Hard Disk Partitioning


         The first sector (logical sector #0) on a hard disk  con-
         tains partition information.

                                              offset
                     +-----------------------+
                     |       hd_siz          | $1c2
                     |                       |
                     +-----------------------+
                     |       p0_flg          | $1c6
                     |       p0_id           | $1c7
                     |       p0_st           | $1ca
                     |                       |
                     |       p0_siz          | $1ce
                     |                       |
                     +-----------------------+
                     |       p1_flg          | $1d2
                     |       p1_id           | $1d3
                     |       p1_st           | $1d6
                     |                       |
                     |       p1_siz          | $1da
                     |                       |
                     +-----------------------+
                     |       p2_flg          | $1de
                     |       p2_id           | $1df
                     |       p2_st           | $1e2
                     |                       |
                     |       p2_siz          | $1e6
                     |                       |
                     +-----------------------+
                     |       p3_flg          | $1ea
                     |       p3_id           | $1eb
                     |       p3_st           | $1ee
                     |                       |
                     |       p3_siz          | $1f2
                     |                       |
                     +-----------------------+
                     |       bsl_st          | $1f6
                     |                       |
                     |       bsl_cnt         | $1fa
                     |                       |
                     +-----------------------+
                     |       (reserved)      | $200
                     +-----------------------+


         'hd_siz' is the total size of the disk, in  logical  sec-
         tors.

         'bsl_st' specifies the starting sector# of the bad sector
         list.   [Typically the bad sector list will be located at
         the end of the device.]

         'bsl_cnt' specifies the /number/ of  bad  sectors.   Each
         bad  sector  is  specified  by  a longword containing the
         sector's number.  The total number of sectors used up  by
         the  bad  sector  list  is  therefore  'bsl_siz' / 4.  If
         'bsl_cnt' is zero, there are no bad sectors.

         A disk may contain up to four partitions.  The first sec-
         tor  in  a  partition is a boot sector (which, on the ST,
         will contain a BPB).


                     +-----------------------+
                     |       root boot       |
                     +-----------------------+

                     +-----------------------+
                     |       partition 0     |
                     |                       |
                     +-----------------------+

                     +-----------------------+
                     |       partition 1     |
                     |       (optional)      |
                     +-----------------------+

                     +-----------------------+
                     |       partition 2     |
                     |       (optional)      |
                     +-----------------------+

                     +-----------------------+
                     |       partition 3     |
                     |       (optional)      |
                     +-----------------------+

                     +-----------------------+
                     |       optional        |
                     |       bad sector      |
                     |       list            |
                     +-----------------------+


         Each partition is described by a 12-byte structure:

                     +-----------------------+
                     |       p*_flg          | (+0)
                     +-----------------------+
                     |       p*_id           | (+1)
                     |                       |
                     |                       |
                     +-----------------------+
                     |       p*_st           | (+4)
                     |                       |
                     |                       |
                     |                       |
                     +-----------------------+
                     |       p*_siz          | (+8)
                     |                       |
                     |                       |
                     |                       |
                     +-----------------------+
                                               (+12.)

         'p*_flg' should be nonzero to indicate that the partition
         exists.   The BIOS will boot the first partition that has
         bit 7 set in this byte.

         'p*_id' is a three-byte field that identifies the  parti-
         tion.  For GEMDOS partitions, the field contain the three
         ascii characters "GEM".

         'p*_st' specifies the logical sector number of the begin-
         ning of the partition.

         'p*_siz' specifies the size of the partition, in  logical
         sectors.