library IEEE;
use IEEE.std_logic_1164.all;

entity Sm is  -- subtractor multiplexor 
  port ( x  : in  std_logic;
         y  : in  std_logic;
         b  : in  std_logic;
         u  : in  std_logic;
         d  : out std_logic;
         bo : out std_logic);
end Sm;

architecture circuits of Sm is
  signal t011, t111, t010, t001, t100, td : std_logic;
begin  -- circuits of Sm
  t011 <= (not x) and y and b;
  t111 <= x and y and b;
  t010 <= (not x) and y and (not b);
  t001 <= (not x) and (not y) and b;
  t100 <= x and (not y) and (not b);
  bo   <= t011 or t111 or t010 or t001;
  td   <= t100 or t001 or t010 or t111;
  d    <= td when u='1' else x;  
end circuits;  -- of Sm