CSS Computer Science (Optional) — Paper-I, Section-A, Topic I, p.11. Covers: Components of A Computer System.

CPU (Central Processing Unit) — the "Brain"

• Control Unit (CU): directs and coordinates all activities of the computer by generating the timing/control signals that drive the fetch-decode-execute cycle — it fetches the next instruction from memory, decodes what operation it specifies, and directs the relevant data to the ALU or memory to carry it out. It does not perform calculations itself; think of it as a traffic controller for data, not a calculator, and without it the ALU and memory would have no coordinated sequence in which to operate.
• Arithmetic Logic Unit (ALU): performs all arithmetic (addition, subtraction, multiplication, division) and logical (AND, OR, NOT, and comparisons such as greater-than/equal-to) operations — it is the only component that actually "computes" a result; the CU merely tells it what operation to perform and on what data. Every spreadsheet formula and every "if" condition in a program is ultimately evaluated here. Modern multi-core CPUs contain one ALU per core so several calculations can run in parallel.
• Registers: small, extremely fast storage locations built directly into the CPU chip (not in RAM) that hold the data/instructions currently being processed — key examples include the Accumulator (holds intermediate arithmetic results), the Program Counter (holds the address of the next instruction to fetch), and the Instruction Register (holds the instruction currently being decoded). Because they sit inside the CPU with no bus delay, registers are the fastest memory in the entire system, faster even than cache — but there are only a handful of them, each holding just one word of data.

Memory (Primary Storage)

• RAM (Random Access Memory): volatile, read-write memory that holds the currently running programs, the active parts of the operating system, and the data those programs are working on; any location can be accessed directly in roughly equal time (hence "random access"), unlike sequential media such as tape. Because it is volatile, all contents vanish the instant power is cut — which is why unsaved work is lost after a crash or power failure — and more RAM lets a computer keep more programs running smoothly at once.
• ROM (Read-Only Memory): non-volatile memory that permanently or semi-permanently holds firmware/boot instructions — e.g., the BIOS/UEFI that starts up a computer before the operating system loads — which don't change during normal operation. Variants include PROM (programmable once by the manufacturer), EPROM (erasable with ultraviolet light), and EEPROM/flash-based ROM (electrically erasable and rewritable, used in most modern firmware). Because it survives power loss, ROM guarantees a computer can always find its startup instructions.
• Cache Memory: a very fast, small memory located between the CPU and RAM, storing frequently and recently accessed data/instructions so the CPU is not kept waiting on comparatively slow RAM — exploiting "locality of reference," the tendency of programs to reuse the same data repeatedly. Modern CPUs use tiered caches (L1: smallest and fastest, built into each core; L2: larger and slightly slower; L3: shared across all cores, largest and slowest of the three) to balance speed against cost.

Secondary Storage

Non-volatile, larger-capacity, but slower than RAM — used for permanent storage of data and programs even when the computer is powered off. Unlike primary memory, secondary storage is not directly accessed by the CPU during processing; data must first be loaded into RAM before the CPU can work on it. Common types include:

• Hard Disk Drive (HDD): stores data magnetically on spinning platters read by a moving head; large capacity at low cost per GB, but mechanically slower and more fragile than solid-state media (e.g., the main storage drive in a budget desktop).
• Solid State Drive (SSD): stores data electronically in flash memory cells with no moving parts, giving much faster access and greater durability than an HDD, historically at a higher cost per GB (e.g., the boot drive in most modern laptops, chosen for fast startup).
• Optical Disks (CD/DVD/Blu-ray): store data as microscopic pits read by a laser; portable and resistant to magnetic damage, but limited in capacity and slower than magnetic or solid-state media (e.g., software/movie distribution).
• USB Flash Drives / Memory Cards: small, portable flash-memory storage connected via a USB port or card slot; convenient for moving files between machines but generally lower capacity than an internal HDD/SSD (e.g., carrying a presentation between office and home).

(See the dedicated Storage Media page for a full comparison of speed, capacity, durability, and cost.)

Input/Output Devices

The physical means by which data enters (input) and results leave (output) the system — covered in full detail on the dedicated Input Devices and Output Devices pages.

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