7.3.5) What should I know about filenames and filename extensions?

Atari disk operating systems generally support file specifications in the form
of a filename of 1 to 8 characters, with an optional filename extension of
zero to 3 characters.

Valid characters for use in filenames/filename extensions:
(varies somewhat by DOS version)
- The letters 'A' to 'Z'
- The digits '0' to '9'
   (Many DOS versions do not permit filenames to start with a digit.)
- The underscore character ('_') (DOS XE,SpartaDOS,RealDOS)
- The "at" symbol ('@') (DOS XE)
Spaces and other characters are generally not permitted.

Filenames, including filename extensions, are generally arbitrary, but file
naming conventions can be helpful.  The most common method is to reserve
specific extensions for certain types of files.  The following list contains
some of the more commonly used extensions and their typical corresponding file
types.

Filenames reserved by some DOS versions for files to be automatically run at
system startup are listed here as well.

File formats are binary unless described below as "text" where:
- Binary format files are made up of a sequence of bytes of any value from
   0-255.
- Text format files (technically, a special case of binary files) are
   limited to printable/displayable alpha-numeric characters and symbols,
   organized into lines.

.ACT  Action! program source code
.ALF  AlfCrunch archive file containing one or more compressed files
.AMP  Antic Music Processor file
.AMS  Advanced MusicSystem file or Advanced MusicSystem II file
.ARC  Archive file containing one or more compressed files, compatible with
      the multi-platform SEA ARC format.  On the Atari, best supported by
      SuperARC/SuperUnArc or the SpartaDOS X ARC external command.
.ASM  Assembler Editor program source code, LISTed.  ATASCII text.
.ASC  ASCII text file
.ATA  ATASCII text file
.BAS  BASIC SAVEd program.  Less common: .SAV
.BAT  Batch file of DOS commands (DOS XE,SpartaDOS,RealDOS).  ATASCII text.
       - AUTOEXEC.BAT batch file runs at system boot
         (DOS XE,SpartaDOS,RealDOS)
.BIN  File transferred using CompuServe CIS 'A' Protocol.  Fully supported on
      the Atari by TSCOPE/MSCOPE/RSCOPE; also download support by Express! 3.x
      (filenaming convention used on CompuServe before February 1987)
.BXL  BASIC XL SAVEd program
.BXE  BASIC XE SAVEd program
.CMD  Batch file of DOS Commands (XDOS).  ATASCII text.
.COM  DOS external Command (DOS 2.5,DOS XE,SpartaDOS,RealDOS,XDOS)
.CTB  Compiled Turbo-BASIC XL program
.DAT  Data file (typically created/used by another program)
.DCM  Disk Communicator 3.2 disk image file.  Less common: .DSK or .DC3
.DOC  Documentation text file (perhaps ASCII or ATASCII)
.DOS  DOS system file (SpartaDOS,RealDOS)
.EXE  Executable ('Load and Go') machine language (object code) program
.HLP  Help text file (perhaps ASCII or ATASCII)
.LGO  LOGO SAVEd workspace file.  ATASCII text.
.LST  LISTed BASIC program.  ATASCII text.  Less common: .LIS
.M65  MAC/65 SAVEd program source code
.MIC  Micro-Painter picture file ("uncompressed"; 62 sectors under DOS 2.0S).
      Format, minus color data, also supported by Illustrator versions using
      the filename PICTURE and [Insert] to save file or [Clear] to load file.
.MUS  Music Composer file
.OBJ  Object code (machine language) file (not 'Load and Go')
.OBX  Modern naming convention for an object code (machine language) file (not
      'Load and Go').  Used to discourage modern computers from thinking that
      Atari .OBJ files might be MS-DOS/MS-Windows object code files.
.PIC  Micro Illustrator/KoalaPainter/AtariArtist "compressed" picture
.PLT  PILOT SAVEd program.  ATASCII text.
.PRN  Text file formatted for copying to Printer (perhaps ASCII)
.SAV  File named MEM.SAV may be employed by DOS to preserve the contents of
      memory to disk when DUP.SYS (DOS 2.0,DOS 2.5) or KCP.SYS (DOS 3) is
      loaded into memory.
.SCP  SpartaDOS SCOPY disk image file.  Used with SCOPY, an external command
      distributed with SpartaDOS 3.2.
.SFX  Self-Extracting archive file.  Created with the CTH SFX program.
.SYS  DOS System file or driver
       - AUTO.SYS can be used to automatically poke data in RAM locations on
         system startup (DOS I)
       - AUTORUN.SYS binary file is launched on system boot
         (DOS 2.0,DOS 3,DOS 2.5,RealDOS,XDOS)
       - AUTORUN.SYS batch file of DOS commands runs at system boot (XDOS)
.TUR  Turbo-BASIC XL SAVEd program.  Less common: .TBS
.TXT  Text file (perhaps ATASCII or ASCII)
.UTL  A DOS external utility program (DOS 3)
.XEX  Modern naming convention for an executable ('Load and Go') machine
      language (object code) program.  Used to discourage modern computers
      from thinking that Atari .EXE files might be MS-DOS/MS-Windows
      executable programs.
.XMO  Binary file, uploaded/download using XMODEM file transfer protocol
      (filenaming convention used on CompuServe before February 1987)

Additional file extensions associated with Atari picture formats are listed at
the website for RECOIL (Retro Computer Image Library), a viewer of pictures in
native formats of the Atari 8-bit (and other platforms) for modern computers:
http://recoil.sourceforge.net/

Additional file extensions associated with Atari music formats are listed at
the website for ASAP (Another Slight Atari Player), a player of Atari 8-bit
music for modern computers: http://asap.sourceforge.net/

File extensions associated with Atari disk image, cassette image, or cartridge
ROM formats normally used with Atari emulators, rather than on the Atari
itself, are described elsewhere in this FAQ list.
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