cckd

cckd(4)                    Kernel Interfaces Manual                    cckd(4)



NAME
       cckd - Hercules Compressed CKD DASD image file


DESCRIPTION
       Hercules DASD image file that emulates IBM mainframe DASD devices on a
       (usually) non-mainframe platform.  Specified in the Hercules
       configuration file (default name hercules.cnf) to describe the DASD
       devices Hercules is to emulate.  Also specified as input and/or output
       file to several Hercules utilities.  Hercules compressed DASD devices
       are frequently referred to as CCKD.  Hercules uncompressed DASD devices
       are frequently referred to as CKD.  Hercules CKD DASD image files are
       largely compatible with P/390 AWS DASD files.


TECHNICAL OVERVIEW
       The following provides a technical overview of CCKD internal
       structures.  Except as indicated below, data in the following
       structures is stored in the byte order indicated by the CDEVHDR.options
       CCKD_BIGENDIAN bit (the 0x02 bit).  This bit is 1 when data is big-
       endian byte order, 0 when data is little-endian byte order.

   DEVHDR
       occupies the first 512 bytes of a CKD or CCKD DASD file.  The DEVHDR
       contains the device type and the number of heads per cylinder.  Its
       contents are the same whether the DASD image is compressed (CCKD) or
       not (CKD).  Described by the CKDDASD_DEVHDR struct; 512 bytes in size.

   CDEVHDR
       immediately follows the DEVHDR, and contains fields describing the
       number of L1ENTs in the L1TAB, the number of L2ENTs in each L2TAB, and
       anchors the free space chain.  Described by the CCKDDASD_DEVHDR struct;
       512 bytes in size.

   L1TAB
       immediately follows the CDEVHDR, and consists of L1ENT entries, each of
       which points to an L2TAB.  Each L1ENT is an U32 (4 bytes) offset into
       the CCKD DASD file.  The CDEVHDR numl1tab field describes how many
       L1ENTs are in the L1TAB.  L1ENTs are described by the CCKD_L1ENT
       typedef; the L1TAB is of variable size.  Conceptually each L1ENT
       describes cdevhdr.numl2tab tracks.

   L2TAB
       is pointed to by an L1ENT, and is composed of L2ENT entries, the number
       of which is described by the CDEVHDR numl2tab field; currently 256
       L2ENTs in each L2TAB.  L2ENTs contain the offset to the TRKHDR, and the
       combined length of the TRKHDR and (optionally) compressed track data.
       The CDEVHDR numl2tab field describes how many L2ENTs are in each L2TAB.
       Currently, there are 256 L2ENTs in each L2TAB.  Described by the
       CCKD_L2ENT struct; (256 * 8) bytes in size.  Conceptually each L2ENT
       describes one track.

   FREEBLK
       describes free space in the CCKD DASD image, anchored by the CDEVHDR
       free field.  Consists of a 4 byte offset to the next free space (or
       zero for end of free space chain), and a 4 byte length of the free
       space (which length includes the 8 bytes occupied by the FREEBLK
       itself), followed by zero or more bytes of residual data.  Described by
       the first 8 bytes of the CCKD_FREEBLK struct; 8 bytes in size.

   TRKHDR
       Occurs once at the beginning of each track.  contains flag (one byte),
       CC (two bytes), and HH (two bytes) of the track.  When the flag byte =
       0x00, TRKHDR is the same as the Home Address on real DASD.  Flag bits
       are described in hercules.h, and are of the format nlllllcc where n=1
       for new track header format, lllll is used for track recovery purposes,
       and cc describes the track compression algorithm.  The compression
       algorithms are: B'00' = uncompressed, B'01' = zlib, B'10' = bzip2,
       B'11' is currently invalid.  Data in TRKHDR is stored in big-endian
       byte order.  Described by the CKDDASD_TRKHDR struct; 5 bytes in size.

   COUNT field
       8 bytes, containing CC (two bytes), HH (two bytes), R (one byte), KL
       (one byte), and DL (two bytes).  CC is the (relative zero) cylinder
       number.  HH is the (relative zero) head number.  R is the (relative
       zero) record number on the track.  KL is the key length; if zero no key
       is present.  DL is the length of the data record.  Data in the COUNT
       field is stored in big-endian byte order.  Described by the
       CKDDASD_RECHDR struct; 8 bytes in size.

   KEY field
       if present, KL bytes of record key; immediately follows the COUNT
       field.  Byte order is not a factor for the KEY field; to the extent it
       is examined by Hercules code it is simply a byte stream.  Size varies.

   DATA field.
       if present, DL bytes of record data; immediately follows the KEY field
       for keyed record, else immediately follows the COUNT field for unkeyed
       records.  Byte order is not a factor for the DATA field; to the extent
       it is examined by Hercules code it is simply a byte stream.  Size
       varies.


GLOSSARY
   CKD
       Count, Key, Data - contents of an track.  Also refers to the Hercules
       uncompressed DASD image file.

   CCKD
       Compressed Count, Key, Data - compressed contents of a track.  Also
       refers to the Hercules compressed DASD image file.

   DASD
       Direct Access Storage Device - term the IBM mainframe world uses to
       refer to hard drives.

   EOT
       End Of Track - indicated by 8X'FF' in the COUNT field.


SEE ALSO
       http://www.hercules-390.org/ the Hercules emulator homepage.

       http://www.hercules-390.org/cckddasd.html which describes
              the Hercules CCKD DASD facility.

       http://www.hercules-390.org/hercconf.html which describes
              the Hercules configuration file.


HISTORY
       2003-02-07 originally written by James M. Morrison





                                  2003-02-03                           cckd(4)