Digital Logic Circuits

• The fundamental building blocks of computer circuits are switches called logic gates

• Logic gates are made out of transistors

• Three types of logic gates are sufficient to build any computer circuit

• AND and OR gates each take two signals as input and produce an output signal

• NOT gates take a single input signal and produce the opposite signal

• A signal is a voltage level on a wire (ON/OFF, HIGH/LOW, 1/0, TRUE/FALSE, etc.)
  
  
• Behavior of AND, OR, and NOT gates can be summarized using truth tables

• Three other gates are fairly common:  NAND ("not AND"), NOR ("not OR"), and XOR ("exclusive OR")

• XOR can be defined in terms of AND, OR, and NOT

• Logic Simulator from The Analytical Engine

• Circuits for binary arithmetic
  
       

• 4-bit Arithmetic-Logic Unit (ALU)

• 4-bit Memory Register