1 0111. A carry came out, so ignore it. Answer = 0111 (7). Check: 13 - 6 = 7.

Overflow happens when the result of a calculation is larger than the maximum value the available bits can hold.

Example: an 8-bit unsigned register holds 11111111 (255). Adding 1 gives 100000000 (256), which needs 9 bits. The 9th bit is lost and the register shows 00000000, a wrong result.

Unsigned numbers have no sign bit; all bits give the value, so they are zero or positive (8 bits: 0 to 255).

Signed numbers use the most significant bit as the sign (0 = positive, 1 = negative) and can store negative numbers (8 bits: -128 to +127).

Unsigned: 0 to 2⁸ - 1 = 0 to 255.

Signed: -2⁷ to 2⁷ - 1 = -128 to +127.

+45 = 00101101. Invert all bits: 11010010. Add 1: 11010011.

Check: 11010011 = 211; 256 - 211 = 45, so the value is -45.

Floating-point numbers are numbers with a fractional part, such as 15.5 or -203.345. In the IEEE 754 standard they are stored as a sign bit, an exponent and a mantissa (32-bit: 1 + 8 + 23 bits).

7-bit ASCII gives 2⁷ = 128 characters. Extended (8-bit) ASCII gives 2⁸ = 256 characters, because n bits give 2ⁿ different codes.

UTF stands for Unicode Transformation Format. Three encodings: UTF-8, UTF-16 and UTF-32. Conversion between them is lossless.

A plain English SMS uses 7-bit codes, so 160 characters fit in one message. Urdu letters are in Unicode and need more bits per character, so the same space holds only about 70 characters.

The binary system is a number system with base 2 that uses only the digits 0 and 1.

Computers use it because their electronic circuits have only two states, ON (1) and OFF (0), which are easy and reliable to build and detect.

An encoding scheme (such as ASCII or Unicode) is a standard that assigns a unique binary code to every character.

Its role is to change text into binary so that it can be stored, processed and exchanged by all computers in the same way.

The binary system has base 2 and uses the digits 0 and 1; computers use it internally. The octal system has base 8 and uses the digits 0 to 7; one octal digit is a short form of 3 binary bits.

• Computers can store only binary (0 and 1), so every letter and symbol must be changed into a binary code.
• A standard code (ASCII, Unicode) makes sure all devices and programs read and show the same text, so files can be shared correctly.

Unicode (or ASCII) is a common encoding scheme. Unicode gives a unique number to every character of every written language, so text in any language can be stored and exchanged.

An operating system is the essential system software that manages memory, processes, hardware and software and lets the user interact with the computer. Examples: Windows, Android (also macOS, Linux).

1. Process management
2. Memory management
3. File management
4. Device management
5. Network management
6. Security management

A program is a passive executable file stored on disk. A process is that program loaded into RAM and running; it is an active unit that uses CPU time, memory and I/O devices.

The five states are New (Start), Ready, Running, Waiting (Blocked) and Terminated.

A deadlock is a situation in which two or more processes wait forever, each for a resource held by another.

Example: P1 holds resource R1 and needs R2, while P2 holds R2 and needs R1. Neither can continue. It is like two cars on a narrow road, each waiting for the other to reverse.

An interrupt is a signal that tells the CPU that an event needs immediate attention (for example a key is pressed).

The CPU pauses its task, saves its state, runs the Interrupt Service Routine (ISR) and then resumes the task.

FCFS (First-Come First-Served), SJN (Shortest Job Next), Priority scheduling and Round Robin.

Round Robin: every process gets the CPU for a fixed time slice in a cycle; unfinished processes go back to the queue.

The file system is the part of the OS that organises, stores, retrieves and manages data on storage devices. Its components are files, directories (folders) and metadata (name, size, type, permissions, dates).

• Contiguous: file in consecutive blocks; fast but causes fragmentation.
• Linked: blocks joined by pointers like a linked list; flexible but slower.
• Indexed: an index block points to all data blocks; fast and balanced.

An embedded OS is built into the hardware of a device and does a specialised task with minimum resources. Examples: microwave oven, washing machine, ATM, TV, camera.

A distributed OS manages a network of computers as a single system and spreads the tasks over many machines (load balancing). The user does not know which machine is doing the work. Example: cloud computing, Google search.

Order 6, 2, 1: finish times 6, 8, 9; average = 23/3 = 7.67 ms.

Order 1, 2, 6: finish times 1, 3, 9; average = 13/3 = 4.33 ms.

Running the shortest jobs first is better.

System software is software that manages the computer hardware and system resources and provides a platform on which application software runs. Examples: Windows, Linux, macOS, Android, device drivers.

• It uses device drivers so the OS can send commands to and receive data from devices such as the keyboard and printer.
• The operating system controls the flow of data between input, output, storage and processing units, for example moving data between RAM and the hard disk.

When a key is pressed or the mouse moved, the device driver receives the signal from the device and gives it to the operating system. The OS then passes the input to the correct program and to the CPU for processing.

The OS uses the file system to organise files and folders on the storage device. It also controls reading from and writing to the disk and the movement of data between RAM and storage, and keeps track of free space.

For output (for example printing or display) the OS sends the processed data to the device driver, which converts it into commands the output device understands. The OS also queues the jobs when many programs use the same device.

Operating systems such as Windows and Linux (also macOS, Android and device drivers).

• The OS organises data on the storage device using the file system (files, folders, metadata).
• It controls the read and write operations, keeps track of free blocks and protects the data integrity.

The OS schedules the processes so that each gets CPU time, allocates memory to them and handles interrupts and deadlocks, so that the CPU is used efficiently and no process interferes with another.

The operating system controls the overall working of a computer. Examples: Windows, Linux and macOS (Android on mobiles).

An application patch is a small piece of software that updates, fixes or improves an existing program. Functions: fix bugs, improve security, enhance performance and add features.

Software hosting means deploying and providing access to software on servers so that users can reach it over a network, usually the internet.

Types: on-premises, shared, dedicated and cloud hosting.

A code repository (for example GitHub) is a place where the source code of a project is stored. It keeps all the versions of the code and lets a team share and work on it together.

A translator is a program that converts source code written in a programming language into machine code. Example: a compiler such as GCC (or the Python interpreter).

An IDE is a software application that combines the tools needed to develop programs in one interface. Two modules: code editor and debugger (also compiler).

• Syntax highlighting makes code readable.
• Code completion suggests the next part of the code.
• Debugging runs the code step by step.
• Error checking underlines mistakes in real time.
• Compiling and project management in one place.

• Word processor (MS Word): essays, reports, letters.
• Spreadsheet (MS Excel): budgets, calculations, charts.
• Presentation software (PowerPoint): lectures and business presentations.

It should use an offline application.

• It works without an internet connection, which the computers do not have.
• It uses the local resources and stores data on the device, so no remote server is needed.

(a) 365 = 256 + 64 + 32 + 8 + 4 + 1 = (101101101)₂.

(b) 10110011: groups of 3: 010 110 011 = 2 6 3 so (263)₈; groups of 4: 1011 0011 = B 3 so (B3)₁₆.

(c) 1BE8 = 1x4096 + 11x256 + 14x16 + 8 = 4096 + 2816 + 224 + 8 = (7144)₁₀.

• CPU: schedules processes (FCFS, SJN, Round Robin), keeps the CPU busy, sets priorities and handles interrupts and deadlocks.
• Memory (RAM): allocates memory to programs, frees it when they close, keeps a record of free memory and swaps processes.
• Storage: manages hard disks and SSDs through the file system: read/write operations, directories, data integrity.
• I/O devices: manages keyboard, mouse, printer through device drivers and buffering.