Description
The IAS machine was a binary computer with a 40 bit word, storing two 20 bit instructions in each word. The memory was 1024 words (5.1 kilobytes). Negative numbers were represented in "two's complement" format. It had two general-purpose registers available: the Accumulator (AC) and Multiplier/Quotient (MQ).
Although some claim the IAS machine was the first design to mix programs and data in a single memory, that had been implemented four years earlier by the 1948 Manchester Small Scale Experimental Machine.
Von Neumann showed how the combination of instructions and data in one memory could be used to implement loops, by modifying branch instructions when a loop was completed, for example. The resultant demand that instructions and data be placed on the memory later came to be known as the Von Neumann Bottleneck.
While the original design called for using a type of vacuum tubes called RCA Selectron tubes for the memory, problems with the development of these complex tubes forced the switch to Williams tubes. Nevertheless, it used about 2300 tubes in its circuitry. The addition time was 62 microseconds and the multiplication time was 713 microseconds. It was an asynchronous machine, meaning that there was no central clock regulating the timing of the instructions. One instruction started executing when the previous one finished.
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