INT 21 - DOS 2+ - "CHMOD" - SET FILE ATTRIBUTES
        AX = 4301h
        CX = new file attributes (see below)
        DS:DX -> ASCIZ filename
Return: CF clear if successful
            AX destroyed
        CF set on error
            AX = error code (01h,02h,03h,05h) (see AH=59h)
Notes:  will not change volume label or directory attribute bits, but will
          change the other attribute bits of a directory (the directory
          bit must be cleared to successfully change the other attributes of a
          directory, but the directory will not be changed to a normal file as
          a result)
        MS-DOS 4.01 reportedly closes the file if it is currently open
        for security reasons, the Novell NetWare execute-only bit can never
          be cleared; the file must be deleted and recreated
        under the FlashTek X-32 DOS extender, the filename pointer is in DS:EDX
        DOS 5.0 SHARE will close the file if it is currently open in sharing-
          compatibility mode, otherwise a sharing violation critical error is
          generated if the file is currently open
SeeAlso: AX=4300h,AX=4311h,INT 2F/AX=110Eh

Bitfields for file attributes:
 bit 7  shareable (Novell NetWare)
 bit 6  unused
 bit 5  archive
 bit 4  directory
 bit 3  volume label
        execute-only (Novell NetWare)
 bit 2  system
 bit 1  hidden
 bit 0  read-only

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