INT 21 - Phar Lap 386/DOS-Extender - GET REAL-MODE LINK INFORMATION
        AX = 250Dh
Return: CF clear
        EAX = CS:IP of real-mode callback procedure that will call through
                from real mode to a protected-mode routine
        EBX = 32-bit real-mode address of intermode call data buffer
        ECX = size in bytes of intermode call data buffer
        ES:EDX = protected-mode address of intermode call data buffer
Notes:  this function is also supported by FlashTek X-32VM
        X-32VM guarantees the intermode buffer to be at least 4 KB

Call real-mode callback with:
        STACK:  DWORD   offset to protected-mode code
                WORD    placeholder for protected-mode CS
                DWORD   pointer to selector structure (see below)
                        or 0000h:0000h for defaults
                var     parameters for protected-mode procedure
Return: via FAR return

Format of selector structure:
Offset  Size    Description
 00h    WORD    protected-mode GS selector
 02h    WORD    protected-mode FS selector
 04h    WORD    protected-mode ES selector
 06h    WORD    protected-mode DS selector