These files contain relocatable data addresses, but fixed code
addresses (instead of tokens). They are specific to the executable
(i.e., gforth file) they were created with. Also, they disable
dynamic native code generation (typically a factor of 2 in speed).
You get a data-relocatable image, if you pass the engine you want to
use through the GFORTHD environment variable to gforthmi
(see gforthmi), e.g.
GFORTHD="/usr/bin/gforth-fast --no-dynamic" gforthmi myimage.fi source.fs
Note that the --no-dynamic is required here for the image to
work (otherwise it will contain references to dynamically generated
code that is not saved in the image).