How can I simulate pull-up and pull-down resistors in the ModelSim® software?

Description

You can use two variable types, tri0 and tri1, to simulate pull-up and pull-down resistors when connecting the module's output/bidirectional net signals. These two variable types simulate the net signals being pulled-down or pulled-up when this signal is not driven. An example declaration is as follows:

module pullup_tb;
    reg a, b, d, clk;              //these are inputs
    tri1 c,q;                         //these are outputs; instead of:    wire c, q;

   weak_pullupmodelsim my_wkpu( .a(a),
.b(b),
.c(c),
.d(d),
.clk(clk),
.q(q)
  );

Simulating the above example will pull-up net signals c and q to simulate the way these signals would act on the board.