Studying the rest of GRUB's drivers for disk-partitioning on different computers...
The BSDs have different drivers sharing most of the same code, which in turn differs to MSDOS's driver with slight variations & a callback for iterating over a given sector. Except plain BSD skips the Microsoft components!
In any case the inner routine reads a header verifying a magic number, extracts a valid pointer, & reads partition headers at a certain stride handing reformatted data to a callback.
1/3?