Defining an array down to a nonzero constant

Question

I have the following sub-module:

module test
(
input [LENGTH : 1] array;
);

...

endmodule

and I'm calling it from a top module as follows:

...
wire [LENGTH-1 : 0] array_top;
test test_i
(
.array (array_top);
);
...

Assume LENGTH is the same in both modules.

1. How will array_top map to array, given array_top goes down to zero but array goes to down 1?
2. Why would anyone define an array down to 1 and not down to 0?
3. What will happen to array[0]?

Answer 1

Your questions can be answered with a small testbench:

module tb;

reg [3:0] atop;
initial begin
    $monitor("array_top=%b, array=%b", array_top, test_i.array);
    #1 atop = 4'b0000;
    #1 atop = 4'b0001;
    #1 atop = 4'b0010;
    #1 atop = 4'b0100;
end

wire [3:0] array_top = atop;
test test_i (.array (array_top));

endmodule

module test (input [4:1] array);
endmodule

Output:

array_top=xxxx, array=xxxx
array_top=0000, array=0000
array_top=0001, array=0001
array_top=0010, array=0010
array_top=0100, array=0100

1. From your connection: array[1] = array_top[0], etc.
2. Sometimes people want to omit connecting a signal's LSB, like an address for a memory, because the LSB has no effect.
3. There is no array[0] signal.