On Sun, Aug 29, 2010 at 1:31 PM, Bert Freudenberg <bert@freudenbergs.de> wrote:

On 29.08.2010, at 22:22, Eliot Miranda wrote:

On Sun, Aug 29, 2010 at 12:56 PM, Ken G. Brown <kbrown@mac.com> wrote:
Running a fresh Squeak4.2-10382-alpha on your latest 5.8b4.

Open image, do Save As new version. then quit.
Attempt to open saved new version image on Squeak 4.2.4beta1U.app by dragging and dropping on VM, it does not open.
Console message:
10/08/29 1:51:57 PM     [0x0-0x2fe2fe].org.squeak.Squeak[8275]  This interpreter (vers. 6502) cannot read image file (vers. 6505).

John's  5.8b4 is a Cog VM.  Once an image is saved on Cog it will only run on a Cog VM.


I'm sure you mentioned it before, but where can I read about the image format changes?

Lazily I've yet to write this up.  But I think the only change above and beyond support for the closure bytecodes (and not using BlockContext at all) is that image float order now depends on platform, so on x86 it is little-endian; this was forced by the JIT implementation of floating-point arithmetic (its hard to byte swap efficiently given the lack of sophistication of the code generator), and by my not wanting to waste cycles converting to/from big-endian byte order on image load/save or image segment export.  Bit 1 of the image header flags word (which used to contain only the full screen flag) is 1 if the image's floats are little-endian.  So the existing VMs would need to read and write this bit and either convert back to big-endian or copy the Cog VMs in keeping floats in platform-specific order.  For me throwing performance away on each floating-point op on x86 is a heinous sin, so at least internally floats should be little-endian.  The basicAt: & basicAt:put: primitives 38 & 39 need to be implemented to present floats as big endian to the image level.

There are other changes to the image header, using unused bits and fields to store Cog-specific info and it would be convenient if the standard VMs preserve these fields.  But they don't prevent loading on a standard VM; IIRC only the float-order changes would cause errors running a Cog image on a closure-enabled Interpreter VM.

If people think this is important enough I could put together a change set for VMMaker that includes the relevant changes (to image load/save, floating-point arithmetic, image segment load/store and float at:/at:put:).


- Bert -