For Loop

Used In

  • Process
  • Function
  • Procedure

Reference Manual

VHDL-93:
  • Section 8.8

Syntax 

optional_label: for parameter in range loop

    -- sequential statements

end loop label;

Rules and Examples

The for loop defines a loop parameter which takes on the type of the range specified. For example, the range 0 to 3 implies an integer:

process (A)
begin

    Z <= "0000";

    for I in o to 3 loop
        if (A = I) then
            Z(I) <= '1';
        end if;
    end loop;

end process;

Attributes such as ‘high, ‘low and ‘range may also be used to define the iterations of a for loop:

process (A)#

    variable TMP : std_ulogic;

begin

    TMP := '0';

    for I in A'low to A'high loop
        TMP := TMP xor A(I);
        end loop;
    ODD <= TMP;

end process;

The range may be any discrete range, e.g. an enumerated type:

type PRIMARY is (RED, GREEN, BLUE);
type COLOUR is ARRAY (PRIMARY) of integer range 0 to 255;

-- other statements

MUX: process
begin

    for SEL in PRIMARY loop
        V_BUS <= VIDEO(SEL);
        wait for 10 ns;
    end loop;

end process MUX;

The loop parameter does not need to be declared: it is implicitly declared within the loop. It may not be modified within the loop:

for I in 1 to 10 loop
    if (REPEAT = '1') then
        I := I-1;    -- Illegal
    end if;
end loop;

Synthesis Issues

The for loop is supported for synthesis, providing:

  • the loop range is static (i.e. implies a definite number of iterations), and
  • the loop contains no wait statements.

See Also