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Types of General Purpose Computers
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Mainframe
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 - 1950's
 - 1947 IBM 700 series (vacuum tubes), later 7000 series (transistors)
 - still in use today, mostly legacy applications
 Attributes
  * large transactional power
  * large input/output (I/O) capability
  * virtualisation (can run multiple OS's)
  * high stability and reliability (restarted once a decade)
  
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Supercomputer
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 - 1960's
 Attributes
  * massive processing power --> arithmetic calculations
  * parallelism

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Mini computer  
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 - Mid 1960's to mid 1980's
 - Took advantage of the cost and size savings of transistors (later models used microprocessors).
 - 10 to 20 thousand dollars US.
 Attributes
  * Native interpreters --> allowed for high level languages --> easier to program
  * Significantly cheaper and smaller.
  * Lower computational and transactional capability, but the cost allowed for wider application.
  
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Microcomputer
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 - Introduced in the 1970's.
 e.g. Altair 8800 that used Intel 8080 (clock 2MHz, ~4500 transistors)
 - Became popular in the 1980's.
 e.g. IBM XT, Apple II, Commodore 64.
 Attributes
  * Microprocessor (much cheaper to produce, much higher transistor density)
  * Today have billions of transistors