GRUB 0.97 stage2/asm.S/prot_to_real undoes Big Real/Flat/Unreal Mode?

[Roger C. Pao (Enmotus)]

I'm writing an INT 13h Option ROM for an AHCI-like PCIe adapter.  I've
noticed that after prot_to_real is called, My INT 13h no longer has
access to addresses like es=0:edi=00067BB4.

How do I regain access to the AHCI BAR (ABAR), located above the 1 MB
line, now that GRUB put the CPU into 20-bit/1-MB real mode?  There are
no I/O ports on this card.

PCI Firmware 3.0 states the CPU will be in Big Real Mode and A20 will be
gated.  It was until GRUB changed the CPU mode.
-- 
rcpao.enmotus