What are the main functions of a general computer?  

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Most modern computers are based on what is called the “von Neumann architecture,” which consists of an input device, a central processing unit (CPU), random access memory (RAM), and an output device.

This layout, however, is not at all fundamental to computation. Other computation devices were used before the invention on the von Neumann architecture, and many other possibilities for computing platforms have been proposed, ranging from arrays of subatomic particles to the structure of DNA. In spite of this seeming complexity, there are only three operations necessary to compute anything that is computable. To understand this, we can look at the “register machine”.

The concept of the register machine was introduced by Hao Wang in 1957 and has helped form the foundation of modern computing. The register machine is an idealized, imaginary (but completely possible) machine that consists of only a finite number of registers (modern computers contain millions or billions) and a single processing unit.

A register is a discrete memory location that contains a single integer (0, 1, 2, 3, to infinity). The processing unit carries out just three possible instructions, one at a time:

  • Increment register ( add 1 to the contents of the current register and go to the next step)

  • Decrement register (subtract 1 from current register and move to next step), or if the register contains 0, branch to next step

  • End (stop the program)

A program written for a register machine, using just these three instructions, can compute anything any computer can compute. Register machines are considered “unlimited” or “universal” machines because they can simulate any other computing machine. They can run any program any computer is capable of running.

There are no limitations...

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