Re: Engineering degree for embedded systems

From	Walter Banks <walter@bytecraft.com>
Newsgroups	comp.arch.embedded
Subject	Re: Engineering degree for embedded systems
Date	2017-08-17 12:15 -0400
Organization	Aioe.org NNTP Server
Message-ID	<on4fff$824$1@gioia.aioe.org> (permalink)
References	(14 earlier) <omhlpo$eo6$1@dont-email.me> <on2hjo$1jsg$1@gioia.aioe.org> <on3gra$84q$1@dont-email.me> <on41ti$1ifm$1@gioia.aioe.org> <on47kq$b3h$1@dont-email.me>

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On 2017-08-17 10:06 AM, David Brown wrote:
> On 17/08/17 14:24, Walter Banks wrote:
> 
> The AVR needs different instructions for accessing data in flash and
> ram, and address spaces provide a neater and less error-prone 
> solution than macros or function calls for flash data access.
> 
> So far, so good - and if that is your work, then well done.  The
> actual text of the document could, IMHO, benefit from a more concrete
> example usage of address spaces (such as for flash access, as that is
> likely to be a very popular usage).

The named address space stuff is essentially all mine.

> 
> 
> The register storage class stuff, however, is not something I would 
> like to see in C standards.  If I had wanted to mess with specific 
> CPU registers such as flag registers, I would be programming in 
> assembly.  C is/not/  assembly - we use C so that we don't have to 
> use assembly. There may be a few specific cases of particular
> awkward processors for which it is occasionally useful to have direct
> access to flag bits - those are very much in the minority.  And they
> are getting more in the minority as painful architectures like COP8
> and PIC16 are being dropped in favour of C-friendly processors.  It
> is absolutely fine to put support for condition code registers (or 
> whatever) into compilers as target extensions.  I can especially see 
> how it can help compiler implementers to write support libraries in
> C rather than assembly.  But it is/not/  something to clutter up C 
> standards or for general embedded C usage.

To be really clear this was a TR and never expected to be added to the C
standards at the time.

In the current environment I would like to see the C standards moved
forward to support the emerging ISA's. There are many current
applications that need additions to the language to describe effective
solutions to some problems. Ad-hoc additions prevent the very thing that
C is promoted for, that is portability. C standards are supposed to
codify existing practice and so often the politics of standards become
arguments about preserving old standards rather than support for newer
processor technology. I know from what I have been doing is both the
spirit and approach to code development in C can deal with changes in
applications and processor technology.

So many of the development tools still are restricted by the technology
limits of development environments of 40 years ago.

> 
> The disappointing part of named address spaces is in Annex B.1.  It 
> is tantalisingly close to allowing user-defined address spaces with
> specific features such as neat access to data stored in other types 
> of memory.  But it is missing all the detail needed to make it work, 
> how and when it could be used, examples, and all the thought into
> how it would interplay with other features of the language.  It also 
> totally ignores some major issues that are very contrary to the 
> spirit and philosophy of C.  When writing C, one expects "x = 1;" to 
> operate immediately as a short sequence of instructions, or even to 
> be removed altogether by the compiler optimiser.  With a
> user-defined address space, such as an SPI eeprom mapping, this could
> take significant time, it could interact badly with other code (such
> as another thread or an interrupt the is also accessing the SPI bus),
> it could depend on setup of things outside the control of the
> compiler, and it could fail.

The named address space has often been used to support diverse forms
of memory. To use your example x = 1; The choice where x is located
and how it is accessed is made where it is declared. How it is handled
after that is made by the compiler. The assumption is that the code
is written with functional intent.

As valid as the SPI conflict is it is a strawman in practice. C is
filled with undefined and ambiguous cases and this type of potential
problem in practice is very rare.

> 
> You need to think long and hard as to whether this is something
> desirable in a C compiler.

I have and it is. Once I passed the general single address space
C model named address space opened a level of flexibility that
allows C to be used in a variety of application environments
that conventional C code does not work well for.

> It would mean giving up the kind of transparency and low-level 
> predictability that are some of the key reasons people choose C over 
> C++ for such work.  If the convenience of being able to access 
> different types of data in the same way in code is worth it, then 
> these issues must be made clear and the mechanisms developed - if 
> not, then the idea should be dropped.  A half-written
> half-thought-out annex is not the answer.

I buy the documentation point.

 From a usage point I disagree. Writing an application program that can
be spread over many processors is a good example.  In the two decades
since this work was initially done things have changed considerably from
consumer products that distributed an application over 3 or 4
processors. (after initially prototyping on a single processor). In
these processor usage was almost always manually allocation using
geographical centers of reference.

This has evolved to compiler analysis that automate this whole process
over many many processors.

w..

Thread

Engineering degree for embedded systems "hogwarts" <122707@EmbeddedRelated> - 2017-07-27 07:35 -0500
  Re: Engineering degree for embedded systems Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2017-07-27 15:25 +0200
    Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-07-30 12:05 -0400
      Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-07-30 19:05 +0100
        Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-01 08:55 -0400
          Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-01 14:23 +0100
            Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-03 11:03 -0400
              Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-06 10:35 +0100
                Re: Engineering degree for embedded systems John Devereux <john@devereux.me.uk> - 2017-08-06 14:40 +0100
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 12:51 -0400
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-06 20:13 +0100
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 19:21 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 12:40 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 15:18 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 15:27 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 16:47 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 17:30 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 18:35 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 18:56 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 19:02 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 19:08 -0400
                Re: Engineering degree for embedded systems Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2017-08-08 01:30 +0200
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 19:32 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 19:53 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 20:02 -0400
                Re: Engineering degree for embedded systems stephen@mpeforth.com (Stephen Pelc) - 2017-08-07 23:49 +0000
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 20:07 -0400
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 08:00 +0100
                Re: Engineering degree for embedded systems George Neuner <gneuner2@comcast.net> - 2017-08-07 01:21 -0400
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-06 17:15 +0300
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-06 20:21 +0100
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-07 10:35 +0300
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-07 12:54 +0200
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-07 12:34 +0200
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 12:43 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 12:50 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 12:36 -0400
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-07 18:40 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 10:23 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 09:46 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 11:26 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 11:09 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 12:56 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 15:56 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 17:11 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 16:33 +0100
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-08 21:07 +0300
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-09 10:03 +0200
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-10 14:30 +0300
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-10 15:11 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-16 18:39 -0400
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-17 09:37 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-17 08:24 -0400
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-17 16:06 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-17 12:15 -0400
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-18 14:45 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-25 09:51 -0400
          Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-01 20:46 -0500
            Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-02 09:30 +0200
              Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-05 15:11 -0500
              Re: Engineering degree for embedded systems Paul Rubin <no.email@nospam.invalid> - 2017-08-08 00:22 -0700
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 08:36 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 12:37 +0200
  Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-07-27 17:46 +0100
    Re: Engineering degree for embedded systems Grant Edwards <invalid@invalid.invalid> - 2017-07-27 16:54 +0000
      Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-07-27 19:57 +0100
  Re: Engineering degree for embedded systems Frank Miles <fpm@u.washington.edu> - 2017-07-27 16:58 +0000
    Re: Engineering degree for embedded systems Theo Markettos <theom+news@chiark.greenend.org.uk> - 2017-07-28 00:43 +0100
    Re: Engineering degree for embedded systems jim.brakefield@ieee.org - 2017-07-30 15:26 -0700
  Re: Engineering degree for embedded systems Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-27 23:45 +0000
  Re: Engineering degree for embedded systems Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2017-08-03 19:50 +0200
    Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-05 15:20 -0500
      Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-06 09:17 +0300
        Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-08-06 10:12 +0100
          Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-06 13:19 +0300
          Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-06 09:59 -0500
            Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-08-06 18:06 +0100
              Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 21:31 -0400
                Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-08-07 09:32 +0100
        Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-06 09:53 -0500
          Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-07 10:59 +0300
            Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-07 20:09 -0500
              Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-08 21:31 +0300
        Re: Engineering degree for embedded systems Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2017-08-21 11:28 +0200

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