Re: Mars Rover Controlled By Java

From: Harry Conover (hhc314_at_yahoo.com)
Date: 01/22/04

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    Date: 22 Jan 2004 09:47:22 -0800

    Jan C. Vorbrüggen <jvorbrueggen@mediasec.de> wrote in message news:<400F7DA7.8891E58C@mediasec.de>...

    Sorry Jan, I couldn't locate the original source of the text you
    quoted and to which I am responding.

    > > d) There is no reason to believe a real time system cannot be virtual
    > > machine based. It is technicaly feasible and all the benefits of seperating
    > > code from hardware/OS are just as relevant on embedded systems as they are
    > > on desktops. Just look at cell phones.

    I believe that accuracy of this statement depends entirely on the
    degree of reaction latency that can be tollerated in the real time
    system. Even at today's fantastic execution rates, critical real-time
    must occur within only a few machine execution cycles.

    The execution latency penalty imposed by my concept of a virtual
    machine will, at least in my mind, knock it from consideration for
    anything approaching that which is normally required for critical
    real-time performance. Here often anything greater than a few
    processor instruction cycle latency leads to a non-recoverable error
    situation.

    This is precisely why interrupt level service routines, fast device
    drivers, and similar time-critical operations are generally
    implemented in assembly or other low-level code optimized to minimize
    the number and duration of required machine instruction executions.

    With today's focus on high-level programming languages and software
    implemented virtual machines, many people lose sight of the very
    fundamental differences that exist between software that simply
    executes quickly and real-time system software where interrupt level
    response processing must be assured to be alway less than some
    critical latency limit. Couple this with the fact that in most
    real-time control and operating systems, these latency requirements
    are often only a few microseconds and the need for very tightly coded
    software becomes obvious.

    For an example of real-time OS design, compare the kernel
    implementation in something like Wind River's VxWorks RTOS vs. that of
    a non-real-time OS such as early Unix or Microsoft Windows.
    Particularly note the fact the the task scheduler in a real-time OS of
    any reasonable complexity will be both dynamic priority weighted and
    perform pre-emptive scheduling because you can't have a slow process
    (like I/O) hogging the system and locking out tasks that need to
    execute in milliseconds or microseconds.

    The above and other considerations in real-time are what cause me to
    believe that the utility of a virtual machine so very unrealistic for
    RTOS use.

                                                         Harry C.

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