>  Another aproach would be to put the bootloader at the top of the code
and
> just jump straight to it from the reset vector, write your bootloader so
> that this address cannot be modified, as well as making sure your
bootloader
> can't be modified.  Then you have complete freedom with interrupt
routines,
> I would think is a better approach in terms of flexibility.

That's what I do on PICs.

Another wrinkle is you have to think about what happens if an upload is
interrupted in the middle or gets a data error.  A single flag only
distiguishes between never uploaded and uploaded at least once.  My
uploader computes the checksum of the uploaded code and passes that to the
bootloader at the end, which the bootloader saves.  On startup, control is
always given to the bootloader, which only runs the application firmware
if its checksum matches the stored value.  If not, the bootloader retains
control and waits for a new upload.


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