Steps to Modularity - Incremental Snapshots

Tim Rowledge rowledge at interval.com
Wed Mar 17 22:31:53 UTC 1999


I did some work not unrelated to this many years ago when working with the
Active Book Company. They had hardware that was an 8(!)MHz ARM-2 (no cache at
all!) with 1Mb ram and 1Mb rom. We got a full ST-80 (based on Eliot's BrouHaHa
VM, along with lots of my work) running on it.
The part relevant to this thread was getting the image into rom - and out agina
as needed!

Since BHH was OT based, it was pretty simple to
a) copy the OT into ram at startup
b) change all the places (not that many in fact) where an object could be
overwritten to move it to ram on write
c) during a snapshot, compare the current live OT to the ROM version and derive
a delta. The delta was the part written back to disc on the development
machines (real active books didn't do snapshot) and typically were only a few
10's or maybe a 100kb long.  This was a big deal in the days when I only had a
20Mb harddisc...

I had dreams of making it possible to keep parts of the image in separate files
and load them on demand through some faulting mechanism, but never got around
to it. Sounds like Dan might have found a way to provide it at last.

tim

-- 
When all else fails, let a = 7.  If that doesn't help, then read the manual.
Tim Rowledge:  rowledge at interval.com (w)  +1 (650) 842-6110 (w)
 tim at sumeru.stanford.edu (h)  <http://sumeru.stanford.edu/tim>





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