Re: "Non-volatile" OS ?

On Mon, 29 Aug 2005 03:06:51 +0100, "Steve at fivetrees"
<steve@xxxxxxxxxxxxxxxxxxxxx> wrote:

>"David R Brooks" <davebXXX@xxxxxxxxxxxx> wrote in message
>news:5s9vg1hfn632quuc2ijf1kj63ifpul3pgg@xxxxxxxxxx
>>I built such a system many years ago, on a smaller scale: Z80 with
>> 64kB. Running a proprietary OS, similar to CP/M.
>>
>> The key was to define two additional device driver functions:
>> "emergency stop" & "resume".
>
>Similarly, I've used a warm-start system that looks like:
> - on failing power, save context, and shut-down cleanly
> - on start-up and stable power, re-initialise hardware, then restore
>context if possible (validated etc); else do hard restart

Many PDP-11 and VAX processors had the option of supplying the DRAM
from a battery and since the operating systems dated from the core
memory days, they could handle the power failures quite nicely as long
as the battery power lasted.

>With a general-purpose RTOS, if you're trying to keep the RTOS task states
>during power-down, it's a different ballgame, and I'm not convinced it's
>either desirable or necessary as a general OS feature - the *actual*
>physical context may be entirely different when the system wakes up,
>seconds, hours, years later.

It depends how the real time clock behaves. As long as the I/O
requests timeout immediately after restoration of power, the operation
can be retried immediately.

For control loops, it might be a good idea e.g. to reinitialize the
PID-controllers, but you would need this feature anyway to handle an
intermittent sensor failure. Thus, it would be enough for the sensor
driver to report "sensor failure" for a few cycles after power-up,
which would also reinitialize the PID controller.

Paul

.

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