Re: Fortran's Equivalence

[Viqar Abbasi <viqar@cae.ca>]

Hi, from the original poster... :)

I've appreciated this dicussion on the "best" way to implement overlays. 
One thing that I forgot to mention was that I am confined to the Ada 83 
environment, regardless of platform.  So, I am attempting a conscious 
effort to enforce "old-style Ada 83 thinking".

My original reason for this post was because when I received the code, 
it used address clause overlays.  However, on my GNAT 3.07 for SGI, the 
overlaying didn't work (or at least, that particular code have the 
wrong results).  So, I added a routine for bit swapping, (localizing the 
effect of changing machines to that one small routine), and then 
proceeded with Unchecked_Conversion.  Later, when I downloaded GNAT 
3.09, I found that it offered a whole new spectrum of address clause 
flexibility.  A quick test showed me that the old address clause overlay 
would work.  So, I thought I'd ask about the "preferred method" to do 
something which I knew is dangerous.  (And actually, I've never used 
EQUIVALENCE in a Fortran program... But I knew about it. :)

<< there's the Prologue :) >>

<< Questions (this should be a standardized form) >>

Can someone please explain to me the effect of representation clauses 
when doing these overlays?  For example,

record My_Record is
    A:  some_type;
    B:  some_other_type;
    C:  still_another_type;
end record;
for My_Record_Type'Size use 32;
for My_Record_Type use
  record
    A at 0 range 0 .. 14;
    B at 0 range 15 .. 24;
    C at 0 range 25 .. 31;
  end record;

Does "range 0 .. 14" mean the Most Signicant Bits?  Or does it mean 
"Left most bits"?  This will clearly have an impact on my overlays.  If 
I'm going to move bits arouund, then I don't want to have to change the 
record representation clauses...  Will I be able to avoid it, by 
only changing my bit-manipulation routine, depending on my machine?

Thanks for being here... :)
V.