diff options
| -rw-r--r-- | doc/fileformat.txt | 49 |
1 files changed, 36 insertions, 13 deletions
diff --git a/doc/fileformat.txt b/doc/fileformat.txt index 5c33f2a..7d87000 100644 --- a/doc/fileformat.txt +++ b/doc/fileformat.txt @@ -1,14 +1,36 @@ -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. +ALF Archive Structure +--------------------- + +An ALF archive is laid out almost exactly like an ARC archive that +only uses compression types 2 or greater: 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. +Header structure: + +Offset | Length | Description +-------+--------+------------------------------------------------------ +0 | 2 | ALF signature bytes: $1A $0F +2 | 13 | Filename (null-terminated) +15 | 4 | 32-bit compressed size (little-endian) +19 | 2 | File date in MS-DOS format (same as ARC) +21 | 2 | File time in MS-DOS format (same as ARC) +23 | 2 | Checksum (simple additivie, *not* a CRC) +25 | 4 | 32-bit original size (little-endian) +-------+--------+------------------------------------------------------ + +The compressed data for the file starts at offset 29. + 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). +- ALF files use $0F for the 'compression type' (offset 1), whereas + ARC files use compression types 1 through 8. + +- ALF always uses the 29-byte header; ARC uses 29-byte headers for + compression types >= 2, but only 25 bytes for type 1 (stored). - The actual compressed data is incompatible with any of the compression types supported by ARC. Although ALF uses an @@ -25,18 +47,17 @@ The differences are: a valid ALF archive... unless there's "junk at EOF" on the first file. -Other things caused by the limitations of the Atari: +- ARC uses CRC-16 for its checksums; ALF just adds the bytes together + and uses the low 16 bits of the result as the checksum. - 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). + an Atari DOS other than SpartaDOS (or, on SpartaDOS, but without + the R-Time 8 cartridge). -- 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). +- ARC and ALF are both limited to 12 character filenames, with a + null terminator. With ALF, any remaining bytes in the field after + the null will be set to $20 (ASCII spaces, *not* more nulls). - Atari filenames with no extensions (e.g. "FOO") are stored with a trailing period (e.g. "FOO.") in the ALF header. Upon extraction, @@ -55,3 +76,5 @@ Other things caused by the limitations of the Atari: maximum size for a single file is 16MB. I have actually tested the Atari ALF and UNALF programs with an emulator (and emulated hard drive) with a file of 200KB in size, and it worked fine. + +Author: B. Watson (urchlay@slackware.uk) |