Both while and do while are looping constructs that execute the given set of statements as long as the given condition is true.
A while loop first checks if the condition is true and then executes the statements if it is true. If the condition turns out to be false, the loop ends right there.
A do while loop first executes the statements once, and then checks for the condition to be true. If the condition is true, the set of statements are executed until the condition turns out to be false. If the condition is false, the loop ends right there.
So the difference between the two is that a do while loop executes the set of statements atleast once.
Syntax
while (<condition>) begin
// Multiple statements
end
do begin
// Multiple statements
end while (<condition>);
Example #1 - while loop
module tb;
initial begin
int cnt = 0;
while (cnt < 5) begin
$display("cnt = %0d", cnt);
cnt++;
end
end
endmodule
Simulation Log ncsim> run cnt = 0 cnt = 1 cnt = 2 cnt = 3 cnt = 4 ncsim: *W,RNQUIE: Simulation is complete.
Example #2
module tb;
initial begin
int cnt;
while (cnt != 0) begin
$display ("cnt = %0d", cnt);
cnt++;
end
end
endmodule
Simulation Log ncsim> run ncsim: *W,RNQUIE: Simulation is complete.
Example #3 - do while loop
module tb;
initial begin
int cnt = 0;
do begin
$display("cnt = %0d", cnt);
cnt++;
end while (cnt < 5);
end
endmodule
Simulation Log ncsim> run cnt = 0 cnt = 1 cnt = 2 cnt = 3 cnt = 4 ncsim: *W,RNQUIE: Simulation is complete.
Example #3 - do while loop
module tb;
initial begin
int cnt = 0;
do begin
$display("cnt = %0d", cnt);
cnt++;
end while (cnt == 0);
end
endmodule
Simulation Log ncsim> run cnt = 0 ncsim: *W,RNQUIE: Simulation is complete.