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When Aiken devised the Mark I, why did he decided to separate data and instructions? It was not mentioned in Wikipedia (or in any other searches I've looked) on how or why Aiken separated data and instructions.

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According to the Wikipedia article on the Harvard Mark I, the data memory consisted of storage for 72 23-digit numbers, and instructions were read from a paper tape loop.

That's the how.

The why is most likely that he hadn't seen the advantages of putting instructions into read/write memory. Also random access read/write memory was extremely expensive to implement ... compared with sequential and / or readonly memory.

Remember we are talking about a machine that was build from electro-magnetic switches and relays, and could do 3 additions per second.

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I doubt my answer is correct too. I'm actually reading Mark I's manual right now to get a better idea. –  Yannis Rizos Apr 7 '13 at 16:05
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You have it exactly backward. The question is not "why was the Mark I like this?". Computing machines were supposed to be like this. As pointed out above, the "von Neumann architecture" wasn't a thing back then, and all predecessors - the Jaquard loom, the Analytical engine etc. - had separate program and data stores, or no program store at all (hard coded programs). The notion that programs can be viewed as data had yet to be conceived, because it isn't obvious at all - even it seems so to us today! –  Kilian Foth Apr 7 '13 at 16:05
    
@KilianFoth - I mentioned that, though not in detail. See first sentence of 3rd paragraph. But it is also doubtful that he could have put the program into memory ... assuming that he had thought of the idea. The Wikipedia article gives us some idea how much effort was involved in building the Mark I. –  Stephen C Apr 7 '13 at 16:07
    
@KilianFoth If I'm not horribly mistaken the notion that instructions could occupy the same storage as data had been published by Konrad Zuse before WW2. How much of his work was known to Aiken (or von Neumann) I wouldn't know. –  Yannis Rizos Apr 7 '13 at 16:11
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Zuse was virtually unknown outside Germany until 1948 when his 'Plankalkül' was published, and he lost out on a great deal of credit because of that. But that is not to take anything away from John von Neumann - he was hands down one of the greatest minds of the century, the idea of the storable-program computer is just one of his many, many brilliant ideas. –  Kilian Foth Apr 7 '13 at 17:03
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