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[ The PC Guide | Systems and Components Reference Guide | The Processor | Roots of the Processor: Digital Logic and the Semiconductor ]

Implementing Digital Logic: The Digital Switch

Digital logic requires hardware that can take ones and zeros as input, and produce from them ones and zeros as output according to the formula required for the function. In digital computers the one and zero values are determined based on the voltage on the line at a given time; normally, a one is positive voltage and a zero is ground. For a +5V system then, a one is +5V and a zero is 0V.

The hardware used to produce the output level from the input level according to a formula is a digital switch. Most functions are made up of many of these switches; the more complex the formula, the more switches required to implement it. A computer system is really a large group of interconnected sets of digital switches, all reacting to inputs of ones and zeros and producing outputs of ones and zeros in response.

The very first computing devices were electromechanical; they used physical switches and relays. They were very slow, unreliable and noisy, because they had a mechanical component, and this meant something would move when a bit changed from a one to a zero. The next generation of devices used vacuum tubes. These were much better than the mechanical switches because they do not have moving parts. Vacuum tubes enabled the creation of the first electronic computers, but they had several major problems associated with them:

  • They were large; you have no doubt heard the old stories about computers filling whole rooms despite providing very limited functionality.
  • They were slow; a computer's performance is directly related to how fast its switches can change state (one to zero or vice-versa).
  • They were expensive, much of this being because of their size and poor reliability.
  • They consumed enormous amounts of power; a single machine could consume tens of thousands of watts.
  • Finally, and perhaps most importantly, they were notoriously unreliable. A large computer using thousands of these switches would often require replacements on a daily basis--or worse.

Next: The Transistor: A Solid State Digital Switch

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