DISKS(2)DISKS(2)NAME
disks: Disk, PCpart, readn, chstext - generic disk and partition inter‐
face
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.
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 repre‐
sent 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 element 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 acci‐
dents. 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 resulting 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; otherwise 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 con‐
trol 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 form‐
ing 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 cylin‐
ders, 8 for the number of heads, and 6 for the number 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
partitioning and formatting disks. Open employs a number of heuristics
to discover this supposed geometry and store it in the c, h, and s ele‐
ments of Disk. Disk offsets in partition 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 reality.
Chssrc names the source of the geometry values: disk (values returned
by disk itself); part (values stored in PC partition 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 boundaries 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 par‐
titions: a partition table at the end of the master boot record defines
up to four partitions. One (or perhaps more) of those can be an
extended partition that heads a chain (or perhaps roots a tree) of par‐
tition tables elsewhere on disk, allowing many more than four parti‐
tions in all. Disks represents a partition table entry by a value of
type PCpart. It provides the following operations and values:
active Has the value Disks->Active if it is bootable, and zero other‐
wise.
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 partition.
pc.bytes(d)
Return an array of bytes containing the PC-format partition ta‐
ble entry corresponding to pc. It will always be TentrySize
bytes long.
Several other values are defined here for convenience:
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 ALSOscsiio(2), floppy(3), sd(3), prep(3)DISKS(2)
