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Minix/CD-ROM-2.0/MINIX/MANUALS/MAN4/HD.4

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+.TH HD 4
+.SH NAME
+hd \- winchester hard disk
+.SH DESCRIPTION
+The
+.B 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 together.
+.PP
+The devices may be divided into three classes:
+.sp
+.RS
+The devices with a minor device number that is a multiple of 5, i.e.
+.B hd0
+or
+.BR 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 disk.
+.sp
+The devices with a minor device number that is not a multiple of 5, i.e.
+.BR hd1 ,
+.BR hd2 ,
+\&...,
+.BR 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.
+.sp
+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
+.B hd3
+for instance, are named
+.B hd3a
+through
+.BR hd3d .
+Their minor device numbers may be calculated as
+128 + 16*drive + 4*partition + subpartition, counting the partitions
+from zero.
+.PP
+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 partitions.
+.SH PARTITIONING
+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>:
+.sp
+.nf
+.ta +2n +29n +37n
+/* 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	*/
+};
+
+.ta +24n +7n +37n
+#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 */
+.fi
+.PP
+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.
+.PP
+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:
+.sp
+.nf
+.ta +2n +25n
+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;
+};
+.fi
+.PP
+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:
+.sp
+.RS
+.ft B
+ioctl(\fIfd\fP, DIOCGETP, &\fIentry\fP);
+.ft R
+.RE
+.sp
+One can change the placement of the device to the lowsec and size fields of
+.I entry
+by using the
+.B DIOCSETP
+call instead.  Only the base and size fields are used for
+.BR DIOCSETP .
+.PP
+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.
+.PP
+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 table.
+.SH FILES
+/dev/hd[0\-9], /dev/hd[1\-46\-9][a\-d]
+.SH "SEE ALSO"
+.BR ioctl (2),
+.BR int64 (3),
+.BR part (8),
+.BR repartition (8).
+.SH BUGS
+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.
+.PP
+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.
+.PP
+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.
+.PP
+The BIOS driver is not slow at all on a buffered disk.
+.PP
+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.
+.SH AUTHOR
+Kees J. Bot (kjb@cs.vu.nl)