Function  0Fh   Open Disk File                                            (FCB) 
      Searches current directory for specified filename and opens it 
entry   AH      0Fh 
	DS:DX   pointer to an unopened FCB 
return  AL      00h     if file found 
		0FFh    if file not not found 
note 1) If the drive code was 0 (default drive) it is changed to the actual  
	drive used (1=A:,2=B:,3=C:, etc). This allows changing the default drive 
	without interfering with subsequent operations on this file.  
     2) The current block field (FCB bytes C-D, offset 0Ch) is set to zero.  
     3) The size of the record to be worked with (FCB bytes E-F, offset 0Eh) is 
	set to the system default of 80h. The size of the file (offset 10h) and 
	the date (offset 14h) are set from information obtained in the root 
	directory. You can change the default value for the record size (FCB 
	bytes E-F) or set the random record size and/or current record field. 
	Perform these actions after the open but before any disk operations. 
     4) The file is opened in compatibility mode.  
     5) Microsoft recommends handle function call 3Dh be used instead. 
     6) This call is also used by the APPEND command in DOS 3.2+ 
     7) Before performing a sequential disk operation on the file, you must  
	set the Current Record field (offset 20h). Before performing a random  
	disk operation on the file, you must set the Relative Record field 
	(offset 21h). If the default record size of 128 bytes is incorrect, set 
	it to the correct value.