Question

A modport expression allows elements of arrays and structures, concatenations of elements, aggregate expressions of elements declared in an interface to be included in a modport list. This modport expression is explicitly named with a port identifier, visible only through the modport connection.

Like explicitly named ports in a module port declaration, port identifiers exist in their own namespace for each modport list. When modport item is just a simple port identifier, that identifier is used as both a reference to an interface item and a port identifier. Once a port identifier has been defined, there shall not be another port definition with this same name.

For example:

interface I;
    logic [7:0] r;
    const int x=1;
    bit R;
    modport A (output .P(r[3:0]), input .Q(x), R);
    modport B (output .P(r[7:4]), input .Q(2), R);
endinterface

module M ( interface i);,
    initial i.P = i.Q;
endmodule

module top;
    I i1;
    M u1 (i1.A);
    M u2 (i1.B);
    initial #1 $display("%b", i1.r); // displays 00010010
endmodule

The self-determined type of the port expression becomes the type for the port. If the port expression is to be an aggregate expression, then a cast must be used since self-determined aggregate expressions are not allowed. The port expression must resolve to a legal expression for type of module port (See section 18.12—Port connection rules). In the example above, the Q port could not be an output or inout because the port expression is a constant. The port expression is optional because ports can be defined that do not connect to anything internal to the port.