CBMFS

Revision as of 21:12, 20 May 2013

The CBM File System was used by disk drives made by Commodore Business Machines for the PET, VIC-20, C64 and C128 range of computers. These drives were called 'intelligent peripherals' meaning the DOS (Disk Operating System) was runs in the drive itself, not on the computer the drive was attached to.

The most popular CBM drive was the 1541, which supported 35 tracks using GCR encoding. The D64 format is most frequently used to capture 1541 disk images, it consists of a sector-for-sector copy of a 1540/1541 disk. 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 GEOS 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 G64 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.

(Content taken from D64.TXT in Peter Scheper's excellent compendium of C64 file format information).