mmm not sure. Compile the VM in 64 bits is not that complicated. Problem is that some plugins do not work with 64 bits VM, like FFI.
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I think we already tried. Maybe it was using SystemTracer. For example, here is a 64 bits image: https://gforge.inria.fr/frs/download.php/26274/PharoCore64-1.1-11196-UNSTABLE.zip

Notice that when using VM + image in 64 bits right now, we are NOT taking advantage of all its 64 bits. I mean, you may be able to make it work out of the box in a 64 bits machine/OS but that doesn't mean that the VM + image is using really using the 64. Maybe 32 bits of them are zero ;)���
Anyway, at some point in the future, we will have real 64 bits vm, a new object format/header, etc. But not sure it will be "soon".

Cheers