Aspects, rep specs, shifts, conditional expressions, expression functions

[Bill Findlay <yaldnif.w@blueyonder.co.uk>]

The topics listed in the subject have been controversial recently.
I wonder what people think of my first attempt at an Ada 2012 package,
which seems to use them all, in what I hope is a judicious mix?

----
with Unchecked_Conversion;
with KDF9.CPU;

package KDF9.K5_stuff is

   -- PHU, the Program Hold-Up register is internal to I/O Control.
   -- A subset is exposed to Director by means of the K5 order.
   -- It has one element for each of the 4 program priority levels.
      
   type blockage is (buffer_busy, locked_out) with Size => 1;
   
   type group_address is mod 1024 with Size => 10;
    
   type PHU_store (hold_up : one_bit := 0;
                   reason  : blockage := buffer_busy) is
      record
         case hold_up is
            when 0 =>
               null;
            when 1 =>
               case reason is
                  when buffer_busy =>
                     buffer_nr  : buffer_number;
                  when locked_out =>
                     group_nr   : group_address;
               end case;
         end case;
      end record;

   PHU : array (priority) of PHU_store;

   type PHU_subset is
      record
         hold_up   : one_bit;
         reason    : blockage;
         parameter : buffer_number;
      end record
   with Size => 6, Bit_Order => Low_Order_First;
   
   for PHU_subset use
      record
         hold_up   at 0 range 5 .. 5;
         reason    at 0 range 4 .. 4;
         parameter at 0 range 0 .. 3;
      end record;
      
   type PHU_as_6_bits is mod 2**6 with Size => 6;

   function as_6_bits is
      new Unchecked_Conversion(Source => PHU_subset,
                               Target => PHU_as_6_bits);

   function K5_word (short_PHU : PHU_subset) return word is
      (word(as_6_bits(short_PHU)));

   function short_PHU (p : priority) return word is
      (K5_word((
                hold_up   => PHU(p).hold_up,
                reason    => (
                              if PHU(p).hold_up = 0 then
                                 buffer_busy  -- A non-significant 0 bit.
                              else
                                 PHU(p).reason
                             ),
                parameter => (
                              if PHU(p).hold_up = 0 then
                                 0
                              elsif PHU(p).reason = buffer_busy then
                                 PHU(p).buffer_nr
                              else
                                 buffer_number(PHU(p).group_nr mod 2**4)
                             )
              ))
      );

   -- A K5_operand is a KDF9 word, D00-D47, with the content:
   --    short_PHU(0) in D00 .. D05
   --    short_PHU(1) in D06 .. D11
   --    short_PHU(2) in D12 .. D17
   --    short_PHU(3) in D18 .. D23

   function K5_operand return word is
      (
       KDF9.CPU.shift_word_left(short_PHU(0), 47-05) or
       KDF9.CPU.shift_word_left(short_PHU(1), 47-11) or
       KDF9.CPU.shift_word_left(short_PHU(2), 47-17) or
       KDF9.CPU.shift_word_left(short_PHU(3), 47-23)
      );

end KDF9.K5_stuff;

-- 
Bill Findlay
with blueyonder.co.uk;
use  surname & forename;