Programming Interface Information: This information is NOT intended to be used as Programming Interfaces of z/VM.

#### FREBK

Prolog

Control Block Contents
FREBK DSECT

Storage Layout

Cross Reference (Contains links to field and bit definitions)

#### FREBK Prolog

     NAME       : HCPFREBK
     DESCRIPTION: FREE STORAGE BLOCK
     DSECT      : FREBK
     FUNCTION   :
                  MAPS A FREE STORAGE BLOCK HEADER AND TRAILER
                  FOR BOTH ACTIVE AND INACTIVE FREE STORAGE BLOCKS.
     CREATED BY : HCPFREE OR HCPFREVM WHEN A BLOCK IS
                  REQUESTED.  THIS BLOCK IS NEVER ALLOCATED
                  AS A SEPARATE BLOCK.
     DELETED BY : HCPFRET WHEN A BLOCK IS RETURNED TO
                  FREE STORAGE.  THIS BLOCK IS NEVER
                  DEALLOCATED AS A SEPARATE BLOCK.
     SERIALIZED :
                  IT IS THE RESPONSIBILITY OF THE REQUESTOR TO
                  ENSURE THAT ACCESS TO THE FREE STORAGE BLOCK IS
                  SERIALIZED.
     NOTES      : WHEN A FREE STORAGE BLOCK IS ISSUED TO SATISFY A
                  FREE STORAGE REQUEST, THE FREE STORAGE MANAGER
                  PLACES A ONE DOUBLEWORD HEADER AND A TWO
                  DOUBLEWORD TRAILER AROUND THE BLOCK. THESE AREAS
                  ARE USED FOR DEBUGGING PURPOSES.
                  WHEN THE BLOCK IS RETURNED, THESE AREAS ARE
                  EXAMINED.  IF EITHER OR BOTH HAVE BEEN CORRUPTED, AN
                  ABEND IS ISSUED.

#### FREBK Control Block Content

#### FREBK DSECT

    Hex   Dec Type/Val   Lng Label (dup)    Comments
    ---- ---- --------- ---- -------------- --------
    0000    0 Structure      FREBK          FREE STORAGE BLOCK
         This is a picture of how the fields in this DSECT are actually used:
        +----------------------------------------------------+
      0 |                     FREHEADR                       |
        +----------------------------------------------------+
      8 :                                                    :
        :                  (control block)                   :
        :                                                    :
        +----------------------------------------------------+
     xx |                     FRETRALR                       |
        |                                                    |
        +----------------------------------------------------+
     xx+10
         The free storage header (FREHEADR) and free storage trailer (FRETRALR)
         areas are maintained both for internal functional uses and as debugging
         aids for free storage related problems. The free storage header
         immediately precedes the start of the free storage block returned in
         response to an obtain free storage request (HCPGETST). The trailer,
         when present (which is always, except for Fast Allocated Blocks),
         follows the end of the free storage requested by the caller (there may
         be a gap between the end of the size of block requested by the caller
         and the free storage trailer, as we round the requested size up to a
         convenient boundary in certain cases). The exact format and content of
         these fields depends on the type of free storage being used and whether
         the free storage block is active or inactive:
                        +- free storage block itself in here
                        |
                        V
     <----FREHEADR-----> <-------------FRETRALR-------------->
         Normal (non-subpooled) and V=R (subpooled) free storage:
         In use (for undefined blocks, cbi == '<<<') +--------+--------+
         +--------+--------+--------+--------+ |0000rqsz| >>>> |-| >>> |-| <--obtainer->|
         Normal subpooled (non-FAB, non-V=R) free storage:
         In use (normal): +--------+--------+
         +--------+--------+--------+--------+ | >> rqsz|1 subbk@|-| > rqsz|0 subbk@|-| <--obtainer->|
         Available (on collection pile): +--------+--------+
         +--------+--------+--------+--------+ | fpnt |0 subbk@|-| <--obtainer->|
         Fast Allocated Block free storage:
         In use (normal): +--------+--------+
         +--------+--------+--------+--------+ |sli 0FAB|1
         subbk@|-|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx| +--------+--------+
         +--------+--------+--------+--------+
         In use (flagged for collection pile): +--------+--------+
         +--------+--------+--------+--------+ |sli 0FAB|0
         subbk@|-|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx| +--------+--------+
         +--------+--------+--------+--------+
         Available (normal): +--------+--------+
         +--------+--------+--------+--------+ | fpnt |1
         subbk@|-|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx| +--------+--------+
         +--------+--------+--------+--------+
         Available (on collection pile): +--------+--------+
         +--------+--------+--------+--------+ | fpnt |0
         subbk@|-|xxxxxxxx|xxxxxxxx|xxxxxxxx|xxxxxxxx| +--------+--------+
         +--------+--------+--------+--------+
         Notes : Free storage trailer not maintained and is unreliable for Fast
         Allocated Blocks!
    0000    0 Dbl-Word     8 FRESTRT (0)    START OF VARIABLE LENGTH DATA
    0000    0 Dbl-Word     8 FREHEADR       FREE STORAGE BLOCK HEADER
              00000008       FREHEADL       *-FREHEADR LENGTH OF HEADER IN
                                            BYTES
    0000    0 Dbl-Word     8 FRETRALR (2)   FREE STORAGE BLOCK TRAILER
              00000010       FRETRALL       *-FRETRALR LENGTH OF TRAILER IN
                                            BYTES
              00000003       FRERECL        (FREHEADL+FRETRALL+7)/8 SIZE IN
                                            DOUBLE WORDS
    0000    0 Signed       4 FREESIZE (0)   # OF DWORDS OF STORAGE REQUESTED
    0000    0 Signed       2 FREHSPSE (0)   Mini-eyecatcher for non-FAB SPS,
                                            0 for normal non-subpooled free
                                            storage, or SLI for FAB SPS:
    0000    0 Signed       2 FREHFSLI       System Location Identifier (SLI)
                                            for Fast Allocated Block
                                            subpooled free storage
    0002    2 Signed       2 FREHSIZE       Actual count of DWs requested
         Notes : The maximum allowable size value for FREESIZE / FREHSIZE is the
         number of doublewords which can fit into a single 4K page (not counting
         the free storage header and trailer), or 509 doublewords, x'1FD' for
         everything other than Fast Allocate Blocks. Since the trailer is not
         maintained and may not even exist for Fast Allocated Blocks, the
         maximum allowable size is 511 doublewords, x'1FF'. For normal
         non-subpooled free storage, and for V=R free storage (which is
         subpooled, but the "old way"), the high order halfword of FREESIZE (aka
         FREHSPSE) must be zero; for normal (non-FAB, non-V=R) subpooled free
         storage, FREHSPSE contains the mini- eyecatcher '>>' == x'6E6E'.
         For Fast Allocated Block subpooled free storage, FREHSPSE contains the
         System Location Identifier (SLI) number, which identifies where in the
         system the block was obtained.
         A FREHSIZE value of x'FAB' in active subpooled free storage blocks is
         not a size, but is used to indicate that the block is an active "fast
         allocated block".
    0004    4 Signed       4 FRESUBBK (0)   Address of SUBBK for subpool blk
                                            The high order bit of FRESUBBK is
                                            normally 1, and is set to 0 by
                                            garbage collection (HCPFRFDX) to
                                            indicate that the block should be
                                            queued on the collection pile
                                            when freed (to attempt to reclaim
                                            the frame)
    0004    4 Character    4 FREESFLG       ALLOCATED BLOCK CHARACTER STRI
         Note that the free storage trailer is not maintained, and is therefore
         unreliable for Fast Allocated Blocks.
    0000    0 Character    4 FREEID         CONTROL BLOCK'S IDENTIFIER (THE
                                            FIRST CHARACTER IS '<', AND
                                            THE ID FOR UNDEFINED BLOCKS IS
                                            '<<<<' )
    0004    4 Signed       4 FREFSCBK (0)   Addr of storage obtainers FSCBK
    0004    4 Character    4 FREEEFLG       CHARACTER STRING '<<<
    0008    8 Signed       4 FREEDISP       REQUEST'S OFFSET IN CALLING
                                            MODULE (1ST HALF WORD = X'00 00')
    000C   12 Character    4 FREEMOD        MODULE ID OF CALLING MODULE THE
                                            FIRST BYTE'S ALWAYS SET TO 0
    0000    0 Signed       4 FREEPNT        PNTR TO NEXT BLOCK IN FREE
                                            STORAGE CHAIN OR 0 (IF
                                            END-OF-CHAIN)
    0004    4 Signed       4 FRETSIZE       Size of available block in DWs or
                                            the size in bytes for a larger
                                            than subpool size block that is
                                            on the free storage chain
         Notes : For for subpooled free storage, FRETSIZE as such is not used.
         Instead the address of the SUBBK which represents this subpool remains
         here in the second word of the free storage header (as field FRESUBBK).
         Since the maximum allowable value for FRETSIZE for normal (non-FAB)
         free storage is 509 (x'1FD'), any larger value in this field indicates
         that the block in question is in a frame used for subpooled free
         storage (since SUBBKs can't reside in low prefix page addresses like
         x'1FD' and below). However, in order to use this fact to distinguish
         between types, caution must be used, since the high order bit of the
         SUBBK address is used as a free storage garbage collection flag.
         Also note that in *active* subpooled free storage blocks, a value of
         x'FAB' in the actual requested size in dws (in FREHSIZE) is not a size,
         but an indicator that the block is an active "fast allocated block"
         (the requested size is not kept for fast allocated blocks, we know the
         actual size allocated because we know which subpool it's from).
         Note that the free storage trailer is not maintained, and may not even
         exist in some cases, and is therefore unreliable for available blocks
         that were last used as Fast Allocated Blocks.
    0000    0 Character    4 FRETID         CONTROL BLOCK'S IDENTIFIER (THE
                                            FIRST CHARACTER IS '<', AND
                                            THE ID FOR UNDEFINED BLOCKS IS
                                            '<<<<' ) THIS FIELD
                                            IS PRESERVED FROM THE HCPFREE
                                            CALL WHICH ORIGINALLY OBTAINED
                                            THIS CONTROL BLOCK
    0004    4 Character    4 FRETMOD        MODID OF HCPFRET CALLER THE FIRST
                                            BYTE'S ALWAYS SET TO 0
    0008    8 Signed       2 FRETDISP       DISPL INTO MODID OF HCPFRET CALL
    000A   10 Bitstring    1 FREECALL (6)   LOW-ORDER 2 BYTES OF "FREEDISP"
                                            AND "FREEMOD PRESERVED FROM THE
                                            HCPFREE CALL WHICH ORIGINALLY
                                            OBTAINED THIS CONTROL BLOCK

#### FREBK Storage Layout

    *** FREBK - FREE STORAGE BLOCK
    *
    *
    *** FREBK - FREE STORAGE BLOCK

    *** Overlay for FRESTRT in FREBK
    *
    *     +-------------------------------------------------------+
    *   0 |                       FREHEADR                        |
    *     +-------------------------------------------------------+
    *   8
    *
    *** Overlay for FRESTRT in FREBK

    *** Overlay for FRESTRT in FREBK
    *
    *     +-------------------------------------------------------+
    *   0 |                       FRETRALR                        |
    *     |                                                       |
    *     +-------------------------------------------------------+
    *  10
    *
    *** Overlay for FRESTRT in FREBK

    *** Overlay for FREHEADR in FREBK
    *
    *     +-------------+-------------+---------------------------+
    *   0 |  FREHFSLI   |  FREHSIZE   |         FREESFLG          |
    *     +-------------+-------------+---------------------------+
    *   8
    *
    *** Overlay for FREHEADR in FREBK

    *** Overlay for FRETRALR in FREBK
    *
    *     +---------------------------+---------------------------+
    *   0 |          FREEID           |         FREEEFLG          |
    *     +---------------------------+---------------------------+
    *   8 |         FREEDISP          |         FREEMOD           |
    *     +---------------------------+---------------------------+
    *  10
    *
    *** Overlay for FRETRALR in FREBK

    *** Overlay for FREHEADR in FREBK
    *
    *     +---------------------------+---------------------------+
    *   0 |         FREEPNT           |         FRETSIZE          |
    *     +---------------------------+---------------------------+
    *   8
    *
    *** Overlay for FREHEADR in FREBK

    *** Overlay for FRETRALR in FREBK
    *
    *     +---------------------------+---------------------------+
    *   0 |          FRETID           |         FRETMOD           |
    *     +-------------+-------------+---------------------------+
    *   8 |  FRETDISP   |                FREECALL                 |
    *     +-------------+-----------------------------------------+
    *  10
    *
    *** Overlay for FRETRALR in FREBK

#### FREBK Cross Reference

    Symbol         Dspl Value
    -------------- ---- -----
    FREECALL       000A
    FREEDISP       0008
    FREEEFLG       0004
    FREEID         0000
    FREEMOD        000C
    FREEPNT        0000
    FREESFLG       0004
    FREESIZE       0000
    FREFSCBK       0004
    FREHEADL       0000 00000008
    FREHEADR       0000
    FREHFSLI       0000
    FREHSIZE       0002
    FREHSPSE       0000
    FRERECL        0000 00000003
    FRESTRT        0000
    FRESUBBK       0004
    FRETDISP       0008
    FRETID         0000
    FRETMOD        0004
    FRETRALL       0000 00000010
    FRETRALR       0000
    FRETSIZE       0004

This information is based on z/VM V4R3.0. Last updated on 1 May 2002 at 08:42:17 EDT.
Copyright IBM Corporation, 1990, 2002