DiskCopy 4.2 format specification

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File format for Apple disk copy 4.2 definition, v0.99
by Jonathan Gevaryahu AKA Lord Nightmare
Much information comes from the CiderPress and MiniVmac source codes.
More authoritave information comes from http://www.nulib.com/library/FTN.e00005.htm
This is also used in DiskCopy 4.1. DiskCopy 6.3.3 uses a version with tags omitted.
DART is a variant which adds compression.

offset      type/size    contents
0x00        byte         Length of image name string ('Pascal name length'); technically this is part of the 0x01-0x3f area, as pascal strings apparently store their length as their first byte. Effectively the address of the last non-NUL byte of the image name. bytes after the end address are ignored, and sometimes are used to hold extra information, or hold garbage (in the case of the 1.44mb system 6.0.8 star
tup disk, this is leftovers in memory from the System Additions disk, so the string ends up being System Startupns)
 Note: A special (bug) case happens when the disk name is "-not a Macintosh disk", which the name is set to when using dc42 on non-mac diskettes. In that case, the length is set one byte too high; this is probably a bug 
in dc42 that was just never fixed.
0x01-0x3f   63 bytes     Image name, in ascii, padded with NULs
0x40-0x43   BE_UINT32    Data size in bytes (of block starting at 0x54)
 This has one of 4 values on most diskettes:
 00 06 40 00 (409600 bytes) for 400k GCR disks 00 0c 80 00 (819200 bytes) for 800k GCR disks
 00 0b 40 00 (737280 bytes) for 720k MFM disks
 00 16 80 00 (1474560 bytes) for 1440k MFM disks
0x44-0x47   BE_UINT32    Tag size in bytes (of block starting after end of Data block)
 This is typically 12 bytes for every 512 bytes in the image, apparently stored in the header of each sector on the media. The format for Tag data appears to be 3 BE_UINT32 words per 512 byte sector. It is supposedly ver
y important on Lisa Diskettes (according to lisaem).
 00 00 25 80 (9600 bytes) for 400k disks w/tags
 00 00 4b 00 (19200 bytes) for 800k disks w/tags
 00 00 00 00 for diskettes with no tags
0x48-0x4b   BE_UINT32    Data checksum, Big Endian
	0x4c-0x4F   BE_UINT32    Tag checksum, Big Endian
 The algorithm for this is the same as for the data, however the first 12 bytes of the tag section, if present, are skipped (probably due to a bug in an older disk copy version and kept for compatibility). Is 00 00 00 00
 if no tag.
0x50        byte         Disk encoding
 00 = GCR CLV ssdd (400k)
 01 = GCR CLV dsdd (800k)
 02 = MFM CAV dsdd (720k)
 03 = MFM CAV dshd (1440k)
 Other encodings may exist, for instance GCR HD format?.
0x51        byte         Format Byte
 This byte has one of two meanings, depending on whether the disk is GCR format 400k or 800k, or MFM format. The byte is completely ignored for GCR-on-HD (which has a 1:1 interleave and is always 2 sided):
    If disk is GCR format 400k or 800k:
        This byte is a copy of the GCR format byte which appears in the headers of every GCR sector.
        $02 = Mac 400k
        $12 = (documentation error claims this is for mac 400k disks, but this is wrong)
        $22 = Disk formatted as Mac 800k
        $24 = Disk formatted as Prodos 800k (AppleII format)
        $96 = INVALID (Disk was misformatted or had gcr fill (0x96 which represents data of 0x00) written to its format byte)
          Values for bitfield:
          |||\\\\\- These 5 bits are sector interleave factor (400k and 800k only; GCR HD disks ignore this):
          |||            setting of 02 means 2:1 interleave: 0  8  1 9  2 10 3 11 4 12 5  13 6  14 7  15
          |||            setting of 04 means 4:1 interleave: 0  4  8 12 1 5  9 13 2 6  10 14 3  7  11 15
          ||\------ This bit indicates whether a disk is 2 sided or not. 0 = 1 sided, 1 = 2 sided.
          |\------- unknown
          \-------- unknown
    If disk is MFM format:
        Interleave is ALWAYS 1:1 for these formats.
        $22 = doublesided MFM diskettes with 512 byte sectors
          Values for bitfield:
          |||\\\\\- These 5 bits are sector size as a multiple of 256 bytes
          |||       i.e. 02 = 2*256 = 512 bytes per sector
          ||\------ This bit indicates whether a disk is 2 sided or not. 0 = 1 sided, 1 = 2 sided.
          |\------- unknown
          \-------- unknown
0x52-0x53   BE_UINT16    '0x01 0x00' ('Private Word')
 This is more or less the 'magic number' of a DC42 format file; if it is not $01 $00, it is not a DC42 format file.
0x54-...    variable     Image data
 size = value at 0x40-0x43
...-EOF     variable     Tag data
 size = value at 0x44-0x47
 The actual format of the tag data is unknown. It is supposedly very important on Lisa Diskettes (according to lisaem).