196 lines
7.2 KiB
Groff
196 lines
7.2 KiB
Groff
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HD(4) Minix Programmer's Manual HD(4)
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NAME
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hd - winchester hard disk
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DESCRIPTION
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The hd* family of devices refer to the Winchester hard disk drivers for
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the IBM XT, AT and PS/2 machines, but may also refer to the generic (and
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slower) BIOS based hard disk driver. These disks are arrays of 512 byte
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sectors, although Minix always works with two sectors at a time due to
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its 1024 byte block size. You can read or write any number of bytes
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however, Minix takes care of cutting and pasting incomplete blocks
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together.
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The devices may be divided into three classes:
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The devices with a minor device number that is a multiple of 5, i.e.
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hd0 or hd5, refer to the whole hard disk 0 and 1. Through these
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devices one has access to any block on the hard disk. Most notably
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the partition table, that can be found in the first sector of the
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disk.
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The devices with a minor device number that is not a multiple of 5,
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i.e. hd1, hd2, ..., hd6, ..., refer to primary partitions of the
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lower numbered whole hard disk device. These devices normally
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contain MS-DOS or Minix file systems. /dev/hd1 is often the MS-DOS
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C: drive.
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Minor devices from 128 up may refer to Minix subpartitions within
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primary partitions if a subpartition table has been placed in a
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Minix primary partition. The subpartitions of hd3 for instance, are
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named hd3a through hd3d. Their minor device numbers may be
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calculated as 128 + 16*drive + 4*partition + subpartition, counting
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the partitions from zero.
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If a primary partition is an extended partition then up to four logical
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partitions can be accessed as subpartitions of that extended partition.
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This allows one to access foreign file systems of other operating
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systems, Minix file systems are not normally placed in logical
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partitions.
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PARTITIONING
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The first sector of a drive (or partition for subpartitioning) contains
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the partition table at byte offset 446. This is what each of the four
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entries looks like as defined in <ibm/partition.h>:
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/* Description of entry in the partition table. */
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struct part_entry {
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unsigned char bootind; /* boot indicator 0/ACTIVE_FLAG */
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unsigned char start_head; /* head value for first sector */
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unsigned char start_sec; /* sector value + high 2 cyl bits */
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unsigned char start_cyl; /* low 8 cylinder bits */
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unsigned char sysind; /* system indicator */
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unsigned char last_head; /* h/s/c for the last sector */
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unsigned char last_sec;
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unsigned char last_cyl;
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unsigned long lowsec; /* logical first sector */
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1
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HD(4) Minix Programmer's Manual HD(4)
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unsigned long size; /* size of partition in sectors */
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};
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#define ACTIVE_FLAG 0x80 /* value for active in bootind field */
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#define NR_PARTITIONS 4 /* number of entries in table */
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#define PART_TABLE_OFF 0x1BE /* offset of table in boot sector */
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/* Partition types (sysind). */
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#define MINIX_PART 0x81 /* Minix partition type */
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#define NO_PART 0x00 /* unused entry */
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#define OLD_MINIX_PART 0x80 /* created before 1.4b, obsolete */
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#define EXT_PART 0x05 /* extended partition */
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The cylinder numbers are encoded in a very strange way, bits 8 and 9 are
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in the high two bits of the sector number. The sector numbers count from
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1, not 0! More useful are the lowsec and size fields however, they
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simply give the location of the partition as an absolute sector offset
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and length within the drive.
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The partition table entry defined above is specific to IBM type disks.
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The device drivers use another partition entry structure to pass
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information on a partition. This is what <minix/partition.h> looks like:
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struct partition {
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u64_t base; /* byte offset to the partition start */
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u64_t size; /* number of bytes in the partition */
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unsigned cylinders; /* disk geometry for partitioning */
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unsigned heads;
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unsigned sectors;
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};
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The base and size fields are the byte offset and length of a partition.
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(These are 64 bit numbers under Minix-vmd, but only 32 bit numbers under
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standard Minix.) The geometry of the disk is also given for the benefit
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of partition table editors. This information can be obtained from an
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open disk device with the call:
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ioctl(fd, DIOCGETP, &entry);
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One can change the placement of the device to the lowsec and size fields
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of entry by using the DIOCSETP call instead. Only the base and size
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fields are used for DIOCSETP.
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The partition tables when read from disk by the driver are checked and
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truncated to fit within the primary partition or drive. The first sector
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should be left free for the partition table.
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The partition tables are read when the in-use count (opens and mounts)
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changes from 0 to 1. So an idle disk is automatically repartitioned on
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the next access. This means that repartitioning programs only have
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effect if a disk stays in use, unless they reload a changed partition
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table.
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2
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HD(4) Minix Programmer's Manual HD(4)
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FILES
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/dev/hd[0-9], /dev/hd[1-46-9][a-d]
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SEE ALSO
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ioctl(2), int64(3), part(8), repartition(8).
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BUGS
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The subpartitioning is incompatible with the MS-DOS method of extended
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partitions. The latter does not map well to the sparse minor device
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number space.
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The primary partition table is sorted by lowsec like MS-DOS does,
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subpartition tables are not. Just think about what happens when you
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delete a partition in the MS-DOS scheme.
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Don't move a partition that is mounted or kept open by some process. The
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file system may write cached blocks to the new location.
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The BIOS driver is not slow at all on a buffered disk.
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Some IDE disks send an interrupt when they spin down under hardware power
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management. The driver acknowledges the interrupt as it is supposed to
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do by reading the status register. The disk then spins up again... You
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have to disable the spin down in the computer setup to fix the problem.
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AUTHOR
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Kees J. Bot (kjb@cs.vu.nl)
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3
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