VHDL Reference Guide

VHDL Reference Guide - Aggregates

**Aggregates**

Expression	---- used in ---->	Entity Architecture Package Package Body

Syntax

(value_1, value_2, ...)

(element_1 => value_1, element_2 => value_2, ...)

See LRM sections 7.3.2

Rules and Examples

Aggregates are a grouping of values to form an array or record expression. The first form is called positional association, where the values are associated with elements from left to right:

signal Z_BUS : bit_vector (3 downto 0);
signal A_BIT, B_BIT, C_BIT, D_BIT : bit;
...
Z_BUS <= (A_BIT, B_BIT, C_BIT, D_BIT);

Equivalent Assignments

Z_BUS(3) <= A_BIT;
Z_BUS(2) <= B_BIT;
Z_BUS(1) <= C_BIT;
Z_BUS(0) <= D_BIT;

Aggregates may be used as the targets of assignments

The second form is named association, where elements are explicitly referenced and order is not important:

signal Z_BUS : bit_vector (3 downto 0);
signal A_BIT, B_BIT, C_BIT, D_BIT : bit;
...
Z_BUS <= ( 2=> B_BIT, 1 => C_BIT, 0 => D_BIT, 3 => A_BIT);

With positional association, elements may be grouped together using the | symbol or a range. The keyword others may be used to refer to all elements not already mentioned:
signal B_BIT : bit; signal BYTE : bit_vector (7 downto 0); ... BYTE<= (7 => '1', 5 downto 1 => '1', 6 => B_BIT, others => '0');

Assignment to a whole record must be done using an aggregate. Positional or named association may be used

type T_PACKET is record
	BYTE_ID : std_ulogic;
	PARITY  : std_ulogic;
	ADDRESS : integer range 0 to 3;
	DATA    : std_ulogic_vector (3 downto 0);
end record
signal TX_DATA : T_PACKET;
...
TX_DATA <= ('1', '0', 2, "0101");

An aggregate containing just others can assign a value to all elements of an array, regardless of size:

type NIBBLE is array (3 downto 0) of std_ulogic;
type MEM is array (0 to 7) of NIBBLE;
variable MEM8X4: MEM := (others => "0000");
variable D_BUS : std_ulogic_vector(63 downto 0) := (others => 'Z');

Synthesis Issues

Logic synthesis tools may not support named association fully. Also, record assignments using aggregates may not be supported.

Whats New in '93

Aggregates have not changed in VHDL-93.