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Chained constructor loops

It is commonly necessary to select the first element of an array satisfying a certain condition, or to perform some other computation involving prioritizing an array. A looping construct called ``chain'' is provided for this purpose. It acts exactly like a ``for'' loop, except that the assignments in one iteration of the loop act as defaults for the assignments in subsequent iterations. This makes it possible to assign a signal in more than one iteration of the loop, with the later assignment taking priority over the earlier assignment.

For example, suppose we want to specify a ``priority encoder'', that inputs an array of signals, and outputs the index of the highest numbered signal that is true. Here is a priority encoder that inputs an array of boolean signals of size ``n'':

  MODULE priority_n(n,inp,out)
  {
    INPUT inp : array 0..(n - 1) of boolean;
    OUTPUT out : 0..(n-1);

    chain(i = 0; i < n; i = i + 1)
      if (inp[i]) out := i;
  }

Depending on the contents of the array ``inp'', the signal ``out'' might be assigned many times in different iterations of the loop. In this case, the last assignment is given precedence.

The construct:

        chain(i = 0; i < 4; i = i + 1)
          <stmt>
is in every way equivalent to:

        default 
          <stmt with i = 0>
        in default
          <stmt with i = 1>
        in default
          <stmt with i = 2>
        in
          <stmt with i = 3>


next up previous contents
Next: Expressions Up: Constructor loops Previous: Creating parameterized modules   Contents
2002-10-28