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I seem to recall a statement to the effect that the number of people that actually understand how a modern Intel cpu works can be counted on one (slightly mutilated) hand.

Don't you want to know how your cpu works? What happens when you're having an issue and wonder why your code doesn't work as expected?

Depth of knowledge is good, but I think the rational choice is to know the most about the domain you're working in -- because the entire domain is already too big for any one person to fully grasp.