CPM(5)								   File formats 							    CPM(5)

NAME

       cpm - CP/M disk and file system format

DESCRIPTION

   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

       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  sec-
       tors, which are stored with the given software skew.

   Device areas
       A CP/M disk contains three areas:

	      System tracks (optional)
	      Directory
	      Data

       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 sys-
       tem 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.

   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.

       Al stores block pointers.  If the disk capacity is less than 256 blocks, Al is interpreted as 16 byte-values, otherwise as  8  double-byte-
       values.	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 necessarily 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.

   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

   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

   Passwords
       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

SEE ALSO

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

CP
/M tools							  March 30, 2010							    CPM(5)