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`timescale 1us/1ns

/* Flull blown BCD adder -- takes number + sign bit and returns result in same format

Note the number is always a positive representation

so that 1001 is -1

Also note that BCD and hex are exactly the same except for legal digits

do 13'h1001 is actually BCD -1 */

module bcdadd(input [16:0] a, input [12:0] b, output [16:0] z);

  wire [16:0] ain;

  wire [12:0] bin;

  wire [16:0] zout;

  wire [16:0] ztemp;

  // The bcdneg modules return either a positive # or a 9's compliment #

  bcdneg17 aneg(a,ain);

  bcdneg13 bneg(b,bin);

  // Add the converted numbers

  usum adder(ain,bin,zout);

  // the bcdneg module is reversible so if we feed it 9's compliment we get

  // our format back out

  bcdneg17 zneg(zout,ztemp);

  // sometimes you get -0 and we don't want that so filter it here

  assign z=(ztemp==17'h10000)?(17'h00000):ztemp;

endmodule