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Absolute addressing on the ARM

Discussion in 'Embedded' started by Nils M Holm, Mar 16, 2014.

  1. Nils M Holm

    Walter Banks Guest

    Paul Rubin has already posted some links related to your question.

    This is an area that at has to a greater or lessor extent been part
    of my life for 30+ years.

    Here are a few places to start.

    A unambiguously defined language. In some was SubC has that.
    Ada, Pascal and the like are better than C and far from perfect.
    Implementation defined is convention but has many side effects
    most of which are unintended.

    Focus on goals. The reason for my FDA reference was because
    they have both goals based on product reliability and safety but
    also recognize that the tools and the people who use them are
    far from perfect.

    A lot more real research on software engineering and implementation
    practices.. A simple example of this is tools that walk the generated
    code applying normal reliability math to the code with any assumption
    about the reliability of an instruction based on pick one ( 1 , size in
    bytes, execution cycles) . This will not give you an MTTF that isn't
    the goal. The goal is to independently evaluate comparative reliabilities.
    Compare two implementations and it will reasonably accurately
    predict which will run most reliably. We have created and used
    such tools and it has had a remarkable positive effect on changes
    in the way we think about and plan software.

    The topic lists for software engineering as opposed to
    software science is now quite large. Some of the things that are
    important that I see our better customers using are.
    - Application design documents.
    - Ram, rom, and execution cycle budgets
    - Formal function interface documentation. (Independent
    component for reliably analysis)
    - Coding practice document that support the application goals
    for a project

    That's where I would start

    Walter Banks
    Byte Craft Limited
    Walter Banks, Mar 23, 2014
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  2. Nils M Holm

    Walter Banks Guest

    Some very bad tools have been used to create some very reliable
    application code. It has a lot to do with design, coding and testing
    practices. Focus on final product.

    One thing I see is companies that standardize on a particular version
    of a tool and although the tool has evolved over many years they use
    the tools they are familiar with. The trade-off is dealing with a single
    set of changes as a product evolves related to their product and not
    side effects of new tools and product changes.

    Walter Banks, Mar 23, 2014
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  3. Nils M Holm

    Paul Rubin Guest

    Embedded is a pretty wide space and in more critical applications, there
    may be prescribed processes that I'd have to follow regardless of my
    preferences. In less critical applications, memory and realtime
    constraints can still be determining factors. Another issue is the
    skill sets of the programmers likely to work on the code. Using DSL's
    can require a level of PL geekery that a typical embedded developer
    would probably not have. I could imagine an approach of prototyping
    using a DSL, then conferring with the customer about whether they were
    comfortable staying with that approach or wanted a more traditional
    approach. If I were the main consumer of the code, then as a PL geek
    I'd probably use DSL's rather freely.
    I don't see why not, if the target CPU is powerful enough to run it, and
    there aren't realtime constraints that would preclude garbage
    collection. I've done embedded stuff in Python which amounts to the
    same thing.
    The two Haskell EDSL's that I mentioned (Atom and ImProve) both compile
    to C or Ada. I've played with Atom. You write your program in a style
    that looks like multitasking (plus you get to use Haskell syntax and
    type safety), and then the Atom library converts it to a C-coded state
    machine with deterministic timing, that you run through a C compiler to
    produce target code. ImProve operates about the same way, though its
    target problem set is a bit different.

    Strictly functional code can have side effects: they just have to be
    known to the type system. So if some function in your program wants to
    print something (or call some other function that prints stuff), it has
    to have "IO" in its type signature. The function output is e.g. a
    "print command" that gets handed off to the runtime library that does
    the printing. So the type system lets you segregate the side-effecting
    code from the "pure" code, which helps write in a style where most of
    the code is pure and easier to reason about.
    Paul Rubin, Mar 23, 2014
  4. Nils M Holm

    Paul Rubin Guest

    Here's a talk about critical systems experience in Haskell:



    I haven't watched the video and I'm not sure if the slides and the video
    are from the exact same presentation, but they're both the same guy
    discussing the same subject matter.
    Paul Rubin, Mar 23, 2014
  5. You should not regard that as a one-or-the-other question, there is a
    Try to minimize the total effort involved. Is the investment in tool
    building and learning (not only your learning, but of all people
    involved) compensated by the effort saved in using the tool and
    writing/reading the (much shorter/readable) solution(s)?

    favours existing paradigms in an existing language:
    - one-time, small problem
    - many people need to understand it, especially if they need to
    understand only a small part, and already know the language/paradigms
    - semantic difference between programming language/paradigms and problem
    domain is small

    favours full special-purpose language:
    - large recurring problems
    - only few people involved, and they all need to understand large
    amounts of it
    - semantic difference is hughe

    IME this is a 'sweet spot' inbetween that is often a good choice. Named
    parameters make complex 'specifications' (more) readable. My
    professional experience was with Ada, but I also used Python for this
    purpose for mopre private projects.

    Wouter van Ooijen, Mar 23, 2014
  6. Nils M Holm

    David Brown Guest

    Just remember that people can write bad software in any language. A
    good choice of language and tools makes it easier to write good software
    and harder to write bad software, but it is no guarantee. And the
    language and tools are very much secondary to factors such as
    development methods, testing procedures, reviews, documentation,
    specifications, and everything else around the actual coding.

    But of course there is a strong correlation - a company or organisation
    that is willing to invest in Ada development rather than C is likely to
    pay attention to the other parts of the development ecosystem.
    Compilers are not bug-free - but bugs in the compiler users' code
    outweigh the bugs in compilers by many orders of magnitude. Since the
    end result is only bug-free if the entirely of the user code is bug
    free, and that it does not use any part of the compiler that contains a
    bug, it seems obvious that reducing the risk of user error (such as by
    using a "safer" language like Ada, or by restricting C to a carefully
    chosen subset) is far more important than worrying about the particular
    choice of toolchain.

    Also, as noted before in this thread, simplicity or complexity of a
    toolchain is no indication of the quality or the risk of hitting a bug.
    What /does/ increase your chance of meeting a bug is using odd or
    complicated structures in your own code - it is not hard to write your C
    code in a way that significantly reduces the risk of hitting a compiler
    bug (while simultaneously making the code clearer and easier to write
    David Brown, Mar 23, 2014
  7. Nils M Holm

    David Brown Guest

    I don't know, but I guess it might be something to do with the
    user-defined literal syntax in newer C++ (that lets you write things
    like "2_hours + 45_minutes" by defining a "_hours" operator to convert a
    number into a time class).
    David Brown, Mar 23, 2014
  8. Nils M Holm

    BartC Guest

    (Hey, I had that over twenty years ago! But mine are just scale factors, and
    are not connected to the literal:

    2 m + 35 cm, 5 miles+375 yds and so on (all these are converted to mm as
    that was my basic unit).

    As I implement it, the "m", "cm" etc don't interfere with the normal
    namespace. Originally used in an application scripting language, these days
    I mainly use the feature to be able to write 5 million, 1 billion and so

    But the ' character as separator is not that terrible, at least it's easy to
    type with no shift needed.

    One extra literal connected thing they might look at, but it's probably too
    late, is the 'bug' where writing leading zeros such as 064 actually gives
    you the number 52 not 64. Data, especially from external sources, can
    sometimes have leading zeros. Perhaps 'avoid leading zeros' should be added
    to the lists that people have posted of how to ensure more bug-free code.
    BartC, Mar 23, 2014
  9. Nils M Holm

    Tom Gardner Guest

    Those are two points that I think are critical: tools and

    There are many very good toolsets (i.e. not just compilers)
    available for standard languages, and appropriate use can
    significantly improve the end result. Re-creating vaguely
    equivalent tools for a special-purpose language is an
    enormous burden, and I've never seen it happen. End result:
    you spend more time on the nitty-gritty crud that good tools
    make easy.

    If your product is wildly successful then you will need to
    recruit more people to get the job done. By definition you
    can't hire people with experience, so you have to spend time
    and energy training them. Plus it is unlikely that really
    good people will want to develop their career around
    something that has no transferable skills.

    But the key point is that /usually/ the combination of standard
    language plus DSLibrary gets /most/ of the benefits of a
    DSLanguage. Without the disadvantages of a DSLanguage.

    In addition, my experience is that a small DSLanguage for
    limited purposes usually grows "organically" like topsy until
    nobody understands it any more! ("Organically" is clearly a
    euphemism, of course). Unfortunately some mainstream
    standard languages also suffer from that problem :(
    Tom Gardner, Mar 23, 2014
  10. Nils M Holm

    Tom Gardner Guest

    Sounds close to the "executable requirements specification" concept.
    I have no problems with that.

    The main issue becomes ensuring the "executable requirements" are
    fully implemeted in the standard language plus DSLibrary. I know
    aerospace companies go to considerable trouble to develop/use
    special-purpose tools to ensure such tracability.

    Nagging doubt: the small, limited-purpose DSLanguages I've
    seen become successful all gradually evolved over the years
    until they were so large and complex that the designers
    didn't really understand them fully - let alone the people
    that used them.

    Naturally DSLibraries also tend to have that problem, but
    at least there should be good tool support for understanding

    Yeah, I'm getting old w.r.t. processor/memory capabilities.
    But my embedded projects have all had a significant soft
    or hard real-time or low-power elements to them.

    I'd need to glimpse the compelling advantages (vs standard
    language plus DSLibrary) of that before I invested my time.

    When it suits me I become a purist, so I'll claim that's merely
    hiding the dirty stuff under the carpet, pretending it isn't there.
    But it is there, so it cannot be strictly functional.
    Tom Gardner, Mar 23, 2014
  11. Nils M Holm

    David Brown Guest

    Just for your interest:

    I'd rather have a _ as a separator than ', but it could be worse -
    someone mentioned another language that uses $.
    Octal literals are intentional by design, but I too think that in most
    programs they are likely to be a mistake. The only common usage I know
    of them is for posix file modes. Personally, I would far rather see
    leading zeros ignored and 0o64 being used for "octal 64", or something
    akin to Ada like 8#64. But octal in C is well established - the best we
    can hope for is optional warning messages added to compilers.
    David Brown, Mar 23, 2014
  12. I've not yet seen anyone bring up the languages designed for various
    research purposes which are designed to explore new scenarios for
    which the current languages would be considered stale or ossified.

    A good example would be Wirth's research with the Oberon range of


    PS: And while I am thinking about Wirth, don't forget that Pascal was
    originally a small scale specialised language designed only for teaching
    before some people decided it was unique enough and brought enough new
    ideas to the table to be developed into a successful commercial language.
    Simon Clubley, Mar 23, 2014
  13. Nils M Holm

    Paul Rubin Guest

    There's not any really strict definition of functional programming
    that's widely accepted. I'd say Haskell does sweep some stuff under the
    carpet, but the way it does i/o is purely functional in the sense that
    the programs are written as state transformers on the external world.
    If you look closely at the type signature of the "print" function, its
    input is an abstract data value of type RealWorld (it's actually written
    that way) and its output is another such value. So given a real world
    that contains a printer on your desk and a blank sheet of paper in the
    printer, the function produces a new real world, where the formerly
    blank paper now has stuff printed on it. This allows various theorems
    about functional programs to keep working, which is what makes it pure.

    In the actual implementation, the i/o operations (such as printing) are
    done by the runtime environment, which is a separate entity from the
    (pure) program evaluator. Your program using the "print" function
    doesn't actually print anything. Instead it computes commands that it
    returns to the runtime environment, and the runtime environment prints
    stuff. It's not just a nomenclature thing--it has a fancy mathematical
    underpinning based on category theory, though programmers don't have to
    be directly concerned with that.

    This is an old but fairly readable explanation:


    This goes into more (practical) detail:

    Paul Rubin, Mar 23, 2014
  14. Nils M Holm

    Tom Gardner Guest

    I could probably make an argument along the lines that
    most DSLanguages effectively just that - but for a very
    narrow domain.

    Hardly a domain specific language!

    Yes indeed. Somewhere I still have the red/silver (2nd edition?)
    version of the language definition, probably from c1977.

    But it was never intended as a domain specific language either!
    Tom Gardner, Mar 24, 2014
  15. Nils M Holm

    Paul Rubin Guest

    Formalization (in the CompCert style) is still a very specialized topic
    that's completely outside the skillset of normal real-world embedded
    developers. Even places that use it (e.g. Galois Corp.) I get the
    impression that it's a separate area of responsibility than messing with
    day to day code resident inside devices.

    It's getting to be more accessible. The book "Software Foundations" is
    pretty readable and has good exercises:


    I haven't spent much time with it yet, but I want to.
    Paul Rubin, Mar 24, 2014

  16. In many years the only time I've had occasion to deal with octal
    constants in C or C++ is when an unwanted leading zero snuck in
    somewhere causing a problem. Usually when a table of numbers was
    copied from an outside source.

    A syntax like 0o123 would have been a far better choice (or an Ada
    -like "base#number"), but the leading-zero form of octal number
    specification predates hex ("0x") in C, so getting rid of it is likely
    to be impossible. It's even propagated into a number of other
    languages (Java, for example).

    Some lints can warn about any octal usage, but I've not noticed that
    feature on any compiler, except as part of MISRA checking (MISRA
    disallows octal constants and literals), but that's just too painful
    for most uses.
    Robert Wessel, Mar 24, 2014

  17. Underscore might be a bit better, but the single quote is perfectly
    workable, and even looks a bit like a comma.
    Robert Wessel, Mar 24, 2014
  18. Those are two points that I think are critical: tools and
    If your 'vaguely equivalent' predicate is applicable the semantic
    distance between the problem domain and the language/paradigm you have
    available is small. This of course votes heavily against anything
    specific, because the main advantage of something specific is that it
    can reduce (the cost associated with) that gap!

    Wouter van Ooijen, Mar 24, 2014
  19. Nils M Holm

    Paul Rubin Guest

    Maybe this is less of an issue with embedded DSL's, where you can
    use the features of the host language as well as the EDSL.
    I think it's fine to use gc'd languages for soft real time, in the
    typical case where deadlines are in the tens of msec and it's ok to miss
    one now and then. It's possible to keep GC latency at that level
    without using any fancy and inefficient methods. If you look at the
    literature on Erlang (which is basically a concurrent Lisp with
    Prolog-like syntax glued on), soft real time applications were the
    design target from the beginning. Hard real time applications
    (microsecond deadlines that must never be missed) are different, of
    The blurbs for ImProve and Atom at http://tomahawkins.org/ might
    interest you. ImProve uses an SMT solver to statically verify that your
    code meets assertions that you specify. I guess SPARK/Ada or some of
    the tools for the new Ada 2012 design-by-contract stuff does similar
    things. Atom transmogrifies your program from one that appears to be
    written with multiple concurrent tasks, into one with a single outer
    loop that (on a realtime cpu) spends the exact same number of cycles in
    each iteration regardless of the data. It gets rid of the need for
    locks, semaphores, etc. while doing all task scheduling statically at
    compile time.

    I found Atom to be reasonably easy to use, given that I was already
    familiar with Haskell. Haskell's learning curve is notoriously steep
    though, and it's not really possible to use Atom without at least some
    Haskell understanding.
    Paul Rubin, Mar 24, 2014
  20. Nils M Holm

    Tom Gardner Guest

    Possibly, but given my experience I'd like to see some!

    Agreed, and I've done just that for telecom systems running
    on a server. Some people didn't believe it was possible even
    when they saw it running.

    I'd have loved the opportunity to use Erlang. I first came
    across it in the late80s/early90s and thought its USPs were
    relevant and important to its market. But I'd find it a hard
    sell at board level, given the alternative ways of achieving
    the same objectives in more "traditional conservative"
    languages and environments. Shame.

    Interesting, but that might be too general purpose to
    be a DSLanguage!

    I'm not sure what a "real-time" CPU is anymore, unless
    you mean one that doesn't have any I/L caches!

    Nearest I've come across are the XMOS processors, where
    the IDE can tell you the real-time performance before
    the code is executed.

    The learning curve can't be ignored when you have
    to hire new bodies. Nor the desire that the bodies
    have to work on something which isn't seen as a
    "dead end" career path.
    Tom Gardner, Mar 24, 2014
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