Modern Computers Are Electronic (continued)

Electronic Switches: Transistors
Transistors replaced vacuum tubes as the preferred electronic switch mechanism. But we can consider transistors the same way we have considered vacuum tubes.

• A transistor is just a microscopic on/off switch
  
• A single transistor switch represents one binary digit ("bit"):
  ON=1 and OFF=0 .
  
• A microprocessor chip is a machine that is built to obey the rules of arithmetic.

Remember how the gears in our example were built to mimic the operation of carrying a one to the next column by having one gear turn another gear? In the same way, when one of the millions of switches (transistors) of a microprocessor is flipped to start an arithmetic operation, this sets off a chain reaction of switches flipping other switches, sort of like falling dominos. When the switches (transistors) are done "flipping," the binary numbers represented by the final state of the switches is the correct result. Why? The machine is built to "flip" that predetermined way, every time. We call this deterministic behavior: the same input always produces the same output.

And this is all a microprocessor is: a collection of flipping switches that mimic the rules of binary arithmetic.