Instance hierarchies

Modules may, of course, contain instances of other modules, and so forth, provided the module references are not circular. So we can, for example, create three-bit counter module, as follows:

MODULE counter3(carry_in, clear, count, carry_out)
{
  INPUT carry_in, clear : boolean;
  OUTPUT count : array 2..0 of boolean;
  OUTPUT carry_out : boolean;

  carry : array 3..1 of boolean;

  bit0 : counter_bit(carry_in, clear, carry[0], carry[1]);
  bit1 : counter_bit(carry[1], clear, carry[1], carry[2]);
  bit2 : counter_bit(carry[2], clear, carry[2], carry[3]);

  carry_out := carry[3];
}

If we then instantiate this module with

        foo : counter(cin,clr,cnt,cout);

we will have, for example, an instance of counter_bit called foo.bit0, which defines signals

        foo.bit0.carry_in
        foo.bit0.clear
        foo.bit0.bit_out
        foo.bit0.carry_out

MODULE declarations may not appear inside other MODULE declarations, however. That is, all MODULE declarations must be in the outermost scope.