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Overview of a Computer System

CSS Optional — Paper-II, Computer Organization & Architecture

1. Definition

A computer system is defined functionally as an interconnected set of hardware units that together perform four operations on data: processing, storing, moving, and controlling it.

2. Classification — The Four Basic Functions (Stallings)

  • Data Processing — transforming data via arithmetic and logic operations (the ALU's job).
  • Data Storage — holding data at rest, whether temporarily (registers, cache, RAM) or permanently (disk).
  • Data Movement — transferring data between components without changing it: I/O operations, inter-component transfer over the bus.
  • Data Control — coordinating the other three functions so they happen in the correct order (the Control Unit's job).

3. Classification — The Four Structural Components

  • Central Processing Unit (CPU) — executes instructions; contains the ALU, Control Unit, and registers.
  • Main Memory — stores the currently running program and its data.
  • Input/Output (I/O) — moves data between the computer and the external world.
  • System Interconnection (Bus) — the pathway connecting the above three.

4. The Stored-Program Concept

  • Instructions and data reside together in the same memory, in the same format, and are fetched and executed sequentially by the CPU.
  • This is the foundational idea that separates a modern, general-purpose computer from a fixed-wiring calculator: reprogramming means loading new data into memory, not rewiring circuits.

🗺️ Architecture Diagram — Structural View

graph LR
    CPU["CPU
ALU + CU + Registers"] <--> BUS["System Bus
Address / Data / Control"]
    MEM["Main Memory
Instructions + Data"] <--> BUS
    IO["I/O Devices"] <--> BUS

🎯 CSS Exam Angle

Frequently opens Section-A as a short, definitional question. Examiners test whether you can distinguish the four functions with an example device for each, and whether you can draw the four-block structural diagram from memory.

🔁 Repeated CSS Questions

  1. Describe the four basic functions of a computer system with a block diagram.
  2. What is the stored-program concept, and why was it revolutionary?
  3. Differentiate between the structural components of a computer system.

⚠️ MCQ Trap

"Data movement" covers I/O and inter-component transfer — do not confuse it with "data storage," which refers to holding data at rest in memory or disk.