An ALF archive is laid out exactly like an ARC version 2 or greater
archive, with a 29-byte header for each file, followed by the
compressed data, followed by either EOF or the next file's header.

See the file Arcinfo for the original ARC file format. For ALF files,
"Byte 2: Compression version" will always be $0F.

The differences are:

- ALF files use $0F for the 'compression type' (Byte 1), whereas
  ARC files use compression types 2 through 8 (or 1, for ARC v1).

- The actual compressed data is incompatible with any of the
  compression types supported by ARC. Although ALF uses an
  implementation of Lempel-Zev, it's not the same implementation
  as any of the ones that ARC uses.

- For ARC, the last file's compressed data is followed by a 0 byte
  (in place of the $1A header), to signal "end of archive". For
  ALF, there's no data after the last byte of the last compressed
  file;

Other things caused by the limitations of the Atari:

- Not really a file format difference, but the dates stored inside
  ALF files might be wrong or gibberish, if they were created on
  an Atari OS other than SpartaDOS (or, on SpartaDOS, but without
  the time set correctly).

- Atari filenames are generally limited to 12 characters, e.g.
  PROGRAM1.BAS, so the filename field (Bytes 3-15, 13 bytes long)
  will never be completely filled. ALF uses a null (0) byte for
  the filename terminator, and any remaining bytes in the field
  will be set to $20 (ASCII spaces, *not* more nulls).

- ALF files are never embedded inside a self-extracting executable,
  so the first file's header always starts at the first byte of
  the file.

- ARC and ALF both store the compressed and uncompressed file lengths
  as 32-bit unsigned integers... but for ALF, the maximum file size
  that can be compressed is probably under 64KB, so both the lengths
  should have their last 2 bytes set fo 0. I haven't yet attempted
  to compress a file larger than 64KB with ALF on the Atari, so I
  may be wrong...