[SOLVED] What is the error in the following Verilog code of an 8bit ALU

timescale 1ns/1ps
module alu(a,b,s,q);
//input [7:0]A,B;
input [7:0]a,b;
input [3:0]s;
output[7:0]q;
reg [15:0]q;
always @(*)
begin
  if(a>b)
  $display("a is greater than b");
  else if(a<b)
  $display("b is greater than a");
  else
  $display("a is equal to b");
case(s)
4'b0000:begin q=a+b; $display("ADDITION performed:");end
4'b0001:begin q=a-b; $display("SUBTRACION performed:");end
4'b0010:begin q=a*b; $display("MULTIPLICATION performed:");end
4'b0011:begin q=a%b; $display("DIVISION performed:");end
4'b0100:begin q=a&b; $display("AND performed:");end
4'b0101:begin q=~(a&b); $display("NAND performed:");end
4'b0110:begin q=(a|b); $display("OR performed:");end
4'b0111:begin q=~(a|b); $display("NOR performed:");end
4'b1000:begin q=(a^b); $display("XOR performed:");end
4'b1001:begin q=~(a^b); $display("XNOR performed:");end
4'b1010:begin q=(a+1);$display("INCREMENT performed:");end
4'b1011:begin q=(a-1);$display("DECREMENT performed:");end
default: q=16'b0;
endcase

end
endmodule

timescale 1ns/1ps
module alu_test();
//inputs, outputs
reg[7:0]a,b;
reg[3:0]s;
wire[15:0]q;
alu a1(.a(a),.b(b),.s(s),.q(q));
initial begin
$display("inputs and outputs are:");
$monitor("time=%t a=%b b=%b s=%b q=%b", $time,a,b,s,q);
end
initial begin
$dumpfile("dumpp.vcd");
$dumpvars(1);
s=4'b0000;
a=9'b10000111;
b=8'b10001110;
#6
s=4'b0001;
a=8'b11111111;
b=8'b11111111;
#6
s=4'b0010;
a=8'b10000101;
b=8'b11111110;
#6
s=4'b0011;
a=8'b10000101;
b=8'b10001110;
#6
s=4'b0100;
a=8'b11001111;
b=8'b11000111;
#6
s=4'b0101;
a=8'b11001101;
b=8'b11000101;
#6
s=4'b0110;
a=8'b01001111;
b=8'b11010110;
#6
s=4'b0111;
a=8'b1110011;
b=8'b10011011;
#6
s=4'b1000;
a=8'b11001011;
b=8'b11111111;
#6
s=4'b1001;
a=8'b01001111;
b=8'b11000011;
#6
s=4'b1010;
a=8'b00001111;
b=8'b1100011111;
#6
s=4'b1011;
a=8'b11001111;
b=8'b11001011;
#6
s=4'b1100;
a=8'b11001111;
b=8'b00001111;
#6
s=4'b1101;
a=8'b11001011;
b=8'b11000111;
#6
s=4'b1110;
a=8'b11011111;
b=8'b11011111;
#6
s=4'b1111;
a=8'b11001111;
b=8'b10001111;
#6 $finish;
end
endmodule