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From: eagle!jerry
Date: Thu Nov  4 02:01:27 1982
Subject: Re: Fixed point data in Ada
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Posted: Wed Nov  3 10:18:38 1982
Received: Thu Nov  4 02:01:27 1982
Date: 1982-11-04T02:01:27+00:00
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This subject is more complicated than meets the eye.

I will present some statements from the LRM

LRM 3.5.9-3 defines the "delta" of an Ada 
fixed point type as the value given in the declaration 
which may be any positive real value.

LRM 3.5.9-4 says that the "model numbers" for
a fixed point type are multiples of "small"

LRM 3.5.9-5 says "the number small is
choosen as the largest power of two that is not greater
than the specified delta.  Alternatively, it is
possible to specify the value of small by a length clause.
(see 13.2)"

LRM 13.2 defines "length clauses" which are a particular
kind of "representation clause".  In particular  LRM 13.2-9
Defines a clause of the form

	for T'small use simple_expression

and says "the type T must be a fixed point type.  The expression
must be a static expression of some real type; its value
must not be greater than the delta of the type.  The effect
of the length clause is to use this value of small for the
representation of values of the fixed point type."

Taking all the above together it would seem that dollars 
could be specified by

	type money is delta .01 ;
	for money'small use .01 ;

This should get a representation in which values of type
money are represented in "cents".  

Great!  But before you get excited let me quote one more
paragraph of the LRM 3.5.9-11: "The fixed point declaration
is illegal if no implementation defined type satisfies
these requirements". (There must be at least
one anonymous fixed point type according to LRM 3.5.9-7)

My interpretation of this is: if your Ada compiler accepts the 
declaration and length clause it will give you the 
representation you want.  But there is no guarantee
that it will accept it.

I hope this clears up some of the confusion.

Jerry Schwarz
harpo!eagle!jerry