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33 .Nd introduction to devices and device drivers
35 This section contains information related to devices, device drivers
36 and miscellaneous hardware.
37 .Ss The device abstraction
38 Device is a term used mostly for hardware-related stuff that belongs
39 to the system, like disks, printers, or a graphics display with its
41 There are also so-called
43 where a device driver emulates the behaviour of a device in software
44 without any particular underlying hardware.
48 a loophole where the physical memory can be accessed using the regular
49 file access semantics.
51 The device abstraction generally provides a common set of system calls
52 layered on top of them, which are dispatched to the corresponding
53 device driver by the upper layers of the kernel.
55 calls available for devices is chosen from
64 Not all drivers implement all system calls, for example, calling
66 on terminal devices is likely to be not useful at all.
68 Most of the devices in a unix-like operating system are accessed
73 They are usually located under the directory
75 in the file system hierarchy
79 Some devices come in two flavors:
83 devices, or to use better terms, buffered and unbuffered
86 The traditional names are reflected by the letters
90 as the file type identification in the output of
92 Buffered devices are being accessed through the buffer cache of the
93 operating system, and they are solely intended to layer a file system
95 They are normally implemented for disks and disk-like
96 devices only and, for historical reasons, for tape devices.
98 Raw devices are available for all drivers, including those that also
99 implement a buffered device.
100 For the latter group of devices, the
101 differentiation is conventionally done by prepending the letter
103 to the path name of the device node, for example
105 denotes the raw device for the first SCSI disk, while
107 is the corresponding device node for the buffered device.
109 Unbuffered devices should be used for all actions that are not related
110 to file system operations, even if the device in question is a disk
112 This includes making backups of entire disk partitions, or
116 (i.e. those used like tapes).
118 Access restrictions to device nodes are usually subject to the regular
119 file permissions of the device node entry, instead of being enforced
120 directly by the drivers in the kernel.
121 .Ss Drivers without device nodes
122 Drivers for network devices do not use device nodes in order to be
124 Their selection is based on other decisions inside the
125 kernel, and instead of calling
127 use of a network device is generally introduced by using the system
130 .Ss Configuring a driver into the kernel
131 For each kernel, there is a configuration file that is used as a base
132 to select the facilities and drivers for that kernel, and to tune
136 for a detailed description of the files involved.
137 The individual manual pages in this section provide a sample line for the
138 configuration file in their synopsis portion. See also the sample
140 .Pa /sys/config/LINT64
158 manual page first appeared in
162 This man page was written by
164 with initial input by
165 .An David E. O'Brien .