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What are the basic building blocks of combinational logic circuits? In this lecture, 
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I am going to introduce you to logic gates that can be used to build any combinational circuits.
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Although designing with these gates is not the goal of HLS, understanding their behaviour and functionality
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helps us to develop an optimum design in HLS.
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Logic gates are fundamental building blocks of combinational circuits. There are three basic logic
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gates: AND, OR and NOT. The output of an AND gate is one if and only if both inputs are 1. The output 
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of an OR gate is one 
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if at least one of the inputs is 1. The NOT gate reverses the state of its input. 
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The combination of an AND and NOT gates is called NAND gate. Also, the combination of an OR and 
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NOT gates is called NOR gate. 
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These gates are important because any complex combinational circuits can be built by using only one
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of them.
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Therefore, they are called functionally complete gates. 
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Also, in some semiconductor technologies manufacturing these gates are easier than basic logic gates. 
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Exclusive-OR, or XOR for short, is another gate that can be built by using basic logic gates or by using 
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functionally complete gates. The output of an exclusive-OR gate is one if and only if exactly one of
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its inputs is one. 
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In other words, if the inputs are not equal. The last special gate is called XNOR,
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that is the negative of XOR. The output of this gate is one
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if and only if the inputs have the same value. 
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In other words, the inputs are equal.
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Let’s see an example designed by these gates. Single bit binary adder is one of the simplest combinational 
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circuits.
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This adder receives two binary inputs and generates two binary outputs, representing sum and carry bits. 
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This circuit can be built using XOR and AND gates. 
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This circuit is also called half-adder. 
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The truth table represents the functionality of this design.
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However, to extend this addition to more than one bit, we should involve the carry bit, as an input. 
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The resulted adder is called the full adder, which is a single-bit binary adder that receives a third input 
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as the carry generated by the predecessor bit. 
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This figure shows the full adder digital circuit and its corresponding truth table.
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Real logic gates cannot react immediately to the input changes. 
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So, what is a delay of a gate or a logic circuit to possible changes on the input values?
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The next lecture covers the concept of delay in logic circuits. 
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These are the takeaway messages. All combinational logic functions can be implemented by three basic 
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gates AND,  OR, NOT, NAND or NOR gates are enough to implement any combinational logic circuit.
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Now the quiz for this lecture. Show these equivalencies also known as De Morgan’s laws.