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     DISKS(2)                                                 DISKS(2)

     NAME
          disks: Disk, PCpart, readn, chstext - generic disk and
          partition interface

     SYNOPSIS
          include "disks.m";

          disks := load Disks Disks->PATH;

          Disk: adt {
              prefix:  string;    # prefix before partition name
              part:    string;    # partition name (nil if not partition)
              fd:      ref Sys->FD;
              wfd:     ref Sys->FD;
              ctlfd:   ref Sys->FD;
              rdonly:  int;       # non-zero if readonly
              dtype:   string;    # device type

              secs:    big;    # number of sectors in device or partition
              secsize: int;    # device's sector size
              size:    big;    # size of device or partition
              offset:  big;    # within larger disk, perhaps
              width:   int;    # of disk size in bytes as decimal string
              c:       int;    # geometry: cyl, head, sectors
              h:       int;
              s:       int;
              chssrc:  string; # source of c/h/s values

              open:    fn(f: string, mode: int, noctl: int): ref Disk;
          };

          PCpart: adt {
              active: int;    # Active or 0
              ptype:  int;
              base:   big;    # base block address
              offset: big;    # block offset from base to partition
              size:   big;    # in sectors

              extract: fn(a: array of byte, d: ref Disk): PCpart;
              bytes:   fn(p: self PCpart, d: ref Disk): array of byte;
          };

          init:      fn();
          readn:   fn(fd: ref Sys->FD, buf: array of byte, n: int): int;
          chstext: fn(p: array of byte): string;

     DESCRIPTION
          Disks provides a simple way to gather and use information
          about floppy(3) and sd(3) disks and disk partitions, as well
          as plain files.

     DISKS(2)                                                 DISKS(2)

          Init must be called before invoking any other operations of
          the module

          Disk.open opens file and returns a reference to a Disk value
          to represent the disk.  Mode should be either Sys->OREAD or
          Sys->ORDWR to establish the open mode.  Open always opens
          file for reading and stores that file descriptor in the ele-
          ment fd. If the mode is not Sys->OREAD, opendisk also opens
          file for writing and stores that file descriptor in wfd.
          The two file descriptors are kept separate to help prevent
          accidents.  If noctl is not set, open looks for a ctl file
          in the same directory as the disk file; if it finds one, it
          declares the disk to be an sd(3) device, setting dtype to
          "sd".  If the passed file is named fdndisk, it looks for a
          file fdnctl, and if it finds that, declares the disk to be a
          floppy disk, of type "floppy".  If either control file is
          found, it is opened for reading and writing, and the result-
          ing file descriptor is saved as ctlfd.  Otherwise the
          returned disk has type "file".

          Open then stores the file's length (as given by sys-stat(2))
          in size.  If the disk is an sd(3) partition, open reads the
          sector size from the control file and stores it in secsize;
          otherwise the sector size is assumed to be 512, as is the
          case for floppy disks.  Open stores the disk size measured
          in sectors in secs.

          If the disk is an sd(3) partition, open parses the control
          file to find the partition's offset within its disk; other-
          wise it sets offset to zero.  If the disk is an ATA disk,
          open reads the disk geometry (number of cylinders, heads,
          and sectors) from the geometry line in the sd control file;
          otherwise it sets these to zero as well.  Name is initial-
          ized with the base name of the disk partition, and is useful
          for forming messages to the sd control file.  Prefix is set
          to the original file name without the name suffix.

          The IBM PC BIOS interface allocates 10 bits for the number
          of cylinders, 8 for the number of heads, and 6 for the num-
          ber of sectors per track.  Disk geometries are not quite so
          simple anymore, but to keep the interface useful, modern
          disks and BIOSes present geometries that still fit within
          these constraints.  These numbers are still used when parti-
          tioning and formatting disks.  Open employs a number of
          heuristics to discover this supposed geometry and store it
          in the c, h, and s elements of Disk.  Disk offsets in parti-
          tion tables and in FAT descriptors are stored in a form
          dependent upon these numbers, so opendisk works hard to
          report numbers that agree with those used by other operating
          systems; the numbers bear little or no resemblance to real-
          ity.

     DISKS(2)                                                 DISKS(2)

          Chssrc names the source of the geometry values: disk (values
          returned by disk itself); part (values stored in PC parti-
          tion table); or guess (calculated by module's heuristics).

          Readn attempts to read exactly n bytes from file fd into
          buf, using as many sys-read(2) calls as required.  It helps
          insulate a program from any peculiar underlying IO bound-
          aries of a device.  It returns less than n only if end-of-
          file is reached.  It returns -1 if the first read fails.

          The PC BIOS and operating systems support an arcane system
          of disk partitions: a partition table at the end of the mas-
          ter boot record defines up to four partitions.  One (or per-
          haps more) of those can be an extended partition that heads
          a chain (or perhaps roots a tree) of partition tables else-
          where on disk, allowing many more than four partitions in
          all.  Disks represents a partition table entry by a value of
          type PCpart.  It provides the following operations and val-
          ues:

          active
               Has the value Disks->Active if it is bootable, and zero
               otherwise.

          ptype
               Partition type; Disks->Type9 is used by Plan 9 and
               Inferno (see prep(8)).

          base Address of the extended partition that started the
               chain (or rooted the tree) containing this partition.
               Zero for primary partitions defined by the master boot
               record.

          offset
               Block address of the start of the partition relative to
               the base.

          size Size of the partition in sectors.

          extract(a, d)
               Extracts the relevant data from an array of bytes a
               containing a PC-format partition table entry on Disk d,
               and returns a PCpart value that represents the parti-
               tion.

          pc.bytes(d)
               Return an array of bytes containing the PC-format par-
               tition table entry corresponding to pc. It will always
               be TentrySize bytes long.

          Several other values are defined here for convenience:

     DISKS(2)                                                 DISKS(2)

          Active
               Value for PCpart.active if the partition is bootable.

          Type9
               Partition type used by Plan 9 and Inferno.

          Toffset
               Offset (in bytes) of the partition table in a master
               boot record or extended partition start sector.

          TentrySize
               Size in bytes of a partition table entry.

          NTentry
               Number of table entries.

          Magic0
          Magic1
               Each sector containing a partition table should end
               with these two bytes (a master boot record must end
               with them).
          Chstext takes a 3-byte array containing the packed
          cylinder/head/sector representation of a disk address and
          returns the corresponding text in the form c/h/s.

     SOURCE
          /appl/lib/disks.b

     SEE ALSO
          scsiio(2), floppy(3), sd(3), prep(3)