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  Description:  Extended Open File
 
  Given an ASCIIZ pathname, opens, creates, or replaces a file in the
  designated or default directory on the designated or default disk drive.
  Returns a handle that can be used by the program for subsequent access
  to the file.
 
  Input                                       Output
  ════════════════════════════════            ════════════════════════════
  AH=6CH                                      If function successful
  AL=00H                                      Carry flag=clear
  BX=open mode                                AX=handle
     Bit(s)  Significance                     CX=action taken
     0-2     access type                         1=file existed and was
             000=read-only                         opened
             001=write-only                      2=file did not exist and
             010=read/write                        was created
     3       reserved (0)                     If function unsuccessful
                                              Carry flag=set
     Bit(s)  Significance (if set)            AX=error code
     4-6     sharing mode
             000=compatibility
             001=deny read/write
              (deny all)
             010=deny write
             011=deny read
             100 = deny none
     7       inheritance
  0=child process inherits handle
  1=child does not inherit handle
     8-12    reserved (0)
     13      critical error hand-
              ling
  0=execute Int 24H
  1=return error to process
     14      write-through
  0=writes may be buffered and
     deferred
  1=physical write at request time
     15      reserved (0)
  CX=file attribute (bits may be
   combined; ignored if open)
     Bit(s)  Significance (if set)
     0       read-only
     1       hidden
     2       system
     3       volume label
     4       reserved (0)
     5       archive
     6-15    reserved (0)
  DX=open flag
     Bits    Significance
     0-3     action if file exists
             0000 = fail
             0001 = open file
             0010 = replace file
     4-7     action if file
              doesn't exist
             0000 = fail
             0001 = create file
     8-15    reserved (0)
  DS:SI=segment:offset of ASCIIZ
   pathname
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