uart(4) driver for Universal Asynchronous Receiver/Transmitter (UART) devices


device uart

device puc device uart

device scc device uart

In /boot/device.hints hint.uart.0.disabled=1 hint.uart.0.baud=38400 hint.uart.0.port=0x3f8 hint.uart.0.flags=0x10

With flags encoded as:

device is potential system console
use this port for remote kernel debugging
set RX FIFO trigger level to ``low'' (NS8250 only)
set RX FIFO trigger level to ``medium low'' (NS8250 only)
set RX FIFO trigger level to ``medium high'' (default, NS8250 only)
set RX FIFO trigger level to ``high'' (NS8250 only)


The 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 separate and independent instance of the 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 puc(4), or scc(4) device drivers. However, the serial interfaces of those devices that are managed by the puc(4), or scc(4) driver are each independently controlled by the driver. As such, the puc(4), or scc(4) driver provides umbrella functionality for the driver and hides the complexities that are inherent when elementary components are packaged together.

The 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.


At the heart of the driver is the core component. It contains the bus attachments and the low-level interrupt handler.


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.


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.


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.


The driver supports the following classes of UARTs:

  • NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or the 16950 UARTs.
  • SCC: serial communications controllers supported by the scc(4) device driver.


for callin ports
corresponding callin initial-state and lock-state devices

for callout ports
corresponding callout initial-state and lock-state devices


The device driver first appeared in Fx 5.2 .


The device driver and this manual page were written by An Marcel Moolenaar Aq [email protected] .