fix: initialize the last 4 bytes of mem to zero

These last 4 bytes are, in fact, used for register `x0`, and must be
zero. In multi-valued simulators, however, without an explicit init,
these bytes start as undefined. Moving `x0` to anywhere results in
those undefined bytes "infecting" other registers.

Issue: #148

Author: filmil

serving/serving_ram.v

@@ -42,4 +42,11 @@ module serving_ram
 	$display("Preloading %m from %s", memfile);
 	$readmemh(memfile, mem);
      end
+
+     // The last 4 bytes are the `x0` (zero) register. Zero them out
+     // to avoid starting the simulation with `x0` undefined.
+     mem[depth-4] = 8'h0;
+     mem[depth-3] = 8'h0;
+     mem[depth-2] = 8'h0;
+     mem[depth-1] = 8'h0;
 endmodule