DOS version 3.0 added networking and file-locking capabilities to the DOS
file system.  The additional complications will normally affect only those
programs which expect to be used in a network environment.

                                           
   DOS 2.x (non-networking) Access Modes   
                                           
  00H = open for reading                   
  01H = open for writing                   
  02H = open for both reading and writing  
                                           

DOS 3.0+ is upwardly compatible with previous versions.  The Access/Open
Mode is treated as a set of bit-flags:

OpenModeRec
 7 6 5 4 3 2 1 0 
 i share 0  r/w  
                  bits mask
                  0-2: 03H Network Access Rights required for access
                            000 = read
                            001 = write
                            010 = read and write
                  4-6: 70H Sharing Mode:
                            000 = compatibility mode            00H
                            001 = deny read/write(exclusive)    10H
                            010 = deny write                    20H
                            011 = deny read                     30H
                            100 = deny none                     40H
                    7: 80H Inheritance:
1 = file is private to this process 80H
0 = inherited by spawned processes

Notes:   If the file attribute indicates Read-Only, then it overrides
these flags.

  The Network Access Rights and Sharing Mode bits have an effect
if the DOS SHARE program has been installed or other file
sharing support is installed.

  The DOS 4.0+ Fn 6cH uses this mode byte as the low-byte of an
Extended Open Mode word.  It provides flags that let you avoid
critical errors and force disk-caching write-through.

fn 3dH (open file)
fn 5cH (lock)
- -

Access Mode / Open Mode