module BCD_Subtractor_2(input [3:0]X, input [3:0]Y, input SignofX, input SignofY, output reg [3:0]result_S0, output reg [3:0]result_S1, output reg Signbit, output reg overflow);
	reg [3:0]result_NmP_0;
	reg [3:0]result_NmP_1;
	reg [3:0]ResultPXPY;
	reg [3:0]ResultNXNY;
	reg [3:0]S0;
	reg [3:0]S1;
	reg [3:0]S0_plus1;
	reg [3:0]S0_plus_X;
	reg [3:0]carry_to_S1_from_S0_plus1_plusX;
	reg [3:0]S1_plus_carry;
	reg [3:0]useless;
	reg [3:0]NX;
	reg [3:0]NY;
	reg [3:0]NS0;
	reg [3:0]NS1;
	reg [3:0]NS0_plus1;
	reg [3:0]NS0_plus_X;
	reg [3:0]Ncarry_to_S1_from_S0_plus1_plusX;
	reg [3:0]NS1_plus_carry;
	reg [3:0]Nuseless;
	always@(*)
		begin
			overflow = (X > 9 | Y > 9) ? 1 : 0; //X > 9 or Y > 9 overflow
			S0 = 9 - Y; //9 - Y
			NX = Y;
			NY = X;
			NS0 = 9 - NY;
		end

	generate
		begin
			BCD_Adder negminuspositive (X, Y, result_NmP_0, result_NmP_1);

			Comparator comparePXandPY (X, Y, ResultPXPY);
			BCD_Adder PXPY1 (S0, 1'b1, S0_plus1, S1); //+1
			BCD_Adder PXPY2 (X, S0_plus1, S0_plus_X, carry_to_S1_from_S0_plus1_plusX); //+A ripple carry adder
			BCD_Adder PXPY2_2 (S1, carry_to_S1_from_S0_plus1_plusX, S1_plus_carry, useless); //+A ripple carry adder

			Comparator compareNXandNY (NX, NY, ResultNXNY);
			BCD_Adder NXNY1 (NS0, 1'b1, NS0_plus1, NS1); //+1
			BCD_Adder NXNY2 (NX, NS0_plus1, NS0_plus_X, Ncarry_to_S1_from_S0_plus1_plusX); //+A ripple carry adder
			BCD_Adder NXNY2_2 (NS1, Ncarry_to_S1_from_S0_plus1_plusX, NS1_plus_carry, Nuseless); //+A ripple carry adder
		end
	endgenerate

	always@(*)
		begin
			if (SignofX & ~SignofY) // - - +
				begin
					result_S0 = result_NmP_0;
					result_S1 = result_NmP_1;
					Signbit = 1'b1;
				end
			else if (~SignofX & SignofY)//+ - -
				begin
					result_S0 = result_NmP_0;
					result_S1 = result_NmP_1;
					Signbit = 1'b0;
				end
			else if (~SignofX & ~SignofY) //+ - +
				begin
					if (ResultPXPY)
						begin
							result_S0 = S0_plus_X;
							result_S1 = S1_plus_carry;
							Signbit = 1'b1;
						end
					else if (~ResultPXPY)
						begin
							result_S0 = S0_plus_X;
							result_S1 = S1_plus_carry + 4'b1111; //-10
							Signbit = 1'b0;
						end
				end
			else if (~SignofX & ~SignofY) // - - -
				begin
					if (ResultPXPY)
						begin
							result_S0 = NS0_plus_X;
							result_S1 = NS1_plus_carry;
							Signbit = 1'b1;
						end
					else if (~ResultPXPY)
						begin
							result_S0 = NS0_plus_X;
							result_S1 = NS1_plus_carry + 4'b1111; //-10
							Signbit = 1'b0;
						end
				end
		end
endmodule

module BCD_Adder(input [3:0]X, input [3:0]Y, output reg [3:0]S, output reg carry);
	always@(*)
		begin
			assign S = X + Y;
			if (S >= 10) //>10
				begin
					{carry, S} = S + 6;
				end
			else if(S < 10)
				begin
					carry = 1'b0;
				end
		end
endmodule
module Comparator(input [3:0]X, input [3:0]Y, output Result); //X >= Y (0) or X < Y (1)
	reg [4:0]Yt;
	reg [4:0]cX;
	reg [4:0]cY;
	reg [4:0]S;
	reg [5:0]c;
	integer i;
	always@(*)
	begin
		cX = X;
		cY = Y;
		Yt = ~cY;
		c[0] = 1'b1;
		for (i = 0; i < 4; i = i + 1)
		begin
			S[i] = cX[i] + cY[i] + c[i];
			c[i+1] = ((cX[i] & cY[i]) | (cX[i] & c[i]) | (cY[i] & c[i]));
		end
	end
	assign O = c[5] ^ c[4];
	assign Z = !S;
	assign N = S[4];
	assign Result = O ^ N;
endmodule