Added files into modelsim folder

ModelSim/sign_extender.sv

@@ -0,0 +1,17 @@
+`ifndef SIGN_EXTENDER_H
+`define SIGN_EXTENDER_H
+
+module sign_extender #(in_width=4, out_width=5)(
+    input logic [0:0] op,
+    input logic [2**in_width-1:0] val,
+    output logic [2**out_width-1:0] ext_val
+);
+    always_comb begin
+        if (op) begin
+            ext_val = { {(2**(out_width-in_width)){val[2**in_width-1]}}, val};
+        end else begin
+            ext_val = ({{(2**(out_width-in_width)){1'b0}}, val});
+        end
+    end
+endmodule
+`endif