Titanic enterprise that Petzold book: describe how a computer from electric current, does all he can do. So we start with a switch that turns on a light bulb to gradually move up the complexity: telegraph, logic gates, making circuits more complicated logical operations (the "machine choose his cat" is a great time) circuits performing an addition, data storing and operation of the RAM ... and counting. It takes a little snag, but withdraws an intellectual satisfaction and admiration for the gigantic human company that developed all this, Babbage up to the iPad