DESCRIPTION

This document describes the ALLIANCE VHDL subset for structural descriptions.

The declaration part of a structural description includes signal decalarations and component declarations.

An internal signal can be declared of any type supported by the present VHDL subset except reg_bit and reg_vector.

A component must be declared with exactly the same port description as in its entity specification. This means that local ports are to be declared with the same name, type and kind and in the same order.

A structural description is a set of component instanciation statements. Instances' ports are connected to each other trough signals in a port map specification. Both explicit and implicit port map specifications are supported by the ALLIANCE VHDL subset.

The present version of the VHDL compiler does not allow unconnected ports (the open mode is not supported).

Only the concatenation operator (&) can be used in the actual part (effective signal conntected to a formal port) of a port map specification.

EXAMPLES

Here is the description of an adder with an accumulator register.

entity add_accu is
port (
  clk      : in  bit;
  command  : in  bit;
  data_in  : in  bit_vector (31 downto 0);
  data_out : out bit_vector (31 downto 0);
  cry_out  : out bit;
  vdd      : in  bit;
  vss      : in  bit
  );
end add_accu;
architecture structural of add_accu is
signal eff_data  : bit_vector (31 downto 0);            -- effective operande
signal adder_out : bit_vector (31 downto 0);            -- adder's result
signal accu_out  : bit_vector (31 downto 0);            -- accumulator
component adder 
port (a   : in  bit_vector (31 downto 0);
      b   : in  bit_vector (31 downto 0);
      res : out bit_vector (31 downto 0));
end component;
component and_32
port (a   : in  bit_vector (31 downto 0);
      cmd : in  bit;
      res : out bit_vector (31 downto 0));
end component;
component falling_edge_reg
port (din  : in  bit_vector (31 downto 0);
      clk  : in  bit;
      dout : out bit_vector (31 downto 0));
end component;
begin
  my_adder : adder
  port map (a => eff_data, b => accu_out, res => adder_out);
  my_mux   : and_32
  port map (cmd => command, a => accu_out, res => eff_data);
  my_reg   : falling_edge_reg
  port map (din => adder_out, clk => clk, dout => accu_out);
end;