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+.\"
+.\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch
+.\"
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" $Id$
+.\"
+.Dd January 20, 1996
+.Dt INTRO 4
+.Os FreeBSD 2.1
+.Sh NAME
+.Nm intro
+.Nd introduction to devices and device drivers
+.Sh DESCRIPTION
+This section contains information related to devices, device driver
+and miscellaneous hardware.  
+.Ss The device abstraction
+Device is a term used mostly for hardware-related stuff that belongs
+to the system, like disks, printers, or a graphics display with its
+keyboard.  There are also so-called
+.Em pseudo-devices
+where a device driver emulates the behaviour of a device in software
+without any particular underlying hardware.  A typical example for
+the latter class is
+.Pa /dev/mem ,
+a loophole where the physical memory can be accessed using the regular
+file access semantics.
+.Pp
+The device abstraction generally provides a common set of system calls
+layered on top of them, which are dispatched to the corresponding
+device driver by the upper layers of the kernel.  The set of system
+calls available for devices is chosen from
+.Xr open 2 ,
+.Xr close 2 ,
+.Xr read 2 ,
+.Xr write 2 ,
+.Xr ioctl 2 ,
+.Xr select 2 ,
+and
+.Xr mmap 2 .
+Not all drivers implement all system calls, for example, calling
+.Xr mmap 2
+on a terminal devices is likely to be not useful at all.
+.Ss Accessing Devices
+Most of the devices in a unix-like operating system are accessed
+through so-called
+.Em device nodes ,
+sometimes also called
+.Em special files .
+They are usually located under the directory
+.Pa /dev
+in the file system hierarchy
+.Pq see also Xr hier 7 .
+.Pp
+Until
+.Xr devfs 5
+is fully functional, each device node must be created statically and
+independently of the existence of the associated device driver,
+usually by running
+.Xr MAKEDEV 8 .
+.Pp
+Note that this could lead to an inconsistent state, where either there
+are device nodes that do not have a configured driver associated with
+them, or there may be drivers that have successfully probed for their
+devices, but cannot be accessed since the corresponding device node is
+still missing.  In the first case, any attempt to reference the device
+through the device node will result in an error, returned by the upper
+layers of the kernel, usually
+.Ql ENXIO .
+In the second case, the device node needs to be created before the
+driver and its device will be usable.
+.Pp
+Some devices come in two flavors:
+.Em block
+and
+.Em character
+devices, or by a better name, buffered and unbuffered
+.Pq raw
+devices.  The traditional names are reflected by the letters
+.Ql b
+and
+.Ql c
+as the file type identification in the output of
+.Ql ls -l .
+Buffered devices are being accessed through the buffer cache of the
+operating system, and they are solely intended to layer a file system
+on top of them.  They are normally implemented for disks and disk-like
+devices only, for historical reasons also for tape devices.
+.Pp
+Raw devices are available for all drivers, including those that also
+implement a buffered device.  For the latter group of devices, the
+differentiation is conventionally done by prepending the latter
+.Ql r
+to the path name of the device node, for example
+.Pa /dev/rsd0
+denotes the raw device for the first SCSI disk, while
+.Pa /dev/sd0
+is the corresponding device node for the buffered device.
+.Pp
+Unbuffered devices should be used for all actions that are not related
+to file system operations, even if the device in question is a disk
+device.  This includes making backups of entire disk partitions, or
+to
+.Em raw
+floppy disks
+.Pq i.e. those used like tapes .
+.Pp
+Access restrictions to device nodes are usually subject of the regular
+file permissions of the device node entry, instead of being implied
+directly by the drivers in the kernel.
+.Ss Drivers without device nodes
+Drivers for network devices do not use device nodes in order to be
+accessed.  Their selection is based on other decisions inside the
+kernel, and instead of calling
+.Xr open 2 ,
+use of a network device is generally introduced by using the system
+call
+.Xr socket 2 .
+.Ss Configuring a driver into the kernel
+For each kernel, there is a configuration file that is used as a base
+to select the facilities and drivers for that kernel, and to tune
+several options.  See
+.Xr config 8
+for a detailed description of the files involved.  The individual
+manual pages in this section provide a sample line for the
+configuration file in their synopsis portion.  See also the sample
+config file
+.Pa /sys/i386/conf/LINT
+.Po
+for the
+.Em i386
+architecture
+.Pc .
+.Sh SEE ALSO
+.Xr open 2 ,
+.Xr close 2 ,
+.Xr read 2 ,
+.Xr write 2 ,
+.Xr ioctl 2 ,
+.Xr select 2 ,
+.Xr mmap 2 ,
+.Xr socket 2 ,
+.Xr hier 7 ,
+.Xr config 8 ,
+.Xr MAKEDEV 7 ,
+.Xr devfs 5 Pq not yet existent .
+.Sh AUTHORS
+This man page has been written by
+.if t J\(:org Wunsch
+.if n Joerg Wunsch
+with initial input by David E. O'Brien.
+.Sh HISTORY
+.Nm intro
+appeared in FreeBSD 2.1.
+