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 alter-
          nate block bad block handling.  The abm structure can be re-
          trieved through an ioctl(2) with a request of GD_GETABM.
          The actual contents of the alternate block map can be re-
          trieved 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     abmsize;      /* size of map (bytes) */
              int     abments;      /* number of used entries
                                        (bytes) */
              daddrt abmstart;     /* start of altblk map
                                        (phys blk num) */
          };
          typedef  struct abm ABM;

          #define ABMENTSIZ (sizeof(long)) /* size of each
                                               map entry */
          #define NOALTMAP  ((daddrt) 0)  /* value of abmoff
                                               field if no map */

          #define   ABMFREE    -1   /* block not used */
          #define   ABMBADBLK  -2   /* block is bad */
          #define   ABMABM     -3   /* part of AltBlkMap */
          #define   ABMMAXVAL  -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 parti-
          tion, 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 al-
          ternate 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 par-
          tition which will be remapped, or it may contain a flag.

          The currently recognized flag values are ABM_FREE for avail-
          able blocks, ABM_BADBLK if the free block is bad, and



     April, 1990                                                     1

     altblk(4)                                               altblk(4)



          ABM_ABM to indicate that the block is allocated to the al-
          ternate block map.  Flag values with in 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 alternate
          blocked through an ioctl(2) with the request GD_MKBAD.

     FIELD DESCRIPTIONS
          abmsize
               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).

          abments
               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.

          abmstart
               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
          badblk(1M), dp(1M), bzb(4), gd(7).






















     2                                                     April, 1990