subtraction of negative number

Thu, 26 Jan 2012 13:35:33 -0700

Hi

i am trying ot subtract a negative number as follows:

use ieee.std_logic_numeric.all

constant a signed(16 downto 0):= '0' & x"8000";
constant b signed(16 downto 0):= '1' & x"0030";

signal c signed(16 downto 0);

c <= a - b;

c should be 32816 but i get 32720

please help

Many thanks

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subtraction of negative number