can i declare a 2D array as following ?

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For a 2D array just have one range in the brackets, instead of two.

type dataout is array (6 downto 0) of std_logic_vector(7 downto 0);

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Nope, thats not a 2d array, thats a 1d array of 1d array. So you access it like this:

output <= dout(x).

A 2d array is declared like the OP posted. He made a 2d array of 1d arrays. You would access it like this:

output <= dout(x,y); --and then add 2nd set of brackets to get individual bits