Function 0Fh	Write sector buffer			     (XT, Portable)
entry   AH      0Fh  
	AL      number of sectors 
	CH      cylinder (bits 8,9 in top two bits of CL) 
	CL      sector number 
	DH      head number 
	DL      drive number 
	ES:BX   pointer to buffer 
return  CF      set if error 
		AH      status code 
	AL      number of sectors actually transferred 
note 1) Should be called before formatting to initialize the controller's 
	sector buffer. 
     2) Used for diagnostics only on PS/2 systems