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.
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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.
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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:
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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)
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