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From: "P. Cnudde VH14 (8218)" <cnuddep@sh.bel.alcatel.be>
Subject: Re: ADA - VHDL
Date: 1996/07/16
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references: <31E2391F.A16BEBD@sh.bel.alcatel.be> <dewar.837092765@schonberg>
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Date: 1996-07-16T00:00:00+00:00
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Brian Cheebie Merchant wrote:
>    There are some extremely useful software construct which
> simply do not translate well to hardware.  (recursive structures is one
> of these)

I would disagree on the point of recursion. I try to make my function calls
in VHDL recursive if it is possible. To my oppinion it generates far better
hardware in general. 

 An example:

   -- Calculate Parity bits
   function HasEvenParity(constant Argument : in bit_vector) 
      return boolean is
      constant High   : integer := Argument'high;
      constant Low    : integer := Argument'low;
      constant Length : integer := Argument'length;
   begin
      if Length=1 then
         return not BitToBoolean(Argument(High));
      else
         return (HasEvenParity(Argument(High downto (High+Low+1)/2))
                 = HasEvenParity(Argument((High+Low-1)/2 downto Low)));
      end if;
   end HasEvenParity;

This generates a very efficient hardware realisation. In software on the other
hand I would NOT use recursion here. (due to function call overhead). 

(Of course when you use recursion you must make sure that the recursion
depth is fixed).


Regards,



-- 


   ___________          Peter Cnudde
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