CSS Computer Science (Optional) — Paper-I, Section-A, Topic I, p.10. Covers: Computer System Components and Communication System.

The Four Functional Components

A computer system is organized around four functional components that must communicate with one another to process data:

1. Input Unit — accepts raw data and instructions from the outside world (via devices such as keyboard, mouse, or scanner) and converts them into the binary form the CPU can process. It is the sole gateway through which a user's commands and external data enter the system — without it the CPU would have nothing to compute on. (Full device breakdown: see the Input Devices page.)
2. Central Processing Unit (CPU) — often called the "brain" of the computer; it fetches each instruction from memory, decodes it, and executes it using its two sub-units — the Control Unit, which coordinates the whole process, and the Arithmetic Logic Unit, which performs the actual calculation. Its speed and architecture (clock speed, number of cores) largely determine overall system performance. (Full CU/ALU/register breakdown: see Components of a Computer System.)
3. Memory Unit — provides temporary, high-speed storage (RAM) for the data and instructions currently in use, so the CPU is not forced to fetch every item from slow secondary storage. It works alongside ROM (permanent boot instructions) and cache (a small, ultra-fast buffer) to keep the CPU continuously supplied with data. (Detail: see Components of a Computer System.)
4. Output Unit — takes the processed results held in memory and converts them into a form usable by a person or another machine — a visual display, a printed page, sound, or a signal sent onward to another device. Common examples are the monitor, printer, and speaker. (Full device breakdown: see the Output Devices page.)

These four form the basic input → process → store → output cycle that defines how any computer operates, regardless of size or generation.

The Communication System (Buses)

The four components are linked by a communication system made of buses — sets of parallel wires that carry signals between the CPU, memory, and I/O devices:

• Data bus: carries the actual data being transferred between CPU, memory, and I/O devices — bidirectional, since data must move both from memory to the CPU (fetching an instruction or value) and from the CPU back out to memory or an output device (storing a result). Its width (e.g., 32-bit or 64-bit) sets how many bits move in one transfer, so a wider data bus directly improves throughput.
• Address bus: carries only the memory address specifying where data should be read from or written to — unidirectional, because only the CPU generates addresses while memory/I-O devices simply respond to the address they're given. An n-bit address bus can address 2ⁿ distinct memory locations, so its width sets the maximum memory the system can use.
• Control bus: carries control and timing signals — read/write commands, clock pulses, interrupt requests, and bus-request/grant signals — that tell the data and address buses when and how to act (e.g., whether a given cycle is a read or a write, or whether a device is requesting the CPU's attention). Without it the other two buses would have no way of coordinating a transfer.

Why this matters: without a communication system, the CPU, memory, and I/O devices would be isolated components with no way to exchange data — the bus system is what turns separate hardware parts into a functioning, coordinated computer system.

MCQ Traps

• The address bus is unidirectional (CPU specifies location); the data bus is bidirectional (data flows both ways) — a commonly reversed detail.
• The control bus carries signals, not data or addresses — don't conflate its role with the other two buses.

Self-Test Questions

1. Name the four functional components of a computer system in their operational order.