Re: Streams in Ada

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

Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE> writes:

> Ted Dennison<dennison@telepath.com> writes:
> 
> > I think you meant, "all possible Ada implementations", which I would
> > disagree with.
> 
> Do impossible Ada implementations exist? ;-)

No, but some possible Ada implementations don't exist.  ;-)

> > On the other hand, if we were to interpret this to say "on all
> > likely implementations", I could agree with you. I'll leave it up to
> > others how likely a conforming Ada95 PDP-10 implementation is.
> 
> I think if you're after practical portability, the PDP-10 doesn't
> matter to you.

True, but the principle of writing byte-size-independent code is still
often a good idea.  It's just as easy to be portable to the PDP-10 as to
any other machine that has "unusual" Storage_Units.

> > Out of curiosity, what exactly constituted a "byte" on a PDP-10?
> 
> According to http://www.inwap.com/pdp10/hbaker/pdp-10/Byte.html, the
> smallest individually addressable storage unit is a bit, so with a
> common definition of a byte, it consists of a single bit. ;-)

Well, sort of.  You could address a single bit with a "byte pointer".
But a normal machine address addressed a 36-bit word.  I believe machine
addresses are more efficient.

> 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.

>...IIRC, the GCC porters are doing a
> similar thing (CHAR_BITS == 8).

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.

In another newsgroup, I recently read a nice quote from G B Shaw that
seems apropos:  "He is a barbarian who thinks the customs of his tribe
are natural laws."

- Bob