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In this lecture, I am going to explain how to use a for-loop statement to describe a repetitive combinational 
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pattern in HLS.
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For this purpose, let’s consider an N-bit combinational binary adder.  It consists of N full-adders connected 
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together, sequentially, as each of which requires the carry bit generated by its predecessor. A set 
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of N separate function 
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calls can implement this design. However, without knowing the value of N, writing this code is not 
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possible.
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Also, this code would not be efficient for a large value of N. 
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This code has two issues. Firstly, It is not parametric, which means it cannot accept N as an input 
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parameter.
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Secondly, it is not easily generalisable to the large size problems.
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Using the software loop statement is a solution for these two issues. 
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This code shows the software implementation that uses a for-loop statement. As the first function call is 
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a little bit different than others, 
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it is outside the loop and the for-loop index iterates from 1 to N.
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The HLS process of this code generates only one instance of the full-adder that will be called several times,
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sequentially to perform the loop task. 
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Therefore the synthesised circuit requires a few internal temporary memory to keep track of intermediate
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carry-bits between two adjacent iterations. 
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In other words, the circuit is not combinational. 
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In order to get the same code similar to our first implementation, we should guide the HLS tool 
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to unroll the loop during the synthesis process. 
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A compiler directive can help us to do that. By adding the 
#pragma HLS UNROLL after the first line of 
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the for-loop, the HLS unrolls the loop and creates a separate logic circuit instance for each loop iteration. 
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In addition to the UNROLL pragma, How can we generally describe a loop statement in an HLS to guarantee
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the combinational circuit implementation?
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There are a few constraints and coding styles that should be followed in using loop statements in HLS 
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to describe a combination of a circuit.
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The following lecture covers these constraints and coding styles.
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The takeaway message from this lecture is: A software loop statement associated with an UNROLL pragma can 
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implement a repetitive combinational logic 
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circuit structure.
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Now the quiz for this lecture. Write a for-loop statement to represent the following combinational 
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circuit.