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▀ Any normal, system, or hidden file with a matching name will be opened
  by this function. If the file is read-only, the success of the operation
  also depends on the access code in bits 0-2 of register AL. After opening
  the file, the file read/write pointer is set to offset zero (the first
  byte of the file).
 
▀ The function fails if:
  - any element of the pathname does not exist.
  - the file is opened with an access mode of read/write and the file has
    the read-only attribute.
  - [3.0+] SHARE.EXE is loaded and the file has already been opened by one
    or more other processes in a sharing mode that is incompatible with
    the current program's request.
 
▀ The file's date and time stamp can be accessed after a successful open
  call with Int 21H Function 57H.
 
▀ The file's attributes (hidden, system, read-only, or archive) can be
  obtained with Int 21H Function 43H.
 
▀ When a file handle is inherited by a child process or is duplicated with
  Int 21H Functions 45H or 46H, all sharing and access restrictions are
  also inherited.
 
▀ Only bits 0-2 of register AL are significant; the remaining bits should
  be zero for upward compatibility.
 
▀ [3.0+] Bit 3 should always be zero; bits 4-7 control access to the file
  by other programs. Bits 4-6 have no effect unless SHARE.EXE is loaded.
 
▀ [3.0+] A file-sharing error causes a critical-error exception (Int 24H)
  with an error code of 02H. Int 21H Function 59H can be used to obtain
  information about the sharing violation.
 
▀ [4.0+] Int 21H Function 6CH combines the services of Functions 3CH,
  3DH, and 5BH.
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