hd - winchester hard disk


     The hd* family of devices refer to the Winchester hard disk  drivers  for
     the  IBM XT, AT and PS/2 machines, but may also refer to the generic (and
     slower) BIOS based hard disk driver.  These disks are arrays of 512  byte
     sectors,  although  Minix  always works with two sectors at a time due to
     its 1024 byte block size.  You can read or  write  any  number  of  bytes
     however,  Minix  takes  care  of  cutting  and  pasting incomplete blocks

     The devices may be divided into three classes:

          The devices with a minor device number that is a multiple of 5, i.e.
          hd0  or  hd5,  refer  to the whole hard disk 0 and 1.  Through these
          devices one has access to any block on the hard disk.  Most  notably
          the  partition  table,  that can be found in the first sector of the

          The devices with a minor device number that is not a multiple of  5,
          i.e.   hd1,  hd2,  ..., hd6, ..., refer to primary partitions of the
          lower numbered whole  hard  disk  device.   These  devices  normally
          contain  MS-DOS or Minix file systems.  /dev/hd1 is often the MS-DOS
          C: drive.

          Minor devices from 128 up may refer to  Minix  subpartitions  within
          primary  partitions  if  a  subpartition  table has been placed in a
          Minix primary partition.  The subpartitions of hd3 for instance, are
          named  hd3a  through  hd3d.   Their  minor  device  numbers  may  be
          calculated as 128 + 16*drive + 4*partition + subpartition,  counting
          the partitions from zero.

     If a primary partition is an extended partition then up to  four  logical
     partitions  can  be accessed as subpartitions of that extended partition.
     This allows one  to  access  foreign  file  systems  of  other  operating
     systems,   Minix   file  systems  are  not  normally  placed  in  logical


     The first sector of a drive (or partition for  subpartitioning)  contains
     the  partition  table  at byte offset 446.  This is what each of the four
     entries looks like as defined in <ibm/partition.h>:

     /* Description of entry in the partition table.  */

     struct part_entry {
       unsigned char bootind;       /* boot indicator 0/ACTIVE_FLAG      */
       unsigned char start_head;    /* head value for first sector       */
       unsigned char start_sec;     /* sector value + high 2 cyl bits    */
       unsigned char start_cyl;     /* low 8 cylinder bits               */
       unsigned char sysind;        /* system indicator                  */
       unsigned char last_head;     /* h/s/c for the last sector         */
       unsigned char last_sec;
       unsigned char last_cyl;
       unsigned long lowsec;        /* logical first sector              */
       unsigned long size;          /* size of partition in sectors      */

     #define ACTIVE_FLAG     0x80   /* value for active in bootind field */
     #define NR_PARTITIONS   4      /* number of entries in table        */
     #define PART_TABLE_OFF  0x1BE  /* offset of table in boot sector    */

     /* Partition types (sysind). */
     #define MINIX_PART      0x81   /* Minix partition type */
     #define NO_PART         0x00   /* unused entry */
     #define OLD_MINIX_PART  0x80   /* created before 1.4b, obsolete */
     #define EXT_PART        0x05   /* extended partition */

     The cylinder numbers are encoded in a very strange way, bits 8 and 9  are
     in the high two bits of the sector number.  The sector numbers count from
     1, not 0!  More useful are the  lowsec  and  size  fields  however,  they
     simply  give  the  location of the partition as an absolute sector offset
     and length within the drive.

     The partition table entry defined above is specific to  IBM  type  disks.
     The  device  drivers  use  another  partition  entry  structure  to  pass
     information on a partition.  This is what <minix/partition.h> looks like:

     struct partition {
       u64_t base;              /* byte offset to the partition start */
       u64_t size;              /* number of bytes in the partition */
       unsigned cylinders;      /* disk geometry for partitioning */
       unsigned heads;
       unsigned sectors;

     The base and size fields are the byte offset and length of  a  partition.
     (These  are 64 bit numbers under Minix-vmd, but only 32 bit numbers under
     standard Minix.)  The geometry of the disk is also given for the  benefit
     of  partition  table  editors.   This information can be obtained from an
     open disk device with the call:

          ioctl(fd, DIOCGETP, &entry);

     One can change the placement of the device to the lowsec and size  fields
     of  entry  by  using  the  DIOCSETP call instead.  Only the base and size
     fields are used for DIOCSETP.

     The partition tables when read from disk by the driver  are  checked  and
     truncated to fit within the primary partition or drive.  The first sector
     should be left free for the partition table.

     The partition tables are read when the in-use count  (opens  and  mounts)
     changes  from  0 to 1.  So an idle disk is automatically repartitioned on
     the next access.  This  means  that  repartitioning  programs  only  have
     effect  if  a  disk  stays in use, unless they reload a changed partition


     /dev/hd[0-9], /dev/hd[1-46-9][a-d]


     ioctl(2), int64(3), part(8), repartition(8).


     The subpartitioning is incompatible with the MS-DOS  method  of  extended
     partitions.   The  latter  does  not  map well to the sparse minor device
     number space.

     The primary partition  table  is  sorted  by  lowsec  like  MS-DOS  does,
     subpartition  tables  are  not.   Just  think about what happens when you
     delete a partition in the MS-DOS scheme.

     Don't move a partition that is mounted or kept open by some process.  The
     file system may write cached blocks to the new location.

     The BIOS driver is not slow at all on a buffered disk.

     Some IDE disks send an interrupt when they spin down under hardware power
     management.   The  driver acknowledges the interrupt as it is supposed to
     do by reading the status register.  The disk then spins up again...   You
     have to disable the spin down in the computer setup to fix the problem.


     Kees J. Bot (kjb@cs.vu.nl)