dev.4

操作系统设计与实现源码

.TH DEV 4
.SH NAME
dev \- device files in /dev
.SH DESCRIPTION
Device files are the eyes and ears of the system.  Through the device files
one has access to the disks, terminals and other parts of the machine.
Single bytes or disk blocks may be transferred to or from a device with
ordinary
.BR read (2)
or
.BR write (2)
calls, byte positions set with
.BR lseek (2),
or more complicated control functions performed with
.BR ioctl(2).
.PP
Device files as found in
.B /dev
have several attributes that must be considered.  Here are two examples as
.B "ls \-l"
shows them:
.PP
.RS
.nf
.if t .ft C
brw-rw-rw-  1 root     operator   2,   1 Jun 10  1995 fd1
crw--w----  1 kjb      tty        4,   0 May 11 09:41 console
.if t .ft P
.fi
.RE
.PP
Most attributes are the same as for a regular file and have the same
function.  The file type and the major and minor device numbers are special
to devices.
.PP
Character devices are marked with a
.B c
as a file type letter.  Any I/O on a character device is sent down to the
device driver without any interpretation.  This means that a process doing
the I/O must know the characteristics of the device and deal with them
appropriately.
.PP
Block devices provoke the file system server into buffering the data on
those devices.  Data read or written by processes is passed through the file
system block cache.  Unaligned bytes read or written are extracted or
reassembled by the file server from or to whole blocks in the cache.  The
file server transfers data to or from the device driver as blocks to
positions at block size boundaries.  These blocks are Minix blocks of 1024
bytes, disk devices usually have a 512 byte block size.  Only block devices
can be mounted as part of the file system tree if they contain a Minix file
system.
.PP
The major device number (2 for
.B fd1
and 4 for
.BR console )
are used by FS to find the device driver that manages a device.  The minor
device number (1 for
.B fd1
and 0 for
.BR console )
is passed to the driver to select a device among a number of related devices
that are all managed by that driver.  The device drivers are usually kernel
tasks under Minix, small processes that are contained within the address
space of the kernel.  The following tasks and associated devices exist:
.SS "Memory (major 1)"
The
.BR ram ,
.BR mem ,
.BR kmem ,
and
.BR null
devices are managed by the memory task.
The
.B ram
device is a block device for a chunk of memory that is the RAM disk.  Any
byte read from or written to the
.B ram
device is copied from or to that memory chunk.
The
.B mem
device is a character device for the entire address space of the system, but
.B kmem
only for the kernel data area.  These two devices allow programs like
.BR ps (1)
to hunt around the system looking for interesting bits.
The
.B null
device is a data sink.  It happily swallows any bytes written to it, and
returns nothing on a read.
.SS "Floppy disk (major 2)"
The
.BR fd0 ,
.BR fd0a ,
.BR fd0b ,
.BR fd0c ,
and
.BR fd0d
block devices are the first floppy disk and the four partitions that may
exist on a that floppy disk.  Likewise are
.BR fd1
and
.BR fd1[a\-d]
the device and partitions for the second floppy disk.  The floppy disk
devices are described in detail in
.BR fd (4).
Partitioning in general is explained in
.BR hd (4).
.SS "Hard disk (major 3)"
The first hard disk can be accessed by block device
.BR hd0 .
This device addresses the entire hard disk from the first to the last
sector.  A hard disk is normally partitioned in up to four primary
partitions,
.BR hd1 ,
.BR hd2 ,
.BR hd3 ,
and
.BR hd4 .
Each of these devices accesses a range of sectors on the
.B hd0
device.  It is customary to give each operating system on a disk a primary
partition.  So the MS-DOS C: "drive" can be on
.BR hd1 ,
and Minix can be on
.BR hd2 .
Minix wants to have several partitions on its own, so
.B hd2
can be further subdivided into the subpartitions
.BR hd2a ,
.BR hd2b ,
.BR hd2c ,
and
.BR hd2d .
.B /dev
contains devices for the first and second hard disk
.RB ( hd0
and
.BR hd5 )
their primary partitions
.RB ( hd[1\-46\-9] )
and subpartitions thereof
.RB ( hd[1\-46\-9][a\-d] ).
More detail can be found in
.BR hd (4).
.SS "Terminals (minor 4)"
The TTY driver manages the system console device, aptly named
.BR console ,
the serial lines,
.BR tty00
and
.BR tty01 ,
and the pseudo ttys.
Through the console device one can display characters on a screen attached
to a monochrome, Hercules, color, or VGA adapter.  The
.BR ttyc1 ,
.BR ttyc2 ,
etc. devices are the so-called "virtual consoles" that share the one
console display.  One can select which virtual console is to be visible on
the screen and take input from the keyboard.
To allow remote login the devices with minor numbers of 128 or higher offer
virtual terminals.  These pseudo ttys come in tty, pty pairs that form a
pipe between processes running under the tty, and a controlling process
attached to the pty side.
See also
.BR console (4),
and
.BR tty (4).
.SS "Anonymous TTY (major 5)"
This is just one device named
.BR tty
that is a synonym for the controlling tty of a process.  This device is not
managed by any device driver, but is handled by FS itself.  A process can
get access to the terminal it is running under by using
.BR /dev/tty .
.SS "Line printer (major 6)"
The
.B lp
device sends any bytes written to it to the printer.
.SS "TCP/IP (major 7)"
The TCP/IP task is not a kernel task, but a server like MM and FS.  It sits
between FS and the DP8390 task that manages the ethernet boards.  Together
they implement the TCP/IP protocol.  See also
.BR ip (4).
.SS "CD-ROM (major 8)"
This is the CD-ROM driver for the Mitsumi proprietary CD-ROM interface.  The
.BR cd0
device addresses the whole CD, with extra
.BR cd[1\-4]
and
.BR cd[1\-4][a\-d]
devices for if the CD also contains partitions with Minix file systems.
.SS "SCSI disks and tapes (major 10)"
The
.B sd*
devices are disks in the same way as the
.B hd*
devices.  Except that these disks are SCSI disks attached to an Adaptec
1540 controller or compatible.  The driver also manages the
.B rst*
and
.B nrst*
tape devices (rewinding or non-rewinding).  See
.BR sd (4).
.SS "Audio (major 13)"
The
.B audio
device can be used to produce or record air vibrations using a Soundblaster
16 type audio card.  See
.BR audio (4).
.SS "Mixer (major 14)"
The
.B mixer
device is used to control the audio driver.
.SH FILES
.TP 10
.B /dev/*
All Minix devices
.SH "SEE ALSO"
.BR read (2),
.BR write (2),
.BR lseek (2),
.BR ioctl (2),
.BR console (4),
.BR fd (4),
.BR hd (4),
.BR ip (4),
.BR sd (4),
.BR tty (4),
.BR MAKEDEV (8).
.SH DIAGNOSTICS
There are five prominent errors that processes accessing device files may
provoke:
.IP "ENODEV \- No such device" 5
There is no driver managing the device class this device belongs to.  Either
the driver is configured out, or it is not loaded (inet).
.IP "ENXIO \- No such device or address"
This device is not available.  Either the driver does not support it at all,
or the hardware isn't available, i.e. accessing the second disk on a system
with only one disk.
.IP "EACCES \- Permission denied"
This error may cause a lot of head scratching if
.B ls \-l
shows a device file to be writable.  The media you are trying to access is
simply physically write protected!
.IP "EINVAL \- Invalid argument"
Devices may not like reads or writes that are not block multiples, or very
big transfers, etc.  The device manual page should list the limits.
.IP "EIO \- I/O error"
This may be a real I/O error, i.e. a read or write on the device failing due
to a media error.  But it may also be the result of an operation that a
device can't do, or an empty tape drive, etc.
.SH NOTES
Some devices are not present by default.  The
.BR MAKEDEV
script knows how to make them.
.SS "MS-DOS equivalents"
The names of MS-DOS devices map to Minix devices as follows:
.PP
.RS
.nf
.ta +10n +16n
A:	fd0
B:	fd1
C:	hd1, sd1	(usually the first partition)
D:	hd6, sd1, sd6	(or an extended partition)
CON	console
COM1	tty00	(UNIX counts from 0)
LPT1	lp
.fi
.RE
.SH AUTHOR
Kees J. Bot ([e-mail])