altblk(4)                                               altblk(4)



NAME
     altblk - alternate block information for bad block handling

SYNOPSIS
     #include <sys/types.h>
     #include <apple/abm.h>

DESCRIPTION
     abm is the data structure used by A/UX disk device drivers
     to handle bad blocks for disk partitions that support
     alternate block bad block handling.  The abm structure can
     be retrieved through an ioctl(2) with a request of
     GD_GETABM.  The actual contents of the alternate block map
     can be retrieved via the abmi structure through an ioctl(2)
     with a request of GD_GETMAP.  The abmi structure is
     described in gd(7).  The format of the abm structure is:

     struct abm            /* altblk map info stored in bzb */
     {
         int     abm_size;      /* size of map (bytes) */
         int     abm_ents;      /* number of used entries
                                   (bytes) */
         daddr_t abm_start;     /* start of altblk map
                                   (phys blk num) */
     };
     typedef  struct abm ABM;

     #define ABM_ENTSIZ (sizeof(long)) /* size of each
                                          map entry */
     #define NO_ALTMAP  ((daddr_t) 0)  /* value of abm_off
                                          field if no map */

     #define   ABM_FREE    -1   /* block not used */
     #define   ABM_BADBLK  -2   /* block is bad */
     #define   ABM_ABM     -3   /* part of AltBlkMap */
     #define   ABM_MAXVAL  -16  /* last reserved map value */

     Normally the alternate block area, that area between the end
     of the logical partition and the end of the physical
     partition, will (optionally) contain an alternate block map
     and alternate block data blocks for alternate block
     handling.  The alternate block map resides anywhere in the
     alternate block area, in a contiguous set of blocks.  The
     format of the alternate block map is an array of long
     integers.  Each indexed location in the array corresponds to
     a potential alternate block in the alternate block area.  A
     location in the alternate block array (map) may either
     contain the number of a block in the logical partition of
     the disk partition that will be remapped, or it may contain
     a flag.





January 1992                                                    1

altblk(4)                                               altblk(4)



     The currently recognized flag values are ABM_FREE for
     available blocks, ABM_BADBLK if the free block is bad, and
     ABM_ABM to indicate that the block is allocated to the
     alternate block map.  Flag values within the range of
     ABM_ABM and ABM_MAXVAL are reserved for future use.

     Alternate block mapping may be disabled through an ioctl(2)
     with the request GD_ALTBLK.  A bad block may be alternately
     blocked through an ioctl(2) with the request GD_MKBAD.

Field descriptions
     abm_size
          This field contains the size of the alternate block map
          as measured in bytes.  This value should always be
          evenly divisible by ABM_ENTSIZ.  The value of this
          field should be consulted when requesting the contents
          of the alternate block map through an ioctl(2).

     abm_ents
          The value of this field represents the byte offset of
          the next available entry in the alternate block map as
          measured from the beginning of the map.  This field is
          maintained by the device driver.

     abm_start
          The value of this field is set to NO_ALTMAP to indicate
          that there is no alternate block map for the
          corresponding partition.  If the value of this field is
          not set to NO_ALTMAP, then the value is the physical
          block number (relative to the start of the physical
          partition) of the first block of the alternate block
          map.

SEE ALSO
     bzb(4), gd(7)

     badblk(1M), dp(1M), in A/UX System Administrator's Reference


















2                                                    January 1992