Function 0FEh   Get Alternate Screen Buffer Address (text mode only) 
			                            (Topview/DesQview/Taskview) 
entry   AH     0FEh 
	ES:DI  segment:offset of assumed video buffer 
return: ES:DI  segment:offset of actual video buffer 
note 1) This alternate video buffer can be written to directly, in the same 
	manner as writing to address -B0000 or address -B8000. The MT 
        program will manage the actual display. 
     2) There is no need to synchronize vertical retrace when writing to the 
	alternate buffer; this is managed by the MT program 
     3) If TopView or DESQview is not running, ES:DI is returned unchanged. 
     4) TopView requires that function 0FFh be called every time you write into 
	the buffer to tell TopView that something changed