Another try at the run-from-Flash issue for MPC8xx/MBX/custom

[Markus Sundberg]

Doug Rogers <rogers at innocon.com> writes:

> I, too, would like to run from Flash. The question for me isn't speed,
> isn't cost, nor EMI effects. It is simply a matter of size. We have a
> very small area in which to squeeze the parts, and the best we could do
> is 32M RAM and 16M ROM. Our application code is in the 10M range (text
> only, even after chopping over half of it out!), but it is expected to
> grow. So for us it's not really a matter of cost or (running) speed,
> since the Flash runs fast enough for our purposes.
>
> As another constraint, our card needs to be able to run very soon after
> power-up. I've hacked the LynxOS startup monitor to detect either gzip'd
> or bzip2'd KDI's. I've timed the decompression of our smaller test image
> and it takes over 40 seconds on a 40MHz MBX860 board. That's too slow.
> We could live with 20, perhaps, but not much more than that. For now I
> boot my images over the network since our MBX board has only 4M Flash.
> It's faster than decompressing!
>
> With LynxOS, we're able to keep in Flash both the text sections of the
> kernel AND of all binaries on the filesystem. Wow! That allows us to put
> only the RAM disk and vector/data/bss sections into RAM. Shweet!
>
> I was hoping that Linux was to that point, but apparently not.

I haven't seen anyone say it couldn't be done. I wrote:

  Well, I see no reason why it shouldn't be possible, but not without
  modifying things.

  You'd need to link the .data, .data.cacheline_aligned, .data.init
  and .bss sections so they are located in RAM, and the former
  three plus the exception vectors would have to be copied from flash
  to their linked location.

I haven't tried it myself though.

//Marcus
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