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VIM variables: $VIM

$VIM belongs to the three special $ variables, the other two being $HOME and VIMRUNTIME.
If the environment variable VIM is not defined, $VIM might be assigned a value that is dependent on the value of exe_name, at least if running on Windows, that is.
Another fallback for $VIM seems to be the value of the global variable default_vim_dir, see vim_getenv().
If vim determined the value for $VIMRUNTIME, it also sets the value of didset_vim to TRUE.

See also

$VIMRUNTIME

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