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Computers

A computer is a device that takes in digital data, processes it by following a program of instructions, stores it and produces results. Inside, everything is represented as bits, each either 0 or 1. Most computers still follow the organisation John von Neumann described in 1945: a processing unit, a control unit, a memory that holds both instructions and data, and input and output.

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Architecture: the five organs

Von Neumann's 1945 report decomposes a computing device into CA (central arithmetic), CC (central control), M (memory), I (input) and O (output). It distinguishes the problem-specific instructions, which must be stored, from the general control organs that execute any instruction presented to them, and considers a single memory organ serving every storage function, instructions included.

  • Program and data share an address space, so a program can be loaded, replaced or even generated by another program.
  • The control unit repeatedly fetches the instruction at the current address, decodes it and directs execution.
  • Throughput is limited by how fast instructions and data can move between memory and processor, because both travel the same route.

Information content

n=⌈log⁡2N⌉n = \lceil \log_{2} N \rceil

Distinguishing N different values needs at least n bits (the base-2 logarithm, rounded up).

Example: distinguishing 1000 values needs ⌈log₂ 1000⌉ = 10 bits, since 2⁹ = 512 is too few and 2¹⁰ = 1024 is enough.

Common misconception: The 'von Neumann architecture' is a design organisation, not a particular technology. It has been built from vacuum tubes, discrete transistors and integrated circuits alike.
Full explanation — the complete reference version every reading depth is based on

What a computer is

A computer is any device that accepts digital data and manipulates it according to a program — a list of instructions saying how the data should be processed. Phones, laptops, games consoles, the microprocessors in cars and room-sized supercomputers all fit this description; they differ in speed and size, not in the basic idea.

Everything is bits

  • A bit (binary digit) has one of two values: 0 or 1.
  • A byte is eight bits, enough to represent any whole number from 0 to 255.
  • Text, images, sound and programs are all stored as patterns of bits. The letter 'A' is typically stored as 01000001.
  • Bits must be held in a physical object with two distinct states, such as a tiny electric switch or a tiny magnet.
N=2nN = 2^{n}

A group of n bits can take N different patterns: 8 bits give 2⁸ = 256 patterns, which is why a byte holds 0 to 255.

The stored-program design

In his 'First Draft of a Report on the EDVAC' (dated 30 June 1945), John von Neumann described a computing device in five parts: a central arithmetic part that does calculations, a central control part that sequences the operations, a memory, and input and output organs that connect it to the outside world. He counted the instructions for a problem among the things the machine must remember, and found it 'tempting' to treat the whole memory as one organ that could hold instructions and numbers alike.

  1. Input brings data and instructions in (keyboard, touchscreen, sensor, network).
  2. Memory stores the instructions and the data they work on.
  3. The processor's control part fetches each instruction in turn and directs what happens.
  4. The processor's arithmetic part carries out calculations and comparisons.
  5. Output delivers results (screen, speaker, motor, network).

Worked example: how many patterns?

How many different values can 3 bits hold? Each bit doubles the possibilities: 2 × 2 × 2 = 2³ = 8 (000, 001, 010, 011, 100, 101, 110, 111). With 8 bits the count is 2⁸ = 256, so a byte can store the whole numbers 0 to 255. The letter 'A' as 01000001 is the number 64 + 1 = 65.

How we know

The definitions of computer, bit and byte used here come from the US National Institute of Standards and Technology's glossary. The five-part design is taken from von Neumann's own 1945 report, which survives and was reprinted in the IEEE Annals of the History of Computing in 1993, so its wording can be checked directly.

Assumptions and limits

The input–process–store–output picture describes classical digital computers. It says nothing about how fast a machine is, and it does not describe quantum computers, which use qubits instead of bits. Real processors also overlap and reorder work internally while keeping the step-by-step result the program expects.

Common misconception: Misconception: 'a computer understands words and pictures directly'. A computer only ever stores and transforms patterns of bits; whether a pattern means a letter, a colour or an instruction depends on the program interpreting it.
Info: Connections: the processing part is the CPU, the storage is Memory, and computers talk to one another through Networks and the Internet. AI systems are programs that run on computers, and Quantum computing replaces bits with qubits.

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Sources and methodology

  • A computer is a device that accepts digital data and manipulates it according to a program: a sequence of instructions for how the data is to be processed. (awaiting scientific review)
  • A bit is a binary digit, having a value of either 0 or 1. (awaiting scientific review)
  • A byte is a sequence of eight bits, so a single byte can represent any whole number from 0 to 255. (awaiting scientific review)
  • John von Neumann's 'First Draft of a Report on the EDVAC', dated 30 June 1945, described a computing device made of a central arithmetic part, a central control part, a memory, and input and output organs. (awaiting scientific review)
    • First Draft of a Report on the EDVAC (J. von Neumann, dated 30 June 1945; reprinted in IEEE Annals of the History of Computing, 1993) — Other (unclassified)
  • The EDVAC report counted the instructions governing a problem among the material the device must remember, and called it 'tempting' to treat the entire memory as one organ whose parts are interchangeable between those functions. (awaiting scientific review)
    • First Draft of a Report on the EDVAC (J. von Neumann, dated 30 June 1945; reprinted in IEEE Annals of the History of Computing, 1993) — Other (unclassified)
  • Digital computers encode text and other information as strings of bits; for example, computers typically represent the letter 'A' with the bit string 01000001. (awaiting scientific review)
  • Bits must be realised in physical objects that can be placed into one of two distinct states, such as tiny bar magnets or electric switches. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.