double_addsub_tb.v

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module double_addsub_tb ();

parameter WIDTH = 10;

reg [WIDTH-1:0] a,b;
wire [WIDTH+1:0] sum_x,sum_y,sum_z;
reg negate_a,negate_b,fail;

double_addsub x (.a(a),.b(b),.negate_a(negate_a),.negate_b(negate_b),.sum(sum_x));
defparam x .WIDTH = WIDTH;
defparam x .HW_CELLS = 1'b0;

double_addsub y (.a(a),.b(b),.negate_a(negate_a),.negate_b(negate_b),.sum(sum_y));
defparam y .WIDTH = WIDTH;
defparam y .HW_CELLS = 1'b1;

assign sum_z = (negate_a ? -a : a) + (negate_b ? -b : b);

initial begin
	a = ~0;
	b = ~0;
	negate_a = 0;
	negate_b = 0;
	fail = 0;
	$random(123);
end

always begin 
	#1000
	a = $random;
	b = $random;
	negate_a = $random;
	negate_b = $random;

	#1000
	if (sum_x != sum_z || sum_y != sum_z) begin
		$display ("Mismatch at time %d",$time);
		fail = 1'b1;
	end
end

initial begin
	#1000000
	if (!fail) $display ("PASS");
	$stop();
end

endmodule