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       cpm - CP/M disk and file system format


   Characteristic sizes
       Each CP/M disk format is described by the following specific sizes:

              Sector size in bytes
              Number of tracks
              Number of sectors
              Block size
              Number of directory entries
              Logical sector skew
              Number of reserved system tracks (optional)
              Offset  to  start of volume (optional and not covered by operating system, but disk
              driver specific)

       A block is the smallest allocatable storage unit.  CP/M  supports  block  sizes  of  1024,
       2048,  4096, 8192 and 16384 bytes.  Unfortunately, this format specification is not stored
       on the disk and there are lots of formats.  Accessing a block is  performed  by  accessing
       its sectors, which are stored with the given software skew.

   Device areas
       A CP/M disk contains four areas:

              Volume  offset  (optional  and  not  covered  by  operating system, but disk driver
              System tracks (optional)

       The system tracks store the boot loader and CP/M itself.  In order  to  save  disk  space,
       there  are  non-bootable  formats  which omit those system tracks.  The term disk capacity
       always excludes the space for system tracks.  Note that there is no  bitmap  or  list  for
       free  blocks.   When accessing a drive for the first time, CP/M builds this bitmap in core
       from the directory.

       A hard disk can have the additional notion of a volume offset to locate the start  of  the
       drive  image  (which may or may not have system tracks associated with it).  The base unit
       for volume offset is byte count from the beginning of the physical disk, but specifiers of
       K,  M,  T  or  S  may  be  appended to denote kilobytes, megabytes, tracks or sectors.  If
       provided, a specifier must immediately follow the numeric value with no  whitespace.   For
       convenience  upper  and  lower  case  are  both  accepted  and  only  the  first letter is
       significant, thus 2KB, 8MB, 1000trk and 16sec are valid values.  The  offset  must  appear
       subsequent  to  track,  sector  and sector length values for the sector and track units to

   Directory entries
       The directory is a sequence of directory entries (also called extents), which  contain  32
       bytes of the following structure:

              St F0 F1 F2 F3 F4 F5 F6 F7 E0 E1 E2 Xl Bc Xh Rc
              Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al

       St is the status; possible values are:

              0-15: used for file, status is the user number
              16-31: used for file, status is the user number (P2DOS) or used for password extent
              (CP/M 3 or higher)
              32: disc label
              33: time stamp (P2DOS)
              0xE5: unused

       F0-E2 are the file name and its extension.  They may consist of any printable 7 bit  ASCII
       character  but: < > . , ; : = ? * [ ].  The file name must not be empty, the extension may
       be empty.  Both are padded with blanks.  The highest bit of each  character  of  the  file
       name and extension is used as attribute.  The attributes have the following meaning:

              F0: requires set wheel byte (Backgrounder II)
              F1: public file (P2DOS, ZSDOS), forground-only command (Backgrounder II)
              F2: date stamp (ZSDOS), background-only commands (Backgrounder II)
              F7: wheel protect (ZSDOS)
              E0: read-only
              E1: system file
              E2: archived

       Public  files (visible under each user number) are not supported by CP/M 2.2, but there is
       a patch and some free CP/M clones support them without any patches.

       The wheel byte is (by default) the memory location at 0x4b.  If  it  is  zero,  only  non-
       privileged commands may be executed.

       Xl  and  Xh  store the extent number.  A file may use more than one directory entry, if it
       contains more blocks than an extent can hold.  In this case, more  extents  are  allocated
       and  each  of  them  is numbered sequentially with an extent number.  If a physical extent
       stores more than 16k, it is considered to contain multiple logical extents, each  pointing
       to  16k data, and the extent number of the last used logical extent is stored.  Note: Some
       formats decided to always store only one logical extent in a physical extent, thus wasting
       extent  space.   CP/M 2.2 allows 512 extents per file, CP/M 3 and higher allow up to 2048.
       Bit 5-7 of Xl are 0, bit 0-4 store the lower bits of the extent number.  Bit 6 and 7 of Xh
       are 0, bit 0-5 store the higher bits of the extent number.

       Rc  and  Bc  determine the length of the data used by this extent.  The physical extent is
       divided into logical extents, each of them being 16k in size (a physical extent must  hold
       at least one logical extent, e.g. a blocksize of 1024 byte with two-byte block pointers is
       not allowed).  Rc stores the number of 128 byte records of the last used  logical  extent.
       Bc stores the number of bytes in the last used record.  The value 0 means 128 for backward
       compatibility with CP/M 2.2, which did not support Bc.  ISX records the number  of  unused
       instead of used bytes in Bc.

       Al  stores  block  pointers.   If  the  disk  capacity minus boot tracks but including the
       directory area is less than 256 blocks, Al is interpreted as 16 byte-values, otherwise  as
       8  double-byte-values.   Since  the  directory  area is not subtracted, the directory area
       starts with block 0 and files can never allocate block 0, which is why this value  can  be
       given  a new meaning: A block pointer of 0 marks a hole in the file.  If a hole covers the
       range of a full extent, the extent will not be allocated.  In particular, the first extent
       of  a  file  does not neccessarily have extent number 0.  A file may not share blocks with
       other files, as its blocks would be freed if the other files is erased without a following
       disk  system  reset.   CP/M returns EOF when it reaches a hole, whereas UNIX returns zero-
       value bytes, which makes holes invisible.

   Native time stamps
       P2DOS and CP/M Plus support time stamps, which are stored in each fourth directory  entry.
       This  entry  contains  the  time stamps for the extents using the previous three directory
       entries.  Note that you really have time stamps for each extent, no matter if  it  is  the
       first extent of a file or not.  The structure of time stamp entries is:

              1 byte status 0x21
              8 bytes time stamp for third-last directory entry
              2 bytes unused
              8 bytes time stamp for second-last directory entry
              2 bytes unused
              8 bytes time stamp for last directory entry

       A  time  stamp  consists  of  two  dates: Creation and modification date (the latter being
       recorded when the file is closed).  CP/M Plus further  allows  optionally  to  record  the
       access instead of creation date as first time stamp.

              2 bytes (little-endian) days starting with 1 at 01-01-1978
              1 byte hour in BCD format
              1 byte minute in BCD format

       All time stamps are stored in local time.

   DateStamper time stamps
       The DateStamper software added functions to the BDOS to manage time stamps by allocating a
       read only file with the name "!!!TIME&.DAT" in the very first  directory  entry,  covering
       the  very first data blocks.  It contains one entry per directory entry with the following
       structure of 16 bytes:

              5 bytes create datefield
              5 bytes access datefield
              5 bytes modify datefield
              1 byte magic number/checksum

       The magic number is used for the first 7 entries of each 128-byte record and contains  the
       characters !, !, !, T, I, M and E.  The checksum is used on every 8th entry (last entry in
       128-byte record) and is the sum of the first 127 bytes of the record.  Each datefield  has
       this structure:

              1  byte BCD coded year (no century, so it is sane assuming any year < 70 means 21st
              1 byte BCD coded month
              1 byte BCD coded day
              1 byte BCD coded hour or, if the high bit is set, the high byte of  a  counter  for
              systems without real time clock
              1 byte BCD coded minute, or the low byte of the counter

   Disc labels
       CP/M  Plus  support  disc  labels,  which are stored in an arbitrary directory entry.  The
       structure of disc labels is:

              1 byte status 0x20
              F0-E2 are the disc label
              1 byte mode: bit 7 activates password protection,  bit  6  causes  time  stamps  on
              access,  but  5  causes  time  stamps on modifications, bit 4 causes time stamps on
              creation and bit 0 is set when a label exists.  Bit 4 and 6 are exclusively set.
              1 byte password decode byte: To  decode  the  password,  xor  this  byte  with  the
              password  bytes  in reverse order.  To encode a password, add its characters to get
              the decode byte.
              2 reserved bytes
              8 password bytes
              4 bytes label creation time stamp
              4 bytes label modification time stamp

       CP/M Plus supports passwords, which are stored  in  an  arbitrary  directory  entry.   The
       structure of these entries is:

              1 byte status (user number plus 16)
              F0-E2 are the file name and its extension.
              1  byte password mode: bit 7 means password required for reading, bit 6 for writing
              and bit 5 for deleting.
              1 byte password decode byte: To  decode  the  password,  xor  this  byte  with  the
              password  bytes  in reverse order.  To encode a password, add its characters to get
              the decode byte.
              2 reserved bytes
              8 password bytes


       mkfs.cpm(1), fsck.cpm(1), fsed.cpm(1), cpmls(1)