Subtractors

Half Subtractor and Full Subtractor : 

(a) Dataflow modelling

`timescale 1ns / 1ps
module half_subtractor_dataflow(a,b,d,bo);
input a,b;

output d,bo;
assign d=a^(~b);
assign bo=(a&(~b));

endmodule

(b) Behavioral Modeling

`timescale 1ns / 1ps
module half_subtractor_behav(a,b,d,bo);
input a,b;

output reg d,bo;

reg [1:0]d1;
reg [1:0]s1;

always@*
begin
d1[0]<=b;
d1[1]<=a;

case(d1)
2'b00: s1<=2'b00;
2'b01: s1<=2'b11;
2'b10: s1<=2'b10;

2'b11: s1<=2'b00;
default: s1<=2'b00;
endcase

d<=s1[1];
bo<=s1[0];
end
endmodule

(c) Structural Modeling

`timescale 1ns / 1ps
module half_subtractor_struct(a,b,d,bo);
input a,b;
output d,bo;

xor21 u1(a,b,d);
not1 u2(b,b1);
and21 u3(a,b1,bo);
endmodule

module not1(a,y);
input a;

output y;
assign y=~a;
endmodule

module and21(a,b,y);
input a,b;
output y;

assign y=a&b;
endmodule

module xor21(a,b,y);

input a,b;
output y;
assign y=a^b;
endmodule

(ii) Full Subtractor (Structural Modeling)

`timescale 1ns / 1ps
module full_subtractor_struct(a,b,bin,d,bo);
input a,b,bin;
output d,bo;

wire c1,c2,c3;
half_subtractor_behav u1(a,b,c1,c2);
half_subtractor_behav u2(c1,bin,d,c3);

or21 u3(c2,c3,bo);

endmodule

module or21(a,b,y);
input a,b;
output y;
assign y=a|b;

endmodule