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Section 3.4 Bits

A bit (short for “BInary digiT”) is a single on/off value. Only these two values are possible, though the two values may go by different names, such as true/false, or 1/0.
0 and 1 are the logical values of a bit. Physically, those values can be represented in many ways. Some examples of ways that bits are implemented in computation include:
  • Voltage on a wire being above or below a threshold.
  • A light bulb being on or off.
  • The state of a single transistor being open or closed.
  • Whether the magnetic charge on a tiny part of the surface of a disk is oriented up or down.
  • The presence or absence of a hole punched in a card.
  • Whether a tiny area on the light-reflecting surface of a CD is reflective or not.
Each hole (or spot where there is no hole punched) in this card represents a bit of information.
Computers do all of their processing in terms of bits - any piece of information a computer stores or manipulates is stored as a sequence of these on/off values. For different purposes, a computer may use any of the mechanisms listed above (or many others) to represent those bits. It may read a bit off a CD, represent that bit as a voltage on a wire, and then store it as a magnetic field on a disk.

Checkpoint 3.4.1.

What is a bit?
  • A single on/off value.
  • A value of "one."
  • A large section of computer memory.
  • Enough memory to store a character.

Checkpoint 3.4.2.

Which one of the following statements is FALSE?
  • The method used to implement a bit does not affect the information it represents
  • A light bulb that is "Off" represents the same thing as an "Off" represented by a hole that is not punched
  • A bit may be implemented as a small part of a magnetic disk.
  • This is how hard drives store information
  • Copying a bit from one medium to another does not affect the information it represents.
  • Computers do this all the time - turn an electrical signal in a circuit into a magnetic field on a disk. Or turn a hole into a punch card into an electrical signal
  • A bit is always implemented as something electrical.
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