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`timescale 1ns / 1ps

// ============================================================================

//        __

//   \\__/ o\    (C) 2022  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

//     \/_//     robfinch@finitron.ca

//       ||

//

//      BCDSubtract.sv

//

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// ============================================================================

module BCDSubtract(clk, a, b, o, sgn);

parameter N=25;

input clk;

input [N*4-1:0] a;

input [N*4-1:0] b;

output reg [N*4-1:0] o;

output reg sgn;

wire [(N+1)*4-1:0] bc;

wire [(N+1)*4-1:0] o1, o2, o3;

wire c;

BCDNinesComplementN #(N+1) u1 (.i({4'h0,b}), .o(bc));

BCDAddNClk #(.N(N+1)) u2 (.clk(clk), .a({8'h00,a}), .b(bc), .o(o1), .ci(1'b0), .co(c));

BCDNinesComplementN #(N) u3 (.i(o1), .o(o2));

BCDAddNClk #(.N(N+1)) u4 (.clk(clk), .a(o1), .b('d0), .o(o3), .ci(c), .co());

always_ff @(posedge clk)

        if (c)

                o <= o3;

        else

                o <= o2;

always_ff @(posedge clk)

        sgn <= ~c;

endmodule

module BCDNinesComplement(i, o);

input [3:0] i;

output reg [3:0] o;

always_comb

        case(i)

        4'd0:   o = 4'd9;

        4'd1:   o = 4'd8;

        4'd2:   o = 4'd7;

        4'd3:   o = 4'd6;

        4'd4: o = 4'd5;

        4'd5:   o = 4'd4;

        4'd6:   o = 4'd3;

        4'd7:   o = 4'd2;

        4'd8:   o = 4'd1;

        4'd9:   o = 4'd0;

        4'd10:  o = 4'd9;

        4'd11:  o = 4'd8;

        4'd12:  o = 4'd7;

        4'd13:  o = 4'd6;

        4'd14:  o = 4'd5;

        4'd15:  o = 4'd4;

        endcase

endmodule

module BCDNinesComplementN(i, o);

parameter N=25;

input [N*4-1:0] i;

output [N*4-1:0] o;

genvar g;

generate begin : gNC

        for (g = 0; g < N; g = g + 1)

                BCDNinesComplement utc1 (i[g*4+3:g*4],o[g*4+3:g*4]);

end

endgenerate

endmodule

module BCDTensComplement(i, o);

input [3:0] i;

output reg [3:0] o;

always_comb

        case(i)

        4'd0:   o = 4'd0;

        4'd1:   o = 4'd9;

        4'd2:   o = 4'd8;

        4'd3:   o = 4'd7;

        4'd4: o = 4'd6;

        4'd5:   o = 4'd5;

        4'd6:   o = 4'd4;

        4'd7:   o = 4'd3;

        4'd8:   o = 4'd2;

        4'd9:   o = 4'd1;

        4'd10:  o = 4'd0;

        4'd11:  o = 4'd9;

        4'd12:  o = 4'd8;

        4'd13:  o = 4'd7;

        4'd14:  o = 4'd6;

        4'd15:  o = 4'd5;

        endcase

endmodule

module BCDTensComplementN(i, o);

parameter N=25;

input [N*4-1:0] i;

output [N*4-1:0] o;

genvar g;

generate begin : gTC

        for (g = 0; g < N; g = g + 1)

                BCDTensComplement utc1 (i[g*4+3:g*4],o[g*4+3:g*4]);

end

endgenerate

endmodule