I'd guess that one of the first stores banks in RAM or I/O or something at F800-FBFF at least.
The read-discard-store pattern on F890 and F891 is suggestive of them being IRQ status/enable registers.
Yes That is my assumption as well (remember my posts are a day or more back in my research )
The rest of the code looks like bit 6 of F892 is possibly VBL status, and that VBL IRQ is enabled by bit 4 of F891.
F88D is probably a watchdog and system control (ie, banking).
Yes I'm pretty sure the whole 100pinQFP/VDC control regs are memmapped as 128 bytes here [F880;F8FF] (subject of my future post)