What a computer is
A computer is an electronic device that accepts data, processes it according to a stored set of instructions, produces output, and can store both data and instructions for later use.
Its defining characteristics are speed (operations measured in millions or billions per second), accuracy (errors come from wrong data or wrong instructions, not from the machine - the idea of "garbage in, garbage out"), diligence (no fatigue or loss of concentration over long runs), versatility (the same hardware runs many different tasks), and storage (large volumes of data kept and retrieved reliably). A computer has no intelligence of its own; it does exactly what its instructions say.
The five functional units
A computer is organised into five units:
- Input unit: converts data and instructions from a human-usable form into the machine's internal form and feeds them in (keyboard, mouse, scanner).
- Memory unit: holds data and instructions currently in use, and intermediate and final results.
- Arithmetic and Logic Unit (ALU): performs calculations and comparisons.
- Control Unit (CU): fetches instructions, decodes them, and directs the other units - it does not process data itself, it coordinates.
- Output unit: converts results from the internal form back to a human-usable form (monitor, printer).
The ALU and CU together, with a set of fast internal registers, form the Central Processing Unit (CPU).
How the units work together
Processing follows an input - process - output cycle, with storage available at every stage.
- The input unit places data and the program into memory.
- The control unit fetches the next instruction from memory, decodes it, and issues signals.
- The ALU carries out the arithmetic or logic the instruction requires, using data from memory or registers, and writes the result back.
- Steps 2 and 3 repeat, instruction by instruction - the fetch-decode-execute cycle.
- The output unit presents the final results; anything to keep goes to secondary storage.
Because both instructions and data live in the same memory, a program can be loaded, replaced or modified without rewiring the machine - the stored-program concept.
Generations of computers - a summary view
Generations are usually described by the switching technology of the era. This is exam shorthand, not a precise timeline:
- First generation: vacuum tubes. Bulky, hot, high power, machine-language programming.
- Second generation: transistors. Smaller, more reliable, assembly and early high-level languages.
- Third generation: integrated circuits (many transistors on one chip). Time-sharing operating systems, keyboards and monitors.
- Fourth generation: the microprocessor (a CPU on a single chip). Personal computers, graphical interfaces, networking.
- Fifth generation: described in terms of massive parallelism and artificial-intelligence techniques; the boundary here is loose.
Treat generation dates and "example machine" names as approximate.
Hardware and software
- Hardware is the physical part you can touch: the processor, memory chips, drives, board, and peripherals.
- Software is the set of instructions that tells the hardware what to do. It has no physical form of its own.
Software divides into system software (the operating system, device drivers, utilities - it runs and manages the machine) and application software (programs a user runs to get work done - a word processor, a spreadsheet, a browser). Firmware is software stored on a chip inside a device, part-way between the two.
Common confusions
- The control unit does not do calculations. The ALU does; the CU coordinates.
- CPU is not the whole box. The CPU is the processor chip (ALU + CU + registers); the "CPU cabinet" in common speech is the system unit.
- Memory is not the same as storage. "Memory" usually means the main memory the CPU works from directly; "storage" usually means secondary storage like a disk.
- A computer is accurate, not intelligent. Wrong output comes from wrong input or wrong instructions.
- Generation boundaries are approximate. Do not treat a single date or machine name as definitive.
- Firmware is software, even though it lives on a chip.
Quick revision
- Characteristics: speed, accuracy, diligence, versatility, storage; no intrinsic intelligence.
- Five units: input, memory, ALU, control, output. ALU + CU + registers = CPU.
- Cycle: input - process - output, driven by fetch-decode-execute; storage available throughout.
- Stored-program concept: instructions and data share memory, so programs can be changed without rewiring.
- Generations by technology: vacuum tube, transistor, integrated circuit, microprocessor, then AI/parallel - dates approximate.
- Hardware is physical; software is instructions (system vs application); firmware is software on a chip.
Practice
Answer in one line each.
- Which part of the CPU does arithmetic and comparisons?
- Which part of the CPU directs the sequence of operations?
- Name the four basic functions of a computer.
- In what unit is CPU clock speed usually measured?
- Which component carries out program instructions and is called the brain of the computer?
- Where are the data and instructions currently being processed held?
- Turning a user's key-presses into a form the CPU can use is which function?
- A set of instructions that tells the computer what to do is called what?