Folks -

First the good news:
We can now generate, from a single set of sources, four different VMs...

	One that will run a 32-bit image on a 32-bit host
		same old VM
	One that will run a 32-bit image on a 64-bit host
		needed to run on 64-bit machines
	One that will run a 64-bit image on a 32-bit host
		great for testing 64-bit VM on 32-bit machines
	One that will run a 64-bit image on a 64-bit host
		the thing we were after

... and all of them work!

Ian and I worked together on this, with Ian focused on issues of C translation rules, and me focused on properly generalizing all the occurrences of 4 (bytes per word), 2 (shift count), and lots of other things derived therefrom.  I also wrote a tracer subclass that will write images in a 64-bit format, and Ian also contributed the brilliant framework that enabled this four-way compatibility.


Now the fine print...
The 64-bit format is the simplest change that could work, not the simplest format that could work.  For those who understand the object current format, here is the change:

	All object pointers are 64 bits as you'd expect.

	SmallIntegers are still 31-bits, although stored sign-extended to 63 bits (plus tag bit).

	32-bit Object header:
	3 bits	reserved for gc (mark, old, dirty)
	12 bits	object hash (for HashSets)
	5 bits	compact class index
	4 bits	object format (includes low bits of size in bytes)
	6 bits	object size in 32-bit words
	2 bits	header type (0: 3-word, 1: 2-word, 2: forbidden, 3: 1-word)

	64-bit Object header:
	32 bits	all zero
	3 bits	reserved for gc (mark, old, dirty)
	12 bits	object hash (for HashSets)
	5 bits	compact class index
	4 bits	object format (includes low bits of size in bytes)
	5 bits	object size in 64-bit words
	1 bit	extra 4-bit of size in bytes or 32-bit words
	2 bits	header type (0: 3-word, 1: 2-word, 2: forbidden, 3: 1-word)

	The object size field had to be in units of 8 bytes, and therefore three
	low-order bits are required instead of the 2 that are in the current format field.
	I simply used the vacated bit of the word size field for the 3rd low bit.
	It's Byzantine, but it works with few changes.

We have only converted the core interpreter and 3 plugins (BitBlt, BalloonEngine and FilePlugin).  This is enough to run Squeak and test it.    Moreover we now have a lot of experience with the conversion process, and we will write down a set of guidelines for converting the remaining plugins. We may ask for help from the Squeak community in converting the remaining plugins, and also image segment code.  In some case there is clearly someone who knows the code much better than us, and for the rest, there may simply be others who want a complete 64-bit system.

We have not done *any* of the Version 4 changes.  The first reason is that we need to release this work soon before we get out of phase with other Squeak progress.  The second is some discussion in our group about the possibility of making *much* more major changes.  As soon as we get clearer about this and confirm our intention, we'll have more to say about it.  If were to go ahead with such a rethink, then any time spent of Version 4 would have been wasted.

We have not done any testing beyond the point of bringing up a morphic screen with Squeak mouse, opening a browser and running the benchmarks.  There is lots to test, and surely a number of bugs yet to be found.  The Squeak community can be of enormous help here.


Next steps...
Ian and I are both pretty busy, but it is our hope to test the build process some more, and put out a complete source tree with this work in the course of the next month.  At that point others can help us to convert the remaining plugins and to test the system as a whole.

	- Dan