    *** D64 (Electronic form of a physical 1541 disk)
    *** Document revision: 1.9
    *** Last updated: March 11, 2004
    *** Contributors/sources: Immers/Neufeld "Inside Commodore  DOS"
                              Wolfgang Moser

    (Note: This document will try to explain the layout of the 1541  disks,  as
    well as some of the files they contain. However, I do not explain  GEOS  or
    REL files here. See the file GEOS.TXT or REL.TXT for  more  information  on
    those file types or disk structure). Special thanks to Wolfgang  Moser  for
    his work on identifying the differences between the various 'Speeder' DOS's
    that exist. This information is included in a table later in this document.

      First and foremost we have D64, which is  basically  a  sector-for-sector
    copy of a 1540/1541 disk. There are several versions of these which I  will
    cover shortly. The standard D64 is a 174848 byte file comprised of 256 byte
    sectors arranged in 35 tracks with a varying number of  sectors  per  track
    for a total of 683 sectors. Track counting starts at 1, not 0, and goes  up
    to 35. Sector counting starts at 0, not 1, for the first sector,  therefore
    a track with 21 sectors will go from 0 to 20.

      The original media (a 5.25" disk) has the tracks  laid  out  in  circles,
    with track 1 on the very outside of the disk  (closest  to  the  sides)  to
    track 35 being on the inside of the disk (closest to the inner  hub  ring).
    Commodore, in their infinite wisdom, varied the number of sectors per track
    and data densities across the disk to optimize available storage, resulting
    in the chart below. It shows the sectors/track for a  standard  D64.  Since
    the outside diameter of a circle is  the  largest  (versus  closer  to  the
    center), the outside tracks have the largest amount of storage.

            Track   Sectors/track   # Sectors   Storage in Bytes
            -----   -------------   ---------   ----------------
             1-17        21            357           7820
            18-24        19            133           7170
            25-30        18            108           6300
            31-40(*)     17             85           6020
                                       ---
                                       683 (for a 35 track image)

      Track #Sect #SectorsIn D64 Offset   Track #Sect #SectorsIn D64 Offset
      ----- ----- ---------- ----------   ----- ----- ---------- ----------
        1     21       0       $00000      21     19     414       $19E00
        2     21      21       $01500      22     19     433       $1B100
        3     21      42       $02A00      23     19     452       $1C400
        4     21      63       $03F00      24     19     471       $1D700
        5     21      84       $05400      25     18     490       $1EA00
        6     21     105       $06900      26     18     508       $1FC00
        7     21     126       $07E00      27     18     526       $20E00
        8     21     147       $09300      28     18     544       $22000
        9     21     168       $0A800      29     18     562       $23200
       10     21     189       $0BD00      30     18     580       $24400
       11     21     210       $0D200      31     17     598       $25600
       12     21     231       $0E700      32     17     615       $26700
       13     21     252       $0FC00      33     17     632       $27800
       14     21     273       $11100      34     17     649       $28900
       15     21     294       $12600      35     17     666       $29A00
       16     21     315       $13B00      36(*)  17     683       $2AB00
       17     21     336       $15000      37(*)  17     700       $2BC00
       18     19     357       $16500      38(*)  17     717       $2CD00
       19     19     376       $17800      39(*)  17     734       $2DE00
       20     19     395       $18B00      40(*)  17     751       $2EF00

      (*)Tracks 36-40 apply to 40-track images only

      The directory track should be contained totally on track 18. Sectors 1-18
    contain the entries and sector 0 contains the BAM (Block Availability  Map)
    and disk name/ID. Since the directory is only 18 sectors large (19 less one
    for the BAM), and each sector can contain only  8  entries  (32  bytes  per
    entry), the maximum number of directory entries is 18 * 8 = 144. The  first
    directory sector is always 18/1, even though the t/s pointer at 18/0 (first
    two bytes) might point somewhere else.  It  then  follows  the  same  chain
    structure as a normal file, using a sector interleave of 3. This makes  the
    chain links go 18/1, 18/4, 18/7 etc.

    Note that you can extend the directory off  of  track  18,  but  only  when
    reading the disk or image. Attempting to write to a directory sector not on
    track 18 will cause directory corruption.

    Each directory sector has the following layout (18/1 partial dump):

    00: 12 04 81 11 00 4E 41 4D 45 53 20 26 20 50 4F 53 <- notice the T/S link
    10: 49 54 A0 A0 A0 00 00 00 00 00 00 00 00 00 15 00 <- to 18/4 ($12/$04)
    20: 00 00 84 11 02 41 44 44 49 54 49 4F 4E 41 4C 20 <- and how its not here
    30: 49 4E 46 4F A0 11 0C FE 00 00 00 00 00 00 61 01 <- ($00/$00)

      The first two bytes of the sector ($12/$04) indicate the location of  the
    next track/sector of the directory (18/4). If the track is set to $00, then
    it is the last sector of the directory. It is possible,  however  unlikely,
    that the directory may *not* be competely on track 18 (some disks do  exist
    like this). Just follow the chain anyhow.

      When the directory is done, the track value will be $00. The sector  link
    should contain a value of $FF, meaning the whole sector is  allocated,  but
    the actual value doesn't matter. The drive will return  all  the  available
    entries anyways. This is a breakdown of a  standard  directory  sector  and
    entry:

      Bytes: $00-1F: First directory entry
              00-01: Track/Sector location of next directory sector ($00 $00 if
                     not the first entry in the sector)
                 02: File type.
                     Typical values for this location are:
                       $00 - Scratched (deleted file entry)
                        80 - DEL
                        81 - SEQ
                        82 - PRG
                        83 - USR
                        84 - REL
                     Bit 0-3: The actual filetype
                              000 (0) - DEL
                              001 (1) - SEQ
                              010 (2) - PRG
                              011 (3) - USR
                              100 (4) - REL
                              Values 5-15 are illegal, but if used will produce
                              very strange results. The 1541 is inconsistent in
                              how it treats these bits. Some routines use all 4
                              bits, others ignore bit 3,  resulting  in  values
                              from 0-7.
                     Bit   4: Not used
                     Bit   5: Used only during SAVE-@ replacement
                     Bit   6: Locked flag (Set produces ">" locked files)
                     Bit   7: Closed flag  (Not  set  produces  "*", or "splat"
                              files)
              03-04: Track/sector location of first sector of file
              05-14: 16 character filename (in PETASCII, padded with $A0)
              15-16: Track/Sector location of first side-sector block (REL file
                     only)
                 17: REL file record length (REL file only, max. value 254)
              18-1D: Unused (except with GEOS disks)
              1E-1F: File size in sectors, low/high byte  order  ($1E+$1F*256).
                     The approx. filesize in bytes is <= #sectors * 254
              20-3F: Second dir entry. From now on the first two bytes of  each
                     entry in this sector  should  be  $00  $00,  as  they  are
                     unused.
              40-5F: Third dir entry
              60-7F: Fourth dir entry
              80-9F: Fifth dir entry
              A0-BF: Sixth dir entry
              C0-DF: Seventh dir entry
              E0-FF: Eighth dir entry

    Files, on a standard 1541, are stored using an interleave of 10. Assuming a
    starting track/sector of 17/0, the chain  would  run  17/0,  17/10,  17/20,
    17/8, 17/18, etc.

    Note: No GEOS entries are listed in the above description. See the GEOS.TXT
          file for GEOS info.

    The layout of the BAM area (sector 18/0) is a bit more complicated...

        00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
        -----------------------------------------------
    00: 12 01 41 00 12 FF F9 17 15 FF FF 1F 15 FF FF 1F
    10: 15 FF FF 1F 12 FF F9 17 00 00 00 00 00 00 00 00
    20: 00 00 00 00 0E FF 74 03 15 FF FF 1F 15 FF FF 1F
    30: 0E 3F FC 11 07 E1 80 01 15 FF FF 1F 15 FF FF 1F
    40: 15 FF FF 1F 15 FF FF 1F 0D C0 FF 07 13 FF FF 07
    50: 13 FF FF 07 11 FF CF 07 13 FF FF 07 12 7F FF 07
    60: 13 FF FF 07 0A 75 55 01 00 00 00 00 00 00 00 00
    70: 00 00 00 00 00 00 00 00 01 08 00 00 03 02 48 00
    80: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01
    90: 53 48 41 52 45 57 41 52 45 20 31 20 20 A0 A0 A0
    A0: A0 A0 56 54 A0 32 41 A0 A0 A0 A0 00 00 00 00 00
    B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

      Bytes:$00-01: Track/Sector location of the first directory sector (should
                    be set to 18/1 but it doesn't matter, and don't trust  what
                    is there, always go to 18/1 for first directory entry)
                02: Disk DOS version type (see note below)
                      $41 ("A")
                03: Unused
             04-8F: BAM entries for each track, in groups  of  four  bytes  per
                    track, starting on track 1 (see below for more details)
             90-9F: Disk Name (padded with $A0)
             A0-A1: Filled with $A0
             A2-A3: Disk ID
                A4: Usually $A0
             A5-A6: DOS type, usually "2A"
             A7-AA: Filled with $A0
             AB-FF: Normally unused ($00), except for 40 track extended format,
                    see the following two entries:
             AC-BF: DOLPHIN DOS track 36-40 BAM entries (only for 40 track)
             C0-D3: SPEED DOS track 36-40 BAM entries (only for 40 track)

    Note: The BAM entries for SPEED, DOLPHIN and  ProLogic  DOS  use  the  same
          layout as standard BAM entries.

      One of the interesting things from the BAM sector is the byte  at  offset
    $02, the DOS version byte. If it is set to anything other than $41 or  $00,
    then we have what is called "soft write protection". Any attempt  to  write
    to the disk will return the "DOS Version" error code 73  ,"CBM  DOS  V  2.6
    1541". The 1541 is simply telling  you  that  it  thinks  the  disk  format
    version is incorrect. This message will normally come  up  when  you  first
    turn on the 1541 and read the error channel. If you write a $00  or  a  $41
    into 1541 memory location $00FF (for device 0),  then  you  can  circumvent
    this type of write-protection, and change the DOS version back to  what  it
    should be.

      The BAM entries require a bit (no pun intended) more of a breakdown. Take
    the first entry at bytes $04-$07 ($12 $FF $F9 $17). The first byte ($12) is
    the number of free sectors on that track. Since we are looking at the track
    1 entry, this means it has 18 (decimal) free sectors. The next three  bytes
    represent the bitmap of which sectors are used/free. Since it is 3 bytes (8
    bits/byte) we have 24 bits of storage. Remember that at  most,  each  track
    only has 21 sectors, so there are a few unused bits.

       Bytes: 04-07: 12 FF F9 17   Track 1 BAM
              08-0B: 15 FF FF FF   Track 2 BAM
              0C-0F: 15 FF FF 1F   Track 3 BAM
              ...
              8C-8F: 11 FF FF 01   Track 35 BAM

      These entries must be viewed in binary to make any sense. We will use the
    first entry (track 1) at bytes 04-07:

         FF=11111111, F9=11111001, 17=00010111

    In order to make any sense from the binary notation, flip the bits around.

                       111111 11112222
            01234567 89012345 67890123
            --------------------------
            11111111 10011111 11101000
            ^                     ^
        sector 0              sector 20

      Since we are on the first track, we have 21 sectors, and only use  up  to
    the bit 20 position. If a bit is on (1), the  sector  is  free.  Therefore,
    track 1 has sectors 9,10 and 19 used, all the rest are free.  Any  leftover
    bits that refer to sectors that don't exist, like bits 21-23 in  the  above
    example, are set to allocated.

      Each filetype has its own unique properties, but most follow  one  simple
    structure. The first file sector is pointed to by the directory and follows
    a t/s chain, until the track value reaches  $00.  When  this  happens,  the
    value in the sector link location indicates how much of the sector is used.
    For example, the following chain indicates a file 6 sectors long, and  ends
    when we encounter the $00/$34 chain. At this point the last sector occupies
    from bytes $02-$34.

        1       2       3       4       5       6
      ----    -----   -----   -----   -----   -----
      17/0    17/10   17/20   17/1    17/11    0/52
     (11/00) (11/0A) (11/14) (11/01) (11/0B)  (0/34)

    ---------------------------------------------------------------------------

    *** Variations on the D64 layout

    These are some variations of the D64 layout:

      1. Standard 35 track layout but with 683 error bytes added on to the  end
         of the file. Each byte of the  error  info  corresponds  to  a  single
         sector stored in the D64, indicating if the  sector  on  the  original
         disk contained an error. The first byte is for track 1/0, and the last
         byte is for track 35/16.

      2. A 40 track layout, following the same layout as a 35 track  disk,  but
         with 5 extra tracks. These contain 17 sectors each, like tracks 31-35.
         Some of the PC utilities do allow you to create and  work  with  these
         files. This can also have error bytes attached like variant #1.

      3. The Commodore 128 allowed for "auto-boot" disks. With  this,  t/s  1/0
         holds a specific byte sequence which the computer recognizes  as  boot
         code. See the document C128BOOT.TXT for more info.

      Below is a small chart detailing the standard file sizes of  D64  images,
    35 or 40 tracks, with or without error bytes.

         Disk type                  Size
         ---------                  ------
         35 track, no errors        174848
         35 track, 683 error bytes  175531
         40 track, no errors        196608
         40 track, 768 error bytes  197376

      The following table (provided by Wolfgang Moser) outlines the differences
    between the standard 1541 DOS and the various "speeder" DOS's  that  exist.
    The 'header 7/8' category is the 'fill bytes' as  the  end  of  the  sector
    header of  a  real  1541  disk  See  the  document  G64.TXT  for  a  better
    explanation of these bytes.

            Disk format             |Tracks|Header 7/8|Dos type|DiskDos
                                    |      |allsechdrs|        |vs.type
           ============================================================
            Original CBM DOS v2.6   |  35  | $0f  $0f |  "2A"  |$41/'A'
           ------------------------------------------------------------
            SpeedDOS+               |  40  | $0f  $0f |  "2A"  |$41/'A'
           ------------------------------------------------------------
            ProfessionalDOS Initial |  35  | $0f  $0f |  "2A"  |$41/'A'
              (Version 1/Prototype) |  40  | $0f  $0f |  "2A"  |$41/'A'
           ------------------------------------------------------------
            ProfDOS Release         |  40  | $0f  $0f |  "4A"  |$41/'A'
           ------------------------------------------------------------
            Dolphin-DOS 2.0/3.0     |  35  | $0f  $0f |  "2A"  |$41/'A'
            Dolphin-DOS 2.0/3.0     |  40  | $0d  $0f |  "2A"  |$41/'A'
           ------------------------------------------------------------
            PrologicDOS 1541        |  35  | $0f  $0f |  "2A"  |$41/'A'
            PrologicDOS 1541        |  40  | $0f  $0f |  "2P"  |$50/'P'
            ProSpeed 1571 2.0       |  35  | $0f  $0f |  "2A"  |$41/'A'
            ProSpeed 1571 2.0       |  40  | $0f  $0f |  "2P"  |$50/'P'
           ------------------------------------------------------------

      The location of the extra BAM information in sector 18/0,  for  40  track
    images, will be different depending on what standard the  disks  have  been
    formatted with. SPEED DOS stores them from $C0 to $D3, DOLPHIN  DOS  stores
    them from $AC to $BF and PrologicDOS stored them right after  the  existing
    BAM entries from $90-A3. PrologicDOS also  moves  the  disk  label  and  ID
    forward from the standard location of $90 to $A4. 64COPY and Star Commander
    let you select from several different types of extended  disk  formats  you
    want to create/work with.

      All three of the speeder DOS's mentioned above don't alter  the  standard
    sector interleave of 10 for files and 3 for directories. The reason is that
    they use a memory cache installed in the drive which reads the entire track
    in one pass. This alleviates the need for custom interleave values. They do
    seem to alter the algorithm that finds the next available  free  sector  so
    that the interleave value can deviate from 10 under certain  circumstances,
    but I don't know why they would bother.

      Below is a HEX dump of a Speed DOS BAM sector. Note the location of the
    extra BAM info from $C0-D3.

          00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F       ASCII
          -----------------------------------------------  ----------------
    0070: 12 FF FF 03 12 FF FF 03 12 FF FF 03 11 FF FF 01  ????????????????
    0080: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01  ????????????????
    0090: A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0  ????????????????
    00A0: A0 A0 30 30 A0 32 41 A0 A0 A0 A0 00 00 00 00 00  ??00?2A?????????
    00B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ????????????????
    00C0: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01  ????????????????
    00D0: 11 FF FF 01 00 00 00 00 00 00 00 00 00 00 00 00  ????????????????

      Below is a HEX dump of a Dolphin DOS BAM sector. Note the location of the
    extra BAM info from $AC-BF.

          00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F       ASCII
          -----------------------------------------------  ----------------
    0070: 12 FF FF 03 12 FF FF 03 12 FF FF 03 11 FF FF 01  ????????????????
    0080: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01  ????????????????
    0090: A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0  ????????????????
    00A0: A0 A0 30 30 A0 32 41 A0 A0 A0 A0 00 11 FF FF 01  ??00?2A?????????
    00B0: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01  ????????????????
    00C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ????????????????
    00D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ????????????????

      Below is a HEX dump of a PrologicDOS BAM sector. Note that the disk  name
    and ID are now located at $A4 instead of starting at $90.

          00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F       ASCII
          -----------------------------------------------  ----------------
    0070: 12 FF FF 03 12 FF FF 03 12 FF FF 03 11 FF FF 01  ????????????????
    0080: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01  ????????????????
    0090: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01  ????????????????
    00A0: 11 FF FF 01 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0  ????????????????
    00B0: A0 A0 A0 A0 A0 A0 30 30 A0 32 50 A0 A0 A0 A0 00  ??????00?2P?????
    00C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ????????????????
    00ED: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ????????????????

    ---------------------------------------------------------------------------

      Here is the meaning of the error bytes added onto the end of any extended
    D64. The CODE is the same as that generated by the 1541 drive controller...
    it reports these numbers, not the error code we usually see when  an  error
    occurs.

      Some of what comes  below  is  taken  from  Immers/Neufeld  book  "Inside
    Commodore DOS". Note the descriptions are not  completely  accurate  as  to
    what the drive DOS is actually doing to seek/read/decode/write sectors, but
    serve as simple examples only. The "type" field is where the error  usually
    occurs, whether it's searching for any SYNC mark, any header ID, any  valid
    header, or reading a sector.

      These first errors are "seek" errors, where the disk controller is simply
    reading headers and looking at descriptor bytes, checksums, format ID's and
    reporting what errors it sees. These errors do *not* necessarily  apply  to
    the exact sector being looked for. This fact  makes  duplication  of  these
    errors very unreliable.

        Code  Error  Type   1541 error description
        ----  -----  ----   ------------------------------
         03    21    Seek   No SYNC sequence found.

                            Each  sector  data  block  and  header  block   are
                            preceeded by SYNC marks. If *no* sync  sequence  is
                            found within 20 milliseconds (only ~1/10 of a  disk
                            rotation!) then this error is generated. This error
                            used to mean the entire track is bad, but  it  does
                            not have to be the case. Only a small area  of  the
                            track needs to be without  a  SYNC  mark  and  this
                            error will be generated.

                            Converting this error to a D64 is very  problematic
                            because it depends on where the physical head is on
                            the disk when a read attempt is made. If it  is  on
                            valid header/sectors then it  won't  occur.  If  it
                            happens over an area without SYNC  marks,  it  will
                            happen.

         02    20    Seek   Header descriptor byte not found (HEX $08, GCR $52)

                            Each sector is preceeded by an  8-byte  GCR  header
                            block, which starts with the value  $52  (GCR).  If
                            this value is not found  after  90  attempts,  this
                            error is generated.

                            Basically, what a track  has  is  SYNC  marks,  and
                            possibly valid data blocks,  but  no  valid  header
                            descriptors.

         09    27    Seek   Checksum error in header block

                            The  header  block  contains  a   checksum   value,
                            calculated by XOR'ing the TRACK,  SECTOR,  ID1  and
                            ID2 values. If this checksum is wrong,  this  error
                            is generated.

         0B    29    Seek   Disk sector ID mismatch

                            The ID's from the header  block  of  the  currently
                            read sector are compared against the ones from  the
                            low-level header of 18/0. If there is  a  mismatch,
                            this error is generated.

         02    20    Seek   Header block not found

                            This error can be reported again when searching for
                            the correct header block. An image of the header is
                            built and searched for, but not found after 90 read
                            attempts.  Note  the  difference  from  the   first
                            occurance. The first one only searches for a  valid
                            ID, not the whole header.

      Note that error 20 occurs twice during this phase. The first time is when
    a header ID is being searched for, the second is  when  the  proper  header
    pattern for the sector being searched for is not found.

      From this point on, all the errors apply to the specific sector  you  are
    looking for. If a read passed all the previous checks, then we are  at  the
    sector being searched for.

      Note that the entire sector is read before  these  errors  are  detected.
    Therefore the data, checksum and off bytes are available.

        Code  Error  Type   1541 error description
        ----  -----  ----   ------------------------------
         04    22    Read   Data descriptor byte not found (HEX $07, GCR $55)

                            Each sector data block is preceeded  by  the  value
                            $07, the "data block" descriptor. If this value  is
                            not there, this error is  generated.  Each  encoded
                            sector  has  actually  260  bytes.  First  is   the
                            descriptor byte, then  follows  the  256  bytes  of
                            data, a checksum, and two "off" bytes.

         05    23    Read   Checksum error in data block

                            The checksum of  the  data  read  of  the  disk  is
                            calculated, and compared against the one stored  at
                            the end of the sector. If  there's  a  discrepancy,
                            this error is generated.

         0F    74    Read   Drive Not Ready (no disk in drive or no device 1)

      These errors only  apply  when  writing  to  a  disk.  I  don't  see  the
    usefulness of having these as they cannot be present when only *reading*  a
    disk.

        Code  Error  Type   1541 error description
        ----  -----  ----   ------------------------------
         06    24    Write  Write verify (on format)

         07    25    Write  Write verify error

                            Once the GCR-encoded sector  is  written  out,  the
                            drive waits for the sector to come around again and
                            verifies the whole 325-byte GCR block.  Any  errors
                            encountered will generate this error.

         08    26    Write  Write protect on

                            Self explanatory. Remove the write-protect tab, and
                            try again.

         0A    28    Write  Write error

                            In actual fact, this error never occurs, but it  is
                            included for completeness.

      This is not an error at all, but when gets reported when the  read  of  a
    sector is ok.

        Code  Error  Type   1541 error description
        ----  -----  ----   ------------------------------
         01    00    N/A    No error.

                            Self explanatory. No errors were  detected  in  the
                            reading and decoding of the sector.

      The advantage with using the 35 track D64  format,  regardless  of  error
    bytes, is that it can be converted directly back to a 1541 disk  by  either
    using the proper cable and software on the PC, or send it down to  the  C64
    and writing it back to a 1541. It is the best documented format since it is
    also native to the C64, with many books explaining the disk layout and  the
    internals of the 1541.

    ---------------------------------------------------------------------------

    What it takes to support D64:

      The D64 layout is reasonably robust,  being  that  it  is  an  electronic
    representation of a physical 1541  disk.  It  shares  *most*  of  the  1541
    attributes and it supports all file formats, since all C64 files came  from
    here. The only file I have found that can't be copied to a D64 is a T64 FRZ
    (FRoZen files), since you lose the extra file type attribute.

      Since the D64 layout seems to be an exact byte copy of a 1541 floppy,  it
    would appear to be the perfect format for *any* emulator. However, it  does
    not contain certain vital bits of information that, as a user, you normally
    don't have access to.

      Preceeding each sector on a real  1541  disk  is  a  header  block  which
    contains the sector ID bytes and checksum. From the  information  contained
    in the header, the drive determines if there's  an  error  on  that  header
    (27-checksum error, 29-disk ID mismatch). The sector itself  also  contains
    info (data block signature, checksum) that result in  error  detection  (23
    checksum, 22 data block not present, etc). The error bytes had to be  added
    on to the D64 image, "extending"  the  format  to  take  into  account  the
    missing info.

      The disk ID is important in the copy protection of  some  programs.  Some
    programs fail to work properly since the D64 doesn't  contain  these  ID's.
    These bytes would be an addition to the format which has  never  been  done
    and would be difficult to do. (As an aside, the  4-pack  ZipCode  files  do
    contain the original master disk ID, but these are lost in  the  conversion
    of a ZipCode to a D64. Only storing *one* of the ID's is  not  enough,  all
    the sector ID's should be kept.)

      The extended track 1541 disks also presented  a  problem,  as  there  are
    several different formats (and how/where to store the extra BAM entries  in
    a sector that was not designed for  them,  yet  still  remain  compatible).
    Because of the additions to the format (error bytes and  40  tracks)  there
    exists 4 different types of D64's, all recognizeable by their size.

      It is also the only format that uses the sector count for the  file  size
    rather than  actual  bytes  used.  This  can  present  some  problems  when
    converting/copying the to another format because you may have to  know  the
    size before you begin (see LBR format).

      It also contains no consistent signature, useful for recognizing  if  D64
    is really what it claims to be. In order to determine if a file is  a  D64,
    you must check the file size.

    ---------------------------------------------------------------------------

    Overall Good/Bad of D64 Files:

      Good
      ----
      * D64 files are the most widely supported and well-defined format, as  it
        is simply an electronic version of a 1541 disk

      * Supports *all* filenames, even those with $00's in them

      * Filenames are padded with the standard $A0 character

      * Supports REL and all GEOS files

      * Allows complete directory customization

      * Because it is a random-access  device,  it  supports  fast-loaders  and
        random sector access

      * PC Cluster slack-space loss is minimized since the  file  is  a  larger
        fixed size

      * Has a label (description) field

      * Format extentible to allow for 40-track disks

      * With  the  inclusion  of  error  bytes,  you  have  support  for  basic
        copy-protection

      * Files on a disk can easily be re-written, as  long  as  there  is  free
        blocks

      Bad
      ---
      * The format doesn't contain *all* the info from the 1541 disk (no sector
        header info like  ID  bytes,  checksums).  This  renders  some  of  the
        original special-loaders and copy-protection useless.

      * You don't *really* know the file size of the  contained  C64  files  in
        bytes, only blocks

      * It can't store C64s FRZ files due to FRZ files needing a  special  flag
        that a D64 can't store. This is by no means a big problem.

      * It is not an expandable filetype, like LNX or T64

      * Unless most of the space on a D64 disk is used,  you  do  end  up  with
        wasted space

      * Directory limited to 144 files maximum

      * Cannot have loadable files with the same names

      * Has no recognizeable file signature (unlike most  other  formats).  The
        only reliable way to know if a file is a D64 is by its size

      * It is too easy for people to muck up the standard layout

      * It is much more difficult to support  fully,  as  you  really  need  to
        emulate the 1541 DOS (sector interleave, REL files, GEOS VLIR files)