VHDL array shift register

[shaiko]

Hello,

Signal x is a 1D array that's defined as follows:

signal x : 1d_array ( 0 to a - 1 ) ( b - 1 downto 0 ) ;

I want to design an array shift register that shift in a vector of '0' every clock. Something like this:

process ( clock ) is 
begin
    if rising_edge ( clock ) then
        if load = '1' then
            x <= input_array ;
        else    
            x <= x ( 1 to a - 1 ) & ( x ( 0 ) ' range => '0' ) ;              	
	end if ;
   end if ;		
end process ;

Unfortunately, the above doesn't work.

Operator "&" is ambiguous

What's the correct syntax to do the same ?

 

[TrickyDicky]

it depends what the base type is. I assume it's ambiguous as it doesnt know if ( x ( 0 ) ' range => '0' ) is a bit_vector, std_logic_vector, signed or unsigned.

use a qualification to say which one you mean:

x <= x ( 1 to a - 1 ) & std_logic_vector'( x ( 0 ) ' range => '0' ) ;

 

[shaiko]

it depends what the base type is.

Well, the base type is already defined in the array type declaration as std_logic_vector.
Why do I need to mention it again explicitly during assignment ?

- - - Updated - - -

And also,
The apostrophe after x ( 0 ) denotes an attribute.
But what's the purpose of the apostrophe after std_logic_vector?

x <= x ( 1 to a - 1 ) & std_logic_vector [COLOR="#FF0000"]'[/COLOR] ( x ( 0 ) ' range => '0' ) ;

 

[TrickyDicky]

Well, the base type is already defined in the array type declaration as std_logic_vector.
Why do I need to mention it again explicitly during assignment ?

The compiler doesnt know that. It just knows that you made an array the length of x(0) containing '0'. That could be an array of bits, a string, an unsigned etc. It doesnt know which one you wanted to create.

- - - Updated - - -

And also,
The apostrophe after x ( 0 ) denotes an attribute.
But what's the purpose of the apostrophe after std_logic_vector?

x <= x ( 1 to a - 1 ) & std_logic_vector [COLOR="#FF0000"]'[/COLOR] ( x ( 0 ) ' range => '0' ) ;

Its a qualifier expression. It qualifies to the compiler what type you meant.