Код:
disk_a <= "0" when pff(1 downto 0) = "00" and motor = '1' else 'Z';
disk_c <= "0" when pff(1 downto 0) = "10" and motor = '1' else 'Z';

process (iorqge, disk_a, disk_c)
 begin
  if iorqge='1' then
     disk0 <= disk_c;
     joy <= disk_a;
  else
     disk0 <= disk_a;
     joy <= disk_c;
 end if;
end process;
как-то так...