Re: Embedded languages based on early Ada (from "Re: Preferred OS, processor family for running embedded Ada?")

[Ray Blaak <rAYblaaK@STRIPCAPStelus.net>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> On Sat, 03 Mar 2007 01:58:35 GMT, Ray Blaak wrote:
> > I am somewhat rusty on my Ada tasking knowledge, but why can't Thing be a
> > protected object?
> 
> I tried to explain it in my previous post.
> 
> When Thing is a protected object, then the procedures and entries of,
> called from the concurrent alternatives are all mutually exclusive. This is
> not the semantics expected from PAR. Probably it would be better to rewrite
> as:

PAR only says that all of its statements run in parallel, nothing more nothing
less (e.g. equivalent to the task bodys you had around each statement before).

Those statements can themselves access synchronization and blockin controls
that affect their execution patterns.

> No. The implied semantics of PAR is such that Thing should be accessed from
> alternatives without interlocking because one *suggests* that the updates
> are mutually independent. 

The updates are independent only if their behaviour truly is independent. If
they access a shared synchronization control then by definition they are
mutually dependent.

It is not PAR that dictates this, but rather the statements themselves.

PAR would only be convenience shorthand for writing task bodies around each
statement.

> When Thing is visible from outside it should be
> blocked by PAR for everyone else. This is not the behaviour of a protected
> object. It is rather a "hierarchical" mutex.

The behaviour of a protected object is defined by its entries and how it is
used.

-- 
Cheers,                                        The Rhythm is around me,
                                               The Rhythm has control.
Ray Blaak                                      The Rhythm is inside me,
rAYblaaK@STRIPCAPStelus.net                    The Rhythm has my soul.