Featured post

Top 5 books to refer for a VHDL beginner

VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Saturday 29 December 2012

Exit Statement

Formal Definition

The exit statement is used to finish or exit the execution of an enclosing loop statement. If the exit statement includes a condition, then the exit from the loop is conditional.

Simplified Syntax


exit loop_label;

exit loop_label when condition;


The exit statement terminates entirely the execution of the loop in which it is located. The execution of the exit statement depends on a condition placed at the end of the statement, right after the when reserved word. When the condition is TRUE (or if there is no condition at all) the exit statement is executed and the control is passed to the first statement after the end loop (example 1).

The loop label in the exit statement is not obligatory and can be used only in case of labeled loops. If no label is present then it is assumed that the exit statement relates to the innermost loop containing it. If an exit from a loop on a higher level of hierarchy is needed then the loop has to be assigned a label, which will be used explicitly in the exit statement. (Example 2).


Example 1

Loop_1: for count_value in 1 to 10 loop
exit Loop_1 when reset = '1';
A_1: A(count_value) := '0';
end loop Loop_1;
A_2: B <= A after 10 ns;

At the beginning of each iteration of the LOOP_1 loop, the reset ='1' ondition is checked. If the condition is FALSE, then the rest of the loop is executed (in this case it is the assignment labeled A_1). Otherwise, the control is passed to the next statement after the loop, denoted with the A_2 label.

Example 2

Loop_X: loop
           a_v := 0;
Loop_Y:    loop
Exit_1:       exit Loop_X when condition_1;
              Output_1(a_v) := Input_1(a_v);
              a_v := a_v + 1;
Exit_2:       exit when condition_2;
    end loop Loop_Y;
           Assign_Y: B(i) <= Output_1(i) after 10 ns;
Exit_3:   exit Loop_X when condition_3;
end loop Loop_X;
        Assign_X: A <=B after 10 ns;

There are two nested loops in the above example. When the condition_1 is TRUE, the Exit_1 statement will be executed. This will cause termination of the Loop_X loop and moving the execution to Assign_X. Next, the Loop_X will be terminated because its label is explicitly listed in the exit statement.

If condition_1 is not TRUE then the two assignments below it are performed and condition_2 is checked. If it is TRUE, then the Loop_Y is exited. Since there is no loop label within the exit statement, therefore it relates to the innermost loop. As a result, the next statement to be executed will be Assign_Y. Finally, when the condition_3 is TRUE the Loop_X is terminated. The Exit_3 loop label could be skipped because this statement is within the boundaries of the exit Loop_X loop.

Important Notes

· The exit statement is often confused with the next statement. The difference between the two is that the exit statement "exits" the loop entirely, while the next statement skips to the "next" loop iteration (in other words, "exits" only the current iteration of the loop).

No comments:

Post a Comment

Please provide valuable comments and suggestions for our motivation. Feel free to write down any query if you have regarding this post.