Initializing an array of records in VHDL

问题:

I have an record defined as follows

type ifx_t is
record
  data                        : std_logic_vector(127 downto 0);
  address                     : std_logic_vector (19 downto 0); 
  WrReq                       : std_logic;-- 
  RdReq                       : std_logic; --
end record;
type Array_ifx_t is array (0 to 2) of ifx_t;

And I have to initialize an instance of this array of records and I have tried the following way and it doesn't work

signal pair_in       : Array_ifx_t:= (others =>((others =>'0'),(others=>'0'),'0','0')); 

Kindly help.

回答1:

As said in the comment, ModelSim work when compiling the code from question with ModelSim. However, other tools may be more strict about using a typed value for elements in Array_ifx_t.

For a typed assign and use of named record elements, which I think gives getter overview and avoid mistakes by position references, you can make the initialization with:

constant IFX_T_0S : ifx_t := (data => (others =>'0'),
                              address => (others=>'0'),
                              WrReq => '0',
                              RdReq => '0');

signal pair_in : Array_ifx_t:= (others => IFX_T_0S);

回答2:

My initial reaction to seeing the aggregate in your default value for pair_in was that there were too many 'others', so I independently wrote one using the record type declaration itself:

library ieee;
use ieee.std_logic_1164.all;

package some_record is

    type ifx_t is
    record
      data                        : std_logic_vector(127 downto 0);
      address                     : std_logic_vector (19 downto 0); 
      WrReq                       : std_logic;-- 
      RdReq                       : std_logic; --
    end record;
    type Array_ifx_t is array (0 to 2) of ifx_t;
    -- positional association in an aggregate used for initialization:
    signal pair_in:    ifx_t := ((others => '0'), (others => '0'),'0','0');
end package;

And this analyzed successfully. An aggregate has two types of association, positional or named. The above default value expression is positional. With a named association:

signal pair_in:    ifx_t := -- named association of record elements:
                           (
                             data => (others => '0'), 
                             address => (others =>'0'), 
                             WrReq => '0',
                             RdReq => '0'
                           );

You'll notice this bears an uncanny resemblance to the value expression found in Morten's accepted answer's constant declaration, and actually tells the story of compatibility for an aggregate expression.

An aggregate compatible with a record type contains a value expression that is compatible with each element of the record type. This is done for the array elements data and address by providing aggregates for their default values, while WrReq and RdReq are provided with their default values directly.

The extra others statement found in the original attempt would have been appropriate had pair_in been a composite type comprised of an array comprised of ifx_t record type elements.

The LRM (e.g. IEEE Std 1076-1993) has a section on Expressions, a subsection on Aggregates with a further subsection on Record aggregates.

There's also a section on Types, a subsection on Composite types, with a further subsection on Record types.