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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Storage space question
Date: 1998/12/11
Message-ID: <m34sr3o4us.fsf@mheaney.ni.net>#1/1
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References: <366FE278.FAF73497@pwfl.com>
 <F3rBC7.JAr.0.-s@inmet.camb.inmet.com> <74pfg4$3s6$1@nnrp1.dejanews.com>
NNTP-Posting-Date: Thu, 10 Dec 1998 17:15:37 PDT
Newsgroups: comp.lang.ada
Date: 1998-12-11T00:00:00+00:00
List-Id: <comp.lang.ada>

callen@space.honeywell.com writes:

> OK, well, maybe I'll elaborate a bit more on what it is I'm trying to
> accomplish.  I have a hardware device that has registers I will
> access.  Lets say one register looks like this (identifiers have been
> changed to protect the innocent):
> 
> 
>    type REGISTER_1_Type is
>    record
>       field_1_Ctrl:             t_3bit;
>       field_2_Ctrl:             t_3bit;
>       field_3_Ctrl:             t_2bit;
>       field_4_Ctrl:             t_2bit;
>       field_5_Ctrl:             t_2bit;
>       DCD_Pin_Ctrl:             t_2bit;
>       field_7_Ctrl:             t_2bit;
>    end record;
> 
> Assume these imaginary t_ types are 3 and 2 bits worth of values
> respectively.  Now, in order to write to this register, I would write
> 
>    Reg.DCD_Pin_Ctrl := 2#01#;
> 
> OK, but I'd like to use more descriptive names for these.  Like so.
> 
>    Reg.DCD_Pin_Ctrl := SYNC_DETECT;
> 
> So, I define and represent the enum above (as 2 separate Ada books
> have suggested...). Now, the code looks nicer, but I have this baggage
> that came with it that I don't need.  I really just want the ability
> to do as a #define (or enum) would do in C - that is to do the
> substitution for me to make the code look nicer, but not increase the
> memory footprint (over using constants in the program). The reason is
> that I would like to make *many* of these.  For the sake of argument,
> say there's 100 registers each with 8 bits worth of possibilities. I
> don't want tables with all that info in there, just the 100 values
> that I'm using to initialize the thing to be put in the right place in
> the code. I liked the use of the enumerated types for these, as their
> namespaces won't clash (Each register can have it's own DEFAULT for
> instance).
> 
> Again, I'm using Ada83, and don't see a pragma that will help me.  I
> understand and agree with the dislike of preprocessors, but is what
> I'm trying to do (which textual subst would work for...) such a bad
> thing to ask for?


Then just ditch the enumeration type, and use named constants of an
integer type:

  type CDC_Control is range 0 .. 3;

  Sync_Control : constant CDC_Control := 2#01#;
 
  ...


You don't really need enumeration types that have rep clauses; most of
the time an integer type will do.