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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)

       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 three areas:

              Volume offset (optional)
              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.  Offset  must  appear
       subsequent to track, sector and sector length values.

   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), foreground-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

   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 checksum

       The  checksum  is  only 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)