.\"	$OpenBSD: ch.4,v 1.6 2000/03/24 14:56:09 niklas Exp $
.\"	$FreeBSD$
.\" Copyright (c) 1996
.\"	Julian Elischer <julian@freebsd.org>.  All rights reserved.
.\"
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.Dd March 28, 1997
.Dt CH 4
.Os
.Sh NAME
.Nm ch
.Nd scsi media-changer (juke box) driver
.Sh SYNOPSIS
.Cd "ch? at scsibus? target ? lun ?"
.Sh DESCRIPTION
The
.Xr ch
driver provides support for a
.Em scsi
juke box. It allows many slots of media to be multiplexed between a number
of drives.
.Pp
A scsi adapter and a logical scsibuss must also be separately configured
into the system before a scsi changer can be configured.
.Pp
As the scsi adapter is probed during boot, the
.Em SCSI
bus is scanned for devices. Any devices found which answer as 'Changer'
type devices will be 'attached' to the
.Nm
driver.
The first device found will be attached as
.Em ch0
and the next,
.Em ch1
etc.
It is also possible to specify what ch unit a device should
come on line as; refer to
.Xr scsi 4
for details on kernel configuration.
.Sh KERNEL CONFIGURATION
In configuring, if an optional
.Ar count
is given in the specification, that number of scsi media changers
are configured; Most storage for them is allocated only when found
so a large number of configured devices is cheap. (once the first
has included the driver).
.Sh IOCTLS
The following
.Xr ioctl 2
call applies to the changer. It is defined in
the header file
.Em sys/chio.h.
.Bl -tag -width DIOCSDINFO
CHIOOP
This appears to be a 'do-everything' call.
.El
.Sh FILES
.Bl -tag -width /dev/ch[0-9] -compact
.It Pa /dev/ch[0-9]
device entries
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr cd 4 ,
.Xr sd 4 ,
.Xr st 4
.Sh DISCLAIMER
These manpages were derived from the
.Fx
documentation of the generic scsi drivers.  Since naming and calling
conventions may have diverged or suffered port specific changes, please
review the appropriate header files and configuration file for your port.
.Sh HISTORY
The
.Nm
driver appeared in 386BSD 0.1.