Subtractor

Half Subtractor: 

module hs(d,bout,a,b);
    output d,bout;
    input a,b;
//assign {bout,d} = a-b;
assign d =  a ^ b;
assign bout = ~a & b;

endmodule

 Full Subtractor:

module fs(d,bout,a,b,c);

    output d,bout;

    input a,b,c;

//assign {bout,d} = a - b - c;

 assign d = a ^ b ^ c;

 assign bout = (~a & b) | (~(a^b)&c);

endmodule