Computer System 11 Computer Science Chapter 1 CBSE Notes 2025-26

A computer system is more than just a machine that processes data; it consists of the central processing unit (CPU), memory, input and output devices, and storage devices. All these components work together to accept input, process it as per instructions, and deliver meaningful output. Modern computer systems range in size and form, from large servers and desktop computers to compact laptops, tablets, and smartphones.

Central Processing Unit (CPU) The CPU, often called the brain of the computer, carries out all processing tasks. It is placed on integrated circuits inside the computer and typically consists of the Arithmetic Logic Unit (ALU), which performs all calculations and logical operations, and the Control Unit (CU), which directs the operations of the processor. During its work, the CPU temporarily stores instructions and data in tiny storage spaces called registers.

Input and Output Devices Input devices like keyboard, mouse, scanner, and touch screen allow users to enter data into the computer. Special devices such as braille keyboards assist visually impaired users. With advances, data can even be entered via voice commands. Output devices, including monitors, printers, speakers, and 3D printers, display or present processed data in forms humans can understand. For example, a braille display monitor helps visually challenged users read content generated by computers, and 3D printers now create prototypes in different industries.

Evolution of Computers From the ancient abacus, through mechanical calculators by Pascal and Babbage’s analytical engine, computers have evolved quickly. Key developments such as punched cards, the Turing machine concept, von Neumann’s architecture (where both program and data are stored in memory), and the progress from vacuum tubes to integrated circuits have led to exponential increases in computing power. Modern personal computers, laptops, smartphones, wearable devices, and smart home appliances all stem from this evolution.

Computer Memory Memory allows storage of both data and instructions. It is categorized as primary (main) memory such as RAM and ROM, and secondary (storage) memory like hard drives, SSDs, CDs, and pen drives. Information in RAM is temporary and lost when power goes off, whereas ROM retains data even when the power is gone. Cache memory, located between RAM and CPU, speeds up data access for frequently used information. Secondary memory is generally larger and more permanent than primary memory.

• 1 Kilobyte (KB) = 1024 Bytes
• 1 Megabyte (MB) = 1024 KB
• 1 Gigabyte (GB) = 1024 MB
• 1 Terabyte (TB) = 1024 GB
• Further units include Petabyte (PB), Exabyte (EB), Zettabyte (ZB), and Yottabyte (YB)

Data Transfer and the System Bus Data moves between the CPU, memory, and storage through “buses”— dedicated electrical paths called the data bus (for actual data), address bus (for addresses), and control bus (for control signals). All together, these form the system bus. While the data bus can move information both ways, the address and control buses are typically one-way.

Microprocessors and Microcontrollers A microprocessor is a CPU on a single chip. Over the decades, microprocessors have rapidly advanced, with increases in word size, clock speed, and number of cores. For example, early processors were 4 or 8-bit, but modern ones are typically 64-bit, with multicore technology for multitasking. Microcontrollers are different—they package the CPU, memory, and peripherals on one chip, ideal for embedded systems like washing machines, remote controls, and digital cameras.

Data, Information, and Their Types Data are unorganized raw facts, which computers process to produce information—organized and meaningful output. Data can be:

• Structured: Well-organized, in tables or rows/columns (like attendance records).
• Unstructured: Lacks predefined format (like videos, images, social posts).
• Semi-structured: Inconsistent format but uses markers or tags (like emails, HTML).

Capturing, Storing, and Retrieving Data Data can be captured using keyboards, barcode readers, or sensors. Storing involves saving it in storage devices like hard drives, servers, or memory cards. Retrieval means accessing required data from storage, a more critical task as database sizes grow. Deleting data usually marks it as free space, but does not erase it right away; recovery is possible unless new data overwrites it.

Software: System, Application, and Programming Tools Computer hardware requires software instructions to function. There are different types of software:

• System Software: Includes the operating system (Windows, Linux), utilities, and device drivers, which make the hardware operable and manage system tasks.
• Programming Tools: Provide a way for users to instruct the computer. Programming languages may be low-level (machine or assembly) or high-level (C++, Java, Python). Language translators like compilers, interpreters, and assemblers convert code into machine language. IDEs (like Visual Studio) offer text editing, building, and debugging support.
• Application Software: Meets user-specific needs and can be general-purpose (word processors, browsers) or customized (like school management tools). Applications can be proprietary (Windows) or open-source (Python, Ubuntu). Some are "freeware"—free to use but without available source code.

Operating System and Its Functions The operating system (OS) manages all system resources and provides the bridge between user and hardware. On startup, the OS loads into memory and starts managing application execution, user interface, and security. It offers various types of interfaces:

• Command-line (e.g. MS-DOS)
• Graphical User Interface (GUI) (e.g. Windows, macOS)
• Touch (Android, iOS), Voice (Siri, Google Assistant), Gesture (motion/eye tracking)

Major roles include process management (allocating CPU time to programs), memory management (handling RAM usage), file management (organizing data on storage), and device management (controlling I/O devices with device drivers).

In summary, computers are complex systems consisting of interacting hardware and software. Primary and secondary memory, microprocessors, input/output devices, and the operating system all have significant roles in processing, storing, and presenting data for various applications and user needs.

Class 11 Computer Science Chapter 1 Notes – Computer System: Key Points for Quick Revision

Our Class 11 Computer Science Chapter 1 notes cover all major concepts on computer systems, including CPU, memory, input/output devices, and types of software. Easily understand the evolution of computers, data handling, and the basics of operating systems with structured explanations.

These revision notes help you recall important terms, diagrams, and tables for CBSE exams. Revise major differences between RAM and ROM, microprocessors and microcontrollers, and types of software easily. Improve understanding of the computer system chapter with clear examples and simple definitions.