[Vm-dev] minor image version numbers?

Eliot Miranda eliot.miranda at gmail.com
Fri Jan 16 19:55:40 UTC 2015

Hi David, Hi All,

    here's an example of an occasional problem.  I'm looking at the free
tree in Spur and I have an unused field in each free chunk that is a
remnant of an old attempt at a compaction algorithm I've long abandoned.
I'd like to eliminate the field (2nd slot in a free chunk) but that means
either breaking the loading of existing Spur images or some version marker
in the image header to indicate whether the image contains old format free
chunks or new format.  This is the kind of internal micro-change that I
don't think warrants taking a bit from the image version field.

However, it /would/ be nice to have a field there-in for a minor version
number that the VM implementor could simply increment when they want to
make such a minor change.  What do you think?  Currently the unused bits in
the image format are

ImageFormat availableBits printStringBase: 2

(the top bit is reserved for an extension bit).

So we could use the top 7 bits for a 0-127 field that will probably last me
for a long time, and it would be easy to mask it out of the version field.

An alternative is for me to use some other field in the Spur image header.
I'm happy to do that if the above scheme smells to anyone.  There's a
16-bit field in Cog image headers I can use (see the2ndUnknownShort in
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