Format of DOS/16M XBRK structure:
Offset	Size	Description	)
 00h	DWORD	linear address of first available byte
 04h	DWORD	linear address of last available byte + 1 ???
 08h	DWORD	real-mode address of XBRK structure???
 0Ch	DWORD	???
 10h  2 BYTEs	???
 12h	WORD	segment of ???
 14h  8 BYTEs	???
 1Ch 512 BYTEs	protected-mode IDT
21Ch  N BYTEs	protected-mode GDT