A function defines a group of sequential statements executed when the function is called. A function returns a single value. Unlike procedures, functions are primarily used in expressions and only have inputs in their argument list. A function call invokes the execution of the function body.

[PURE | IMPURE] function function_name RETURN type_name

…declarations

BEGIN

…sequential statements

END [FUNCTION] [function_name];

…

function_name[actual_parameter] -- funtion call

PURE | IMPURE

Pure functions return the same value each time they are called with the same values as actual parameters (default). Impure functions can return a different value each time they are called, even when multiple calls have the same actual parameter values.

formal_parameters

Specifies a list of parameters (constants, signals, or variables with the mode in, out, or inout).

type_name

Data type or data subtype name, for example BIT, INTEGER, REAL, … of the return value.

declarations

Declarations include data types, constants, signals, files, components, attributes, subprograms, and other information used in the model description.

sequential statements

Statements that are executed in the order in which they appear, that define algorithms for the execution of a subprogram or process.

-- declaration with formal parameters i and length and data type of return value

FUNCTION i2bv (i: INTEGER; length: INTEGER) RETURN BIT_VECTOR IS

VARIABLE bv: BIT_VECTOR(length-1 DOWNTO 0);

VARIABLE temp: INTEGER;

BEGIN -- start of sequential procedure statements

temp := i;

FOR j IN 0 TO (bv'LENGTH-1) LOOP

IF (temp mod 2 = 1) THEN

bv(j) := '1';

ELSE

bv(j) := '0';

END IF;

temp := temp/2;

END LOOP;

return bv;

END i2bv; -- end of sequential procedure statements

…

ARCHITECTURE bench OF entity_name IS

SIGNAL in_var: INTEGER;

SIGNAL out_vec: BIT_VECTOR (1 to 8);

…

BEGIN

in_var <= 10;

-- function call with actual parameters in_var and value 8

out_vec <= i2bv (in_var,8) AFTER 1ms;

END ARCHITECTURE bench;