Re: Streams in Ada

[Robert A Duff <bobduff@world.std.com>]

Florian Weimer <fw@deneb.enyo.de> writes:

> Robert A Duff <bobduff@world.std.com> writes:
> 
> > > If I were writing an Ada compiler for the PDP-10, I would probably
> > > choose Storage_Element'Size = 9.
> > 
> > Sounds like a bad idea to me.  Storage_Element'Size should be 36 on the
> > PDP-10.  I believe anything else would be unnecessarily inefficient.
> 
> Are you sure?

No.

>...  There are quite a few Ada compilers out there which use
> Storage_Element'Size = 8 even though it's inefficient on that
> particular platform.

Which machines?  I've never heard of such a thing.

>...I think the size of a storage element is pretty
> irrelevant for efficiency concerns.

Well, maybe I misunderstood.  I was presuming that if
Storage_Element'Size = 8, then the compiler behaves accordingly (eg,
for laying out arrays).  *That's* what would affect efficiency.
I guess it's true that Storage_Element'Size in and of itself is
irrelevent unless you're using 'Address and related stuff.

> > >...IIRC, the GCC porters are doing a
> > > similar thing (CHAR_BITS == 8).
> 
> Oops, CHAR_BITS == 9 of course.
>  
> > That's because of various problems with the C language, which don't
> > exist in Ada.  C has no packed arrays, for example.  Plus the fact that
> > C culture has grown up in such a way that there's a huge amount of code
> > that implicitly assumes that char is 8 bits.
> 
> An 8 bit char on a PDP-10 doesn't work.  Only values which are
> divisors of 36 are acceptable.

I don't see why.  The PDP-10 hardware can deal with 8-bit bytes just
fine (although you waste approx 1/9 of the bits).  Is there some
subtlety of C I'm missing?

Or do I have to say you waste 1.0/9.0 of the bits?  ;-)

- Bob