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`timescale 1ns / 1ps
// ============================================================================
//        __
//   \\__/ o\    (C) 2022  Robert Finch, Waterloo
//    \  __ /    All rights reserved.
//     \/_//     robfinch<remove>@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)*4-1:0] bc;
wire [(N)*4-1:0] o1, o2, o3;
wire c;

BCDNinesComplementN #(N) u1 (.i({4'h0,b}), .o(bc));
BCDAddNClk #(.N(N)) 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)) 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 <= |o ? ~c : 1'b0;

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