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Logic gates are made up of transistors. Transistors are used along with other electronic devices and are setup on a silicon wafer. They are called the Integrated Circuits or the ICs. Transistors setup in a specific way, results in what are called logic gates.

The logic gates, as the name tells, performs logical operations like NOT, AND, OR and few others. Logic gates form the basic units that would give chip level units - logic units, arithmetic units, memory units.

The different Logic gates are listed below. There are a few more than the list given.

• NOT
• AND
• OR
• XOR
• NAND
• NOR

The inputs to these gates are electronic signals. They are either high or low. The high state is said to be ‘1′ and the low state is said to be ‘0′. You may notice that we are slipping into the world of binary information as opposed to the analog information present in electronic signal. This is a layer of abstration, that will be repeated many times in the next technology layers.

NOT

The NOT gate is one of the basic gates. It has one input and one output. Its fuction is to invert the input. If it is a ‘1′, its output is ‘0′. If the input is ‘0′, then the ouptut is ‘1′.

The rest of the logical gates take more than one inputs. Let us, for sake of illustration, see the input to output mapping for gates with two inputs.

AND, OR, XOR

NAND, NOR

These logic gates are used to make combinational and sequential (logic) circuits. Combinational circuits are used generally to make the logic units and arithmetic units. Sequential circuits are used to make memory units.

The combinational and sequential circuits in effect form the higher arithmetic and logical units, and also the different types of memory elements. We will look at some of the chip level