INT 67 - VIDEMS.SYS v1.31+ - RETURN HIDDEN BLOCK SIZE                           
	AX = 2763h
	CL = 04h
	BX = signature AAFFh
Return: AH = 00h if successful
	    DX = block size in kilobytes
Desc:	Returns the amount of EMSsee Expanded Memory Specification which can be safely used at any time, and
	  can't be destroyed by writing anything to the B800:0000 buffer.
Note:	this function normally returns 184K, while the driver provides up to
	  240K of EMSsee Expanded Memory Specification.
SeeAlso: AX=2763h/CL=01h,AX=2763h/CL=02h