Now I have a 4.0 vm & image working nicely on my RISC OS Pi I need to move forward to more modern images and VMs. I find myslef a touch confused about what image format changes might have happened and when.

On my iMac it seems that I can run - 
OK 3.9 image on 3.8 vm
OK 3.9 image on 4.2 vm
NO 3.9 image on Cog
NO 4.1 image on 3.8 vm
OK 4.1 image on 4.2 vm
?? 4.1 image on cog (crashed after mostly starting up)
NO 4.3 image on 3.8 vm
OK 4.3 image on 4.2 vm
OK 4.3 image on cog 
OK scratch image (2.8 image, I think) on 3.8 vm
OK scratch image on 4.2 vm
NO scratch image on cog

Couldn't spot anything very noticeable on the sites I scanned. Pointer to history of these changes?

tim
--
tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim
"#define QUESTION ((bb) || !(bb))  - Shakespeare."