The allocation control XAB (XABALL) provides additional control
    over file or area space allocation on disk devices in order to
    optimize performance.

    When RMS uses a XAB to create or extend an area, the following
    XABALL fields duplicate and take precedence over associated
    fields in the related FAB:

    o  The allocation quantity (ALQ) field, XAB$L_ALQ, overrides
       FAB$L_ALQ

    o  The bucket size (BKZ) field, XAB$B_BKZ, overrides FAB$B_BKS

    o  The default extension quantity (DEQ) field, XAB$W_DEQ,
       overrides FAB$W_DEQ

    o  The XAB$V_CBT and XAB$V_CTG options of the allocation options
       field, XAB$B_AOP, override the FAB$V_CBT and FAB$V_CTG options
       of the file-processing options field, FAB$L_FOP

    The following table lists the symbolic offset, the size in bytes,
    the FDL equivalent, and a brief description of each field.

    Offset      Size   FDL Equivalent      Description

    XAB$B_AID   1      AREA n              Area identification number
    XAB$B_ALN   1      AREA POSITION       Alignment boundary type
    XAB$L_ALQ   4      AREA ALLOCATION     Allocation quantity
    XAB$B_AOP   1      AREA                Allocation options
    XAB$B_BKZ   1      AREA BUCKET_SIZE    Bucket size
    XAB$B_BLN   1      None                Block length
    XAB$B_COD   1      None                Type code
    XAB$W_DEQ   2      AREA EXTENSION      Default extension quantity
    XAB$L_LOC   4      AREA POSITION       Location
    XAB$L_NXT   4      None                Next XAB address
    XAB$W_RFI   6      AREA POSITION       Related file identifier
                       FILE_ID
                       or FILE_NAME
    XAB$W_VOL   2      AREA VOLUME         Related volume number