Generate Statement

Used In

• Architecture

Reference Manual

VHDL-93:

• Section 9.7

Syntax

label: for parameter in range generate

    -- concurrent statements

end generate label;

Rules and Examples

The for generate statement is usually used to instantiate “arrays” of components. The generate parameter may be used to index array-type signals associated with component ports:

architecture GEN of REG_BANK is

    component REG
        port(
            D, CLK, RESET : in std_ulogic;
            Q : out std_ulogic
        );
    end component;

begin

    GEN_REG:
    for I in 0 to 3 generate
        REGX : REG port map ( DIN(I), CLK, RESET, DOUT(I));
    end generate GEN_REG;

end GEN;

A label is compulsory with a generate statement.

The for generate statement is particularly powerful when used with integer generics.

Instance labels inside a generate statement do not need to have an index:

REGX(I): -- Illegal

For generate statements may be nested to create two-dimensional instance “arrays”.

Another form of generate is the if generate statement. This is usually used within a for generate statement, to account for irregularity. For instance, a ripple-carry adder with no carry-in:

architecture GEN of RIPPLE is

    component FULLADD
        port (
            A, B, CIN : in bit;
            SUM, CARRY : out bit
        );
    end component;

    component HALFADD
        port(
            A, B : in bit;
            SUM, CARRY : out bit
        );
    end component;

    signal C : bit_vector(0 to 7);

begin

    GEN_ADD: for I in 0 to 7 generate

        LOWER_BIT: if I=0 generate
            U0: HALFADD port map ( A(I), B(I), S(I), C(I));
        end generate LOWER_BIT;

        UPPER_BITS: if I>0 generate
            UX: FULLADD port map ( A(I), B(I), C(I-1), S(I), C(I));
        end generate UPPER_BITS;

    end generate GEN_ADD;

    COUT <= C(7);

end GEN;

A generate statement may contain local declarations, followed by the keyword begin.

Synthesis Issues

Generate statements are usually supported for synthesis.

See Also

• Generic