Re: conversions between fixed-point types

[Dirk Herrmann <fight_spam@invalid.invalid>]

Jeffrey R. Carter wrote:
> You are complicating matters with your functions and your conversions to 
> and from Float. Since IIRC your target H/W doesn't support 
> floating-point, you will be able to see what happens and formulate your 
> questions better if you deal only with the fixed-point types and direct 
> type conversions between them:

Thanks, Jeff, for the suggestions, I have tried them out.  I'd like to add 
that our target H/W does not have a floating point unit, but there certainly 
is a floating point library.

> What you have called a "direct cast" is a type conversion. Also, it is 
> never necessary to convert a real numeric literal to a real type. Your 
> 1st line could be written as
> 
> FIO.Put (Float (Fpb (Float'(-1.5) ) ) );
> 
> With the apostrophe, this is a "qualified expression" and tells the 
> compiler that the expression in the parentheses is of type Float. 
> However, I suspect that
> 
> FIO.Put (Float (Fpb'(-1.5) ) );
> 
> is even better.

FpB'Image(FpB'(-1.5))                evaluates to -1.2
FpB'Image(FpB(FpA'(-1.5)))           evaluates to -1.2
FpB'Image(MakeBFromA(FpA'(-1.5)))    evaluates to -1.6

Thus, even avoiding all floating point operations, the difference is still 
there.  It appears to be a question of whether the conversion from FpA to FpB 
or the conversion from Float to FpB is computed statically or dynamically.

Best regards,
Dirk