INT 21 - Phar Lap 386/DOS-Extender - CONVERT PROTECTED-MODE ADDRESS TO MS-DOS
        AX = 250Fh
        ES:EBX = 48-bit protected-mode address to convert
Return: CF clear if successful (address < 1MB)
            ECX = 32-bit real-mode MS-DOS address
        CF set on error (address >= 1MB)
            ECX = linear address
Note:   this function is also supported by FlashTek X-32VM