DEV(4)							     Kernel Interfaces Manual							    DEV(4)

NAME

       dev - device files in /dev

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 read(2) or write(2) calls,  byte  positions  set
       with lseek(2), or more complicated control functions performed with ioctl(2).

       Device files as found in /dev have several attributes that must be considered.  Here are two examples as ls -l shows them:

	      brw-rw-rw-  1 root     operator	2,   1 Jun 10  1995 fd1
	      crw--w----  1 kjb      tty	4,   0 May 11 09:41 console

       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.

       Character devices are marked with a 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.

       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.

       The  major  device  number  (2 for fd1 and 4 for console) are used by FS to find the device driver that manages a device.  The minor device
       number (1 for fd1 and 0 for 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:

   Memory (major 1)
       The ram, mem, kmem, and null devices are managed by the memory task.  The ram device is a block device for a chunk of memory  that  is  the
       RAM  disk.   Any  byte read from or written to the ram device is copied from or to that memory chunk.  The mem device is a character device
       for the entire address space of the system, but kmem only for the kernel data area.  These two devices allow programs like  ps(1)  to  hunt
       around  the system looking for interesting bits.  The null device is a data sink.  It happily swallows any bytes written to it, and returns
       nothing on a read.

   Floppy disk (major 2)
       The fd0, fd0a, fd0b, fd0c, and fd0d block devices are the first floppy disk and the four partitions that may exist on a that  floppy  disk.
       Likewise  are  fd1  and	fd1[a-d] the device and partitions for the second floppy disk.	The floppy disk devices are described in detail in
       fd(4).  Partitioning in general is explained in hd(4).

   Hard disk (major 3)
       The first hard disk can be accessed by block device 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, hd1, hd2, hd3, and hd4.  Each of these devices accesses a range of sec-
       tors on the 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
       hd1,  and  Minix can be on hd2.	Minix wants to have several partitions on its own, so hd2 can be further subdivided into the subpartitions
       hd2a, hd2b, hd2c, and hd2d.  /dev contains devices for the first and second hard disk (hd0 and hd5) their primary  partitions  (hd[1-46-9])
       and subpartitions thereof (hd[1-46-9][a-d]).  More detail can be found in hd(4).

   Terminals (minor 4)
       The TTY driver manages the system console device, aptly named console, the serial lines, tty00 and 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  ttyc1,  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  termi-
       nals.   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 console(4), and tty(4).

   Anonymous TTY (major 5)
       This is just one device named 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 /dev/tty.

   Line printer (major 6)
       The lp device sends any bytes written to it to the printer.

   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 ip(4).

   CD-ROM (major 8)
       This is the CD-ROM driver for the Mitsumi proprietary CD-ROM interface.	The cd0 device addresses the whole  CD,  with  extra  cd[1-4]  and
       cd[1-4][a-d] devices for if the CD also contains partitions with Minix file systems.

   SCSI disks and tapes (major 10)
       The  sd*  devices  are  disks  in the same way as the hd* devices.  Except that these disks are SCSI disks attached to an Adaptec 1540 con-
       troller or compatible.  The driver also manages the rst* and nrst* tape devices (rewinding or non-rewinding).  See sd(4).

   Audio (major 13)
       The audio device can be used to produce or record air vibrations using a Soundblaster 16 type audio card.  See audio(4).

   Mixer (major 14)
       The mixer device is used to control the audio driver.

FILES

       /dev/*	 All Minix devices

SEE ALSO

       read(2), write(2), lseek(2), ioctl(2), console(4), fd(4), hd(4), ip(4), sd(4), tty(4), MAKEDEV(8).

DIAGNOSTICS

       There are five prominent errors that processes accessing device files may provoke:

       ENODEV - No such device
	    There is no driver managing the device class this device belongs to.  Either the driver is configured out, or it is not loaded (inet).

       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  sec-
	    ond disk on a system with only one disk.

       EACCES - Permission denied
	    This error may cause a lot of head scratching if ls -l shows a device file to be writable.	The media you are trying to access is sim-
	    ply physically write protected!

       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.

       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.

NOTES

       Some devices are not present by default.  The MAKEDEV script knows how to make them.

   MS-DOS equivalents
       The names of MS-DOS devices map to Minix devices as follows:

	      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

AUTHOR

       Kees J. Bot (kjb@cs.vu.nl)

																	    DEV(4)