Vacuum Tube Computer Part 20: Conditional Execution Logic

Much of the power of digital computers stems from their ability to make decisions and alter processing based upon the data they are manipulating. In high level languages, this is often expressed as if—then—else statements, and in machine language by conditional jump instructions such as “jump if less than zero”.

The Motorola MC14500 microcontroller upon which this computer is modeled has a very unconventional way of handling conditional execution, in which store or output instructions can either execute or perform no operation depending on the setting of a condition code bit. This allows separating the conditional code from instruction fetch, which simplifies integrating the processor into lightweight industrial control applications.

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