Statement that includes a sequence
of statements that is to be executed repeatedly, zero or more times.
loop_label: while condition loop
end loop loop_label;
loop_label: for loop_parameter
in range loop
end loop loop_label;
The loop statement contains
a sequence of statements, which are supposed to be repeated many
times. The statement also lists the conditions for repeating the
sequence or specifies the number of iterations.
A loop statement can have
several different forms depending on the iteration scheme preceding
the reserved word loop. In
its simplest form, no iteration scheme is specified and the loop is
repeated indefinitely (Example 1).
In order to exit from an infinite loop, an exit statement has to be
used. See exit statement
for details. An exit statement can be specified with a condition that
must be met to exit the loop (Example 2).
Instead of specifying an infinite loop with a conditional exit
statement, a while loop can be used. In such a case the reserved word while
with a condition precede the keyword loop.
The sequence of statements inside the loop will be executed if the
condition of the iteration scheme is true. The condition is evaluated
before each execution of the sequence of statements. When the
condition is false, the loop is not entered and the control is passed
to the next statement after the end loop clause (Example 3).
Another iteration scheme is useful when a discrete range can define
the number of iterations. In this case the keyword for
with a loop parameter precede the keyword loop.
The header of the loop also specifies the discrete range for the
loop parameter. In each iteration the parameter takes one value from
the specified range, starting from the leftmost value within the
range (Example 4).
The discrete range can also be declared in the form of a discrete
type (example 5), including an enumerated type.
signal Clock : BIT := '0';
Clk_1: process (Clock)
Clock <= not
Clock after 5 ns;
end process Clk_1;
The process (which is a clocking signal generator) contains a loop
without an iteration scheme, which will iterate indefinitely.
L2 when A > 10;
end loop L2;
The infinite loop becomes in practice a finite, as the iterations
will terminate as soon as the variable A becomes greater than 10.
Shift_3: process (Input_X)
variable i : POSITIVE := 1;
L3: while i <=
Input_X(i+8) after 5 ns;
i := i + 1;
end loop L3;
end process Shift_3;
The loop L3 will be repeated as long as the value of the variable i
is not greater than 8. When i reaches the value of 9 the loop is no
Shift_4: process (Input_X)
count_value in 1 to
Input_X(count_value + 8) after
end loop L4;
end process Shift_4;
In the above example the loop statement parameter count_value will
cause the loop to execute 8 times, with the value of count_value
changing from 1 to 8.
Shift_5: process (Input_X)
subtype Range_Type is
POSITIVE range 1 to 8;
count_value in Range_Type loop
Input_X(count_value +8) after
end loop L5;
end process Shift_5;
The subtype Range_Type defines the range of integer values from 1 to
8. The parameter count_value changes according to this specification,
i.e. from 1 to 8.
The parameter for a 'for' loop does not need to be specified - the
loop declaration implicitly declares it.
The loop parameter is a constant within a loop, which means that it
may not be assigned any values inside the loop.