INT 15 - Rational Systems DOS/16M - INSTALLATION CHECK
        AX = BF02h
        DX = 0000h
Return: DX = nonzero if installed
            DX:SI -> XBRK structure (see below)
Note:   this function is also supported by DOS/4G
SeeAlso: AX=BFDCh,AX=BFDEh/BX=0000h

Format of 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