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


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;


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 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 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 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 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 control file; otherwise it sets these to zero as well. Name is initialized 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 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 elements 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 partitions: 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 partition tables elsewhere 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 values:

Has the value Disks->Active if it is bootable, and zero otherwise.
Partition type; Disks->Type9 is used by Plan 9 and Inferno (see prep(8)).
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.
Block address of the start of the partition relative to the base.
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.
Return an array of bytes containing the PC-format partition table entry corresponding to pc. It will always be TentrySize bytes long.

Several other values are defined here for convenience:

Value for PCpart.active if the partition is bootable.
Partition type used by Plan 9 and Inferno.
Offset (in bytes) of the partition table in a master boot record or extended partition start sector.
Size in bytes of a partition table entry.
Number of table entries.
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.




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

DISKS(2 ) Rev:  Thu Feb 15 14:43:27 GMT 2007