On Tue, Jun 22, 2010 at 6:52 PM, David T. Lewis lewis@mail.msen.com wrote:

Does the Cog VM update to a new image format version number when snapshotting?

Yes it does. It and the Stack VM set the image format to 6505. This reflects the platform-order floats change which is not backwards-compatible.

Note that Cog also insists on a 6504 or 6505 format image to start-u, i.e. it won;t attempt to start-up non-closure images.

If not, then I have the following proposal:

An image that is snapshotted by a Cog VM is going to expect Cog support when next loaded. It would be good if the image snapshot declares this explicitly so the VM can exit in a controlled manner if unable to provide the required support.

I propose that we add a Cog bit to the image format version number, as illustrated in the attached change set. This can be extended in the future such that an image that requires "foo" support from a VM will set the "foo bit".

The original Squeak images had image format number 6502. With closure support, this became 6504. Requiring Cog implies a requirement for closure support, so an image that requires Cog support would be 6505. For 64-bit images, the corresponding image format numbers are 68000, 68002, and 68003. Note that bit 0 was previously unused, which means that Cog would become the recipient of the highly-coveted bit zero position ;)

:) Looks like I accidentally chose the right bit :)

There are plenty of free bits available to be allocated for future capabilities, and the high order bit would be reserved for expansion if the need ever arises.

Dave