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+.\"
+.\" Copyright (c) 2003 Marcel Moolenaar
+.\" 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 AUTHOR ``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 AUTHOR 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.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd August 25, 2003
+.Dt UART 4
+.Os
+.\"
+.Sh NAME
+.Nm uart
+.Nd driver for Universal Asynchronous Receiver/Transmitter (UART) devices
+.\"
+.Sh SYNOPSIS
+.Cd "device uart"
+.Pp
+.Cd "device puc"
+.Cd "device uart"
+.Pp
+.\"
+.Sh DESCRIPTION
+The
+.Nm
+device driver provides support for various classes of UARTs implementing the
+EIA RS-232C (CCITT V.24) serial communications interface.
+Each such interface is controlled by a seperate and independent instance of
+the
+.Nm
+driver.
+The primary support for devices that contain multiple serial interfaces or
+that contain other functionality besides one or more serial interfaces is
+provided by the
+.Xr puc 4
+device driver.
+However, the serial interfaces of those devices that are managed by the
+.Xr puc 4
+driver are controlled by the
+.Nm
+driver.
+As such, the
+.Xr puc 4
+driver provides umbrella functionality for the
+.Nm
+driver and hides the complexities that are inherent when elementary components
+are packaged together.
+.Pp
+The
+.Nm
+driver has a modular design to allow it to be used on differing hardware and
+for various purposes.
+In the following sections the components are discussed in detail.
+Options are described in the section that covers the component to which each
+option applies.
+.\"
+.Ss CORE COMPONENT
+At the heart of the
+.Nm
+driver is the core component. It contains the bus attachments and the low-level
+interrupt handler.
+.\"
+.Ss HARDWARE DRIVERS
+The core component and the kernel interfaces talk to the hardware through the
+hardware interface.
+This interface serves as an abstraction of the hardware and allows varying
+UARTs to be used for serial communications.
+.\"
+.Ss SYSTEM DEVICES
+System devices are UARTs that have a special purpose by way of hardware
+design or software setup.
+For example, Sun UltraSparc machines use UARTs as their keyboard interface.
+Such an UART cannot be used for general purpose communications.
+Likewise, when the kernel is configured for a serial console, the
+corresponding UART will in turn be a system device so that the kernel can
+output boot messages early on in the boot process.
+.\"
+.Ss KERNEL INTERFACES
+The last but not least of the components is the kernel interface.
+This component ultimately determines how the UART is made visible to the
+kernel in particular and to users in general.
+The default kernel interface is the TTY interface.
+This allows the UART to be used for terminals, modems and serial line IP
+applications.
+System devices, with the notable exception of serial consoles, generally
+have specialized kernel interfaces.
+.\"
+.Sh SEE ALSO
+.Xr puc 4
+.\"
+.Sh HISTORY
+The
+.Nm
+device driver first appeared in
+.Fx 5.2 .