The Four Generation of Computers. First Generation 1951 1958 First generation computers, often referred to as mainframes, were large, costly to buy, expensive to power, and often unreliable because of their delicacy and complexity. They were slow compared with today's computers, and their internal storage capacity was limited. Vacuum tubes controlled the internal operations. The ENIAC contained more than 18,000 of these tubes and is rumored to have dimmed the lights of Philadelphia when it was turned on. Vacuumtubes took up a lot of space and generated considerable heat. Special air conditioning had to be installed to cool the surroundings. Vacuum tubes could switch on and off thousands of times per second, but one tube would fail about every fifteen minutes. Too much time was wasted looking for the burned out tubes.