td0notes.txt

                                    Teledisk

                               Image File Format

                                     Notes





                                  PRELIMINARY




















                                 Dave Dunfield
                                 April 2, 2007

                          Last revised: July 28, 2008



                              Teledisk .TD0 notes

                               TABLE OF CONTENTS


                                                                         Page

     1. Introduction                                                        1

        1.1 Acknowlegements                                                 1

     2. Image file Format                                                   2


     3. Image Header                                                        2

        3.1 Signature (2 bytes)                                             2
        3.2 Sequence (1 byte)                                               2
        3.3 CheckSequence (1 byte)                                          3
        3.4 Teledisk version (1 byte)                                       3
        3.5 Data rate (1 byte)                                              3
        3.6 Drive type (1 byte)                                             3
        3.7 Stepping (1 byte)                                               3
        3.8 DOS allocation flag (1 byte)                                    4
        3.9 Sides (1 byte)                                                  4
        3.10 Cyclic Redundancy Check (2 bytes)                              4

     4. Comment Header / Data block                                         4

        4.1 Cyclic Redundancy Check (2 bytes)                               4
        4.2 Data length (2 bytes)                                           4
        4.3 Year (1 byte)                                                   5
        4.4 Month (1 byte)                                                  5
        4.5 Day (1 byte)                                                    5
        4.6 Hour, Minite, Second (1 byte each)                              5
        4.7 Comment data block (variable size)                              5

     5. Track Header                                                        6

        5.1 Number of sectors (1 byte)                                      6
        5.2 Cylinder number (1 byte)                                        6
        5.3 Side/Head number (1 byte)                                       6
        5.4 Cyclic Redundancy Check (1 byte)                                6

     6. Sector Header                                                       7

        6.1 Cylinder number (1 byte)                                        7
        6.2 Side/Head (1 byte)                                              7
        6.3 Sector number (1 byte)                                          7
        6.4 Sector size (1 byte)                                            8
        6.5 Flags                                                           8
        6.6 Cyclic Redundancy Check (1 byte)                                8

     7. Sector Data Header                                                  9

    Teledisk .TD0 notes                                    Table of Contents

                                                                         Page
        7.1 Data block size (2 bytes)                                       9
        7.2 Encoding method (1 byte)                                        9
    Teledisk .TD0 notes                                              Page: 1


    1. Introduction

       Teledisk is a program which reads non-PC format diskettes into  image
       files for archival and can later recreate a copy of the original disk
       from the image file.  Once popular for archival of  classic  computer
       software,  Teledisk has  been  withdrawn  from  the  market  by  it's
       manufacturer and is no longer  legally  available.  This  presents  a
       problem for those  people  who  have  data  archived  with  Teledisk,
       because the file format is proprietary and not  documented  rendering
       the data useless without the program.

       In my development of ImageDisk  (a replacement for Teledisk),  I have
       created a utility to convert Teledisk .TD0 images into ImageDisk .IMD
       format - in doing so,  I have researched the  Teledisk  format,  read
       other peoples notes,  done some reverse engineering myself,  and come
       to what I  believe  is  a  somewhat  complete  understanding  of  the
       contents of a .TD0 image file.

       This document presents my notes  on  the  Teledisk  .TD0  image  file
       format.

       This document is a work in progress.  If you have any information  to
       add,  corrections etc.  Please contact me.  I can be reached via  the
       contact information on my web site.

                       http://www.classiccmp.org/dunfield

       1.1 Acknowlegements

          The following people whom I have never met have saved me  tons  of
          time by  making  the  results  of  their  related  efforts  freely
          available.

        Will Krantz    - Wrote a program called wteledisk which converts
                         TX50 .TD0 into binary files for an emulator.
                         Published a web page with his notes and source.

        Sergey Erokhin - Provided more details for Wills web page.

        Simon Owen     - Published source code to read some .TD0 files
                         for his SimCoupe emulator.

        Haruhika       - Published LZSS-Huffman source code which has
        Okumura          become "the reference" for many implementations.
    Teledisk .TD0 notes                                              Page: 2


    2. Image file Format

       The overall disk image file has this format:

        Image header                (12 bytes)
        ;If the image was created using "Advanced Compression", everything
        ;below this line is compressed with LZSS-Huffman encoding.
        Optional comment header     (10 bytes)
        Optional comment data       (Variable size)
          ;For each track on the disk ...
          Track header              (4 bytes)
            ;For each sector within the track
            Sector header           (6 bytes)
            Optional data header    (3 bytes)
            Optional data block     (variable size)
        ;Image ends with Trackheader beginning with 255 (FF hex)

       If the Teledisk image was generated with "Advanced Data Compression",
       all parts of the file following  "Image Header"  are compressed as  a
       single block with LZSS-Huffman encoding with the string lookup buffer
       preset to all spaces  (ASCII  20).  With  "normal"  compression,  the
       remainder of the file  is  not  compressed/encoded  (other  than  the
       sector RLE compression).

    3. Image Header

       The image header describes global information about the  disk  image.
       It is never compressed, and is laid out in the following format:

        Signature                   (2 bytes)
        Sequence                    (1 byte)
        Checksequence               (1 byte)
        Teledisk version            (1 byte)
        Data rate                   (1 byte)
        Drive type                  (1 byte)
        Stepping                    (1 byte)
        DOS allocation flag         (1 byte)
        Sides                       (1 byte)
        Cyclic Redundancy Check     (2 bytes)

       3.1 Signature (2 bytes)

          - Contains 'TD' for normal compression

          - Contains 'td' for advanced compression

       3.2 Sequence (1 byte)

        - Early Teledisk document indicates that this  begins  with  00  and
          increments for each member of a multi-volume set.

        - TDCHECK reports  "bad header"  if this value is  set  to  anything
          other than 00.
    Teledisk .TD0 notes                                              Page: 3


       3.3 CheckSequence (1 byte)

        - Early Teledisk document indicates that this must be the  same  for
          each member of a multi-volume set

       3.4 Teledisk version (1 byte)

        - Encodes the version number of the Teledisk program  which  created
          the image in the form High-nibble.low-nibble. eg: 15 = 1.5

       3.5 Data rate (1 byte)

        - Encodes the data rate used for the diskette in lower 2 bits.

             0 = 250kbps
             1 = 300kbps
             2 = 500kbps

        - High bit indicates single-density diskette  (I believe this is for
          older versions only which did not support mixed density disks).

       3.6 Drive type (1 byte)

        - Indicates the type of drive the disk was made on.

        - Early Teledisk document indicates the encoding is:

             1 = 360k
             2 = 1.2M
             3 = 720k
             4 = 1.44M

        - Experimentation with TDCHECK indicates  that  the  text  generated
          from the various encoding is:

             0 = 5.25 96 tpi disk in 48 tpi drive
             1 = 5.25
             2 = 5.25 48-tpi
             3 = 3.5
             4 = 3.5
             5 = 8-inch
             6 = 3.5

        - Use Data rate to determine appropriate drive density.

       3.7 Stepping (1 byte)

        - Encodes step type in lower 2 bits

             0 = Single-Step
             1 = Double-step
             2 = Even-only step (96 tpi disk in 48 tpi drive)

        - High bit indicates presence of optional comment block
    Teledisk .TD0 notes                                              Page: 4


       3.8 DOS allocation flag (1 byte)

        - non-zero means the disk was read using the DOS FAT table  to  skip
          unallocted sectors.

       3.9 Sides (1 byte)

        - Encodes the number of sides read from the disk.

             01 = One
             anything-else = Two

       3.10 Cyclic Redundancy Check (2 bytes)

          This field contains the error-checking cyclic redundancy check for
          the header calculated with the polynomial value 41111  (A097  hex)
          using an input preset value of 0.  The CRC is calculated over  the
          first 10 bytes of the header, and should match the value stored in
          this field.

    4. Comment Header / Data block

       The comment block encodes an ASCII comment as well  as  the  creation
       date.  It's presence is indicated by the high bit of  the  "Stepping"
       field in the image header being set.

       When present,  it occurs immediately after the Image  header  in  the
       following format:

        Cyclic Redundancy Check     (2 bytes)
        Data length                 (2 bytes)
        Year since 1900             (1 byte)
        Month                       (1 byte)
        Day                         (1 byte)
        Hour                        (1 byte)
        Minite                      (1 byte)
        Second                      (1 byte)

       Following the comment header are comment line records,  consisting of
       ASCII text terminated by NUL (00) bytes.

       4.1 Cyclic Redundancy Check (2 bytes)

          This 16-bit field contains the  error-checking  cyclic  redundancy
          check for the header calculated with the  polynomial  value  41111
          (A097 hex) using an input preset value of 0. The CRC is calculated
          over the entire header block  (beginning at offset 2 - just  after
          the CRC) and the data records.

       4.2 Data length (2 bytes)

          This is the length of the comment data  block  which  follows  the
          comment header.  To display the comment data, read and output this
          many bytes following the header,  translating NUL (00)  bytes into
          newline sequences.
    Teledisk .TD0 notes                                              Page: 5


       4.3 Year (1 byte)

          Gives the year the image was created as an offset from  1900.  eg:
          2007 is encoded as 2007 - 1900 = 107 (6B hex).

       4.4 Month (1 byte)

          Gives the month the image was created  using  a  zero  index.  ie:
          0=January, 11=December.

       4.5 Day (1 byte)

          Gives the day  (of the month)  the image was created using a range
          of 1-31.

       4.6 Hour, Minite, Second (1 byte each)

          Gives the time of day the image was created using 24-hour time.

       4.7 Comment data block (variable size)

          Contains the ASCII text of the  comment  as  NUL  (00)  terminated
          lines.  The size of this block is given by  "Data length"  in  the
          comment header.  To display the comment,  read  and  output  "Data
          length bytes"  from this block,  translating NUL  (00)  bytes into
          newline sequences.
    Teledisk .TD0 notes                                              Page: 6


    5. Track Header

       Every disk track recorded in  the  image  will  begin  with  a  track
       header, which has the following format:

        Number of sectors           (1 byte)
        Cylinder number             (1 byte)
        Side/Head number            (1 byte)
        Cyclic Redundancy Check     (1 byte)

       5.1 Number of sectors (1 byte)

          This field indicates how many sectors are recorded for this track.
          This also indicates how many sector headers  to  expect  following
          the track header.

          A number of sectors of 255 (FF hex) indicates the end of the track
          list.  No other fields occur in this record,  and the CRC  is  not
          checked.

       5.2 Cylinder number (1 byte)

          This field encodes the physical cylinder  number  (head  position)
          for this track, in a range of 0-(#tracks on drive-1).

       5.3 Side/Head number (1 byte)

          This field encodes the disk side  (0 or 1)  that this track occurs
          on in it's lower bit.

          The high bit of this field is  used  to  indicate  the  track  was
          recorded in single-density.  This allows mixed-density disks to be
          represented (FM on some tracks, and MFM on others).

          FM disks that I recorded had this bit set for every track, and NOT
          the FM indicator in bit 7 of the  "Data rate"  field of the  image
          header.  I cannot confirm this,  but I suspect that early versions
          of Teledisk did not support mixed density disks, using only the FM
          bit in the image header.  If this is the case, then a track should
          be interpreted as single density if either of the two FM indicator
          bits are set.

       5.4 Cyclic Redundancy Check (1 byte)

          This  8-bit  field  contains  the   lower   byte   of   a   16-bit
          error-checking cyclic redundancy check for the  header  calculated
          with the polynomial value 41111  (A097 hex)  using an input preset
          value of 0.  The CRC is calculated over the first three  bytes  of
          the header and should match the forth byte.

       Track headers and sector block lists occur until all  tracks  on  the
       disk have been accounted for.  When the last track record and  sector
       block list has been read,  a 255  (FF hex)  byte indicates the end of
       the image.
    Teledisk .TD0 notes                                              Page: 7


    6. Sector Header

       Following the  Track  header  will  be  a  number  of  sector  blocks
       consisting of a sector header and optional  data  header/data  block.
       The number of sector blocks is indicated by the  "Number of  sectors"
       field in the track header.

       Each sector header has the following format:

        Cylinder number             (1 byte)
        Side/Head                   (1 byte)
        Sector number               (1 byte)
        Sector size                 (1 byte)
        Flags                       (1 byte)
        Cyclic Redundancy Check     (1 byte)

       6.1 Cylinder number (1 byte)

          This field indicates the logical cylinder number which is  written
          in the ID field of the disk  sector.  For  most  disk  formats  it
          matches the Cylinder number indicated in the track header, however
          this  does  NOT  have  to  be  the  case  -  some  formats  encode
          non-physical cylinder numbers.

       6.2 Side/Head (1 byte)

          This field indicates the  logical  Side/Head  indicator  which  is
          written in the ID field of the disk sector.  For most disk formats
          it matches the Side/Head number indicated  in  the  track  header,
          however this does NOT have to be the case -  some  formats  encode
          non-physical Side/Head numbers.

       6.3 Sector number (1 byte)

          This field indicates the logical sector number which is wrtten  in
          the ID field of the disk sector.  Sector numbers do not have to be
          in any particular order  (the ordering of the  sectors  determines
          the interleave factor of the track), do not necessarily begin at 0
          or 1, and are not necessarily an unbroken series of numbers.  Some
          formats encode seemingly arbitrary sector numbers.

          Teledisk sometimes creates bogus sectors headers to describe  data
          that is not in a properly formatted sector.  These  extra  sectors
          appear to be created with sector numbers begining at 100.
    Teledisk .TD0 notes                                              Page: 8


       6.4 Sector size (1 byte)

          Indicates the size of  the  sector,  according  to  the  following
          table:

            0 = 128 bytes           4 = 2048 bytes
            1 = 256 bytes           5 = 4096 bytes
            2 = 512 bytes           6 = 8192 bytes
            3 = 1024 bytes

          Note that disk formats exist which  have  different  sector  sizes
          within the same track,  and Teledisk will encode  them  this  way,
          however the PC 765  floppy  disk  controller  cannot  format  such
          tracks, and the disk can not be recreated.

       6.5 Flags

          This is a bit field indicating characteristics that Teledisk noted
          about the sector when it  was  recorded.  The  field  contain  the
          logical OR of the following byte values:

             01 = Sector was duplicated within a track
             02 = Sector was read with a CRC error
             04 = Sector has a "deleted-data" address mark
             10 = Sector data was skipped based on DOS allocation [note]
             20 = Sector had an ID field but not data [note]
             40 = Sector had data but no ID field (bogus header)

          note:  Bit values 20 or 10 indicate  that  NO  SECTOR  DATA  BLOCK
          FOLLOWS.

          The meaning of some of these bits was taken  from  early  Teledisk
          documentation,  and may not be accurate - For example,  I've  seen
          images where sectors were duplicated within a track and the 01 bit
          was NOT set.

       6.6 Cyclic Redundancy Check (1 byte)

          This  8-bit  field  contains  the   lower   byte   of   a   16-bit
          error-checking cyclic redundancy check for the sector header, data
          header and sector data calculated with the polynomial value  41111
          (A097 hex) using an input preset value of 0. The CRC is calculated
          over the first five bytes of the sector header,  the entire sector
          data header and the sector data block.  The calculated CRC  should
          match the value stored in the fourth byte of the sector header.
    Teledisk .TD0 notes                                              Page: 9


    7. Sector Data Header

       The sector data header occurs following the sector header  only  when
       sector data is present.  This is indicated by bits 10 and 20  of  the
       Flags value NOT being set. When present it has the following format:

        Data block size             (2 bytes)
        Encoding method             (1 byte)

       7.1 Data block size (2 bytes)

          This indicates the size of the sector data  block,  including  the
          encoding method (ie: data block size + 1).

       7.2 Encoding method (1 byte)

          This field describes how the sector data is encoded.  It can  have
          three possible values:

          7.2.1 0 - Raw sector data

             Encoding method == 0 indicates that "sector size"  bytes of raw
             sector data follow.  This is the actual data  content  for  the
             sector.

          7.2.2 1 - Repeated 2-byte pattern

             Encoding method == 1 indicates that a repeated  2-byte  pattern
             is used.  Note that this may occur  multiple  times  until  the
             entire sector has been  recreated,  as  determined  by  "sector
             size" in the sector header.

             Each entry consits of two 16-bit values.  The first is a  count
             value indicating how many times the second (the data value)  is
             repeated.

          7.2.3 2 - Run Length Encoded data

             Encoding == 2 indicates that an RLE  data  block  occurs.  Note
             that this may occur multiple times until the entire sector  has
             been recreated,  as determined by  "sector size"  in the sector
             header.

             Each entry begins with a 1 byte length value or 00.

             If 00,  then this entry is for a literal block.  The next  byte
             indicates a length 'n',  and the following 'n' bytes are copied
             into  the  sector  data  as  raw  bytes  (similar  to  Encoding
             method==0 except for only a portion of the sector).

             If not 00,  then the length 'l'  is determined as the value * 2
             (ie:  2-510).  The next byte indicates a repeat  count  'r'.  A
             block of  'l'  bytes is then  read  once  from  the  file,  and
             repeated in the sector data 'r' times.

          Sector headers and data blocks occur until  all  sectors  for  the
          track have been accounted for.