Magnetic cores were the basic technology for computer main memory from the 1950s into the early 1970s. The tiny iron oxide cores could be magnetized clockwise or counterclockwise to represent bits of information. Data could be retrieved in millionths of a second. This array is from an IBM-built Semi-Automatic Ground Environment (SAGE) computer from the 1950s. SAGE, an early U.S. air defense system, was the first large computer network to provide man-machine interaction in "real time" - as events were occurring.