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
	ECX = 00000000h or length of data in bytes
Return: CF clear if successful (address < 1MB and contiguous)
	    ECX = 32-bit real-mode MS-DOS address
	CF set on error (address >= 1MB or not contiguous)
	    ECX = linear address
Note:	this function is also supported by FlashTek X-32VM
SeeAlso: AX=2510h