SERIAL CONSOLE USAGE NOTES
Written by
Bill Paul <wpaul@ctr.columbia.edu>
The FreeBSD boot block can now be used to boot FreeBSD on a system with
only a dumb terminal on a serial port (COM1) as a console. This feature
is provided for the benefit of people who wish to install FreeBSD on
dedicated file/compute/terminal server machines that have no keyboard
(or monitor) attached, just as is possible with Sun workstations and
servers. People who don't need this extra functionality shouldn't notice
the changes at all (unless I've screwed something up horribly).
Note that 'options COMCONSOLE' can still be used to force the kernel to
boot in 'serial console' mode regardless of what boot options you use.
To boot FreeBSD in serial console mode, you must do the following:
- UNPLUG YOUR KEYBOARD. Most PC systems probe for the keyboard during the
Power-On Self-Test (POST) and will generate an error if the keyboard
isn't detected. Additionally, many machines will pause the boot process
and wait for you to reattach the keyboard and press a key before
proceeding any further. If your computer complains about the lack of a
keyboard but boots anyway, then you don't have to do anything special.
(One machine with a PHOENIX BIOS that I have here merely says 'Keyboard
failed' then continues to boot normally.) If your machine complains
loudly about the lack of a keyboard and won't continue to boot until you
plug it back in, you'll have to go into your CMOS configuration menu and
change the 'Keyboard' setting to 'Not installed' in order to bypass the
keyboard probe.
NOTE #1:
Setting the keyboard to 'Not installed' in the CMOS configuration
does *NOT* mean that you won't be able to use your keyboard. All this
does is tell the BIOS not to probe for a keyboard at power-on so that
it won't bitch and moan if the keyboard isn't plugged in. You can leave the
keyboard plugged in even with this flag set to 'Not installed' and the
keyboard will still work. I repeat: changing the CMOS 'keyboard' setting
to 'Not installed' only disables the BIOS's keyboard probe; it does
*NOT* actually disable the keyboard.
NOTE #2:
If your system has a PS/2 mouse, chances are very good that you will
need to unplug your mouse as well as your keyboard. This is because
PS/2 mice share some hardware with the keyboard, and leaving the mouse
plugged in can fool the keyboard probe into thinking the keyboard is
still there. I have access to a Gateway 2000 Pentium 90Mhz system with
an AMI BIOS that behaves this way. In general this is not a problem
since the mouse isn't much good without the keyboard anyway.
- PLUG A DUMB TERMINAL INTO COM1. If you don't have a dumb terminal, you
can use an old PC/XT with a modem program, or the serial port on
another UNIX box. If you don't have a COM1, get one. At this time,
there is no way to select a port other than COM1 without recompiling
both the kernel and the boot blocks. If you're already using COM1 for
another device, you'll have to temporarily remove that device and
install a new boot block and kernel once you get FreeBSD up and running.
(It is assumed that COM1 will be available on a file/compute/terminal
server anyway; if you really need COM1 for something else (and you can't
switch that something else to COM2), then you probably shouldn't even
be bothering with all this in the first place.)
NOTE #1:
The serial port settings are hardcoded to 9600 baud, 8 bits, no parity,
1 stop bit.
NOTE #2:
In addition to a serial cable, you will need a null modem adapter
in order to connect the terminal to the PC's serial port. If you don't
have one, go to Radio Shack and buy one: they're cheap.
NOTE #3:
If you wish to drop into the kernel debugger from the serial console
(useful for remote diagnostics, but also dangerous if you generate a
spurious BREAK on the serial port!) then you should compile your kernel
with the following options:
options BREAK_TO_DEBUGGER
options DDB
- BOOT THE MACHINE. The boot block will probe for a keyboard on your
system. If it fails to find one, you'll see a prompt appear on the
terminal that looks something like this:
No keyboard found.
>> FreeBSD BOOT @ 0x10000: 640/7168 k of memory
Usage: [[[0:][fd](0,a)]/kernel][-abcCdhrsv]
Use 1:sd(0,a)kernel to boot sd0 if it is BIOS drive 1
Use ? for file list or press Enter for defaults
Boot:
This is identical to the prompt that normally appears on the VGA console,
except for the 'No keyboard found' message that indicates a keyboard
couldn't be detected. (If a keyboard is detected, the boot prompt will
appear on the VGA display as usual.)
From here you can boot the system (or let it autoboot by itself) just
like you can from the VGA console and the kernel will automatically
use COM1 as the console device. No recompilation or 'options COMCONSOLE'
is required. This is done by passing a special flag to the kernel in
the 'boothowto' word. (The curious can refer to <sys/reboot.h> and the
sio driver sources for details.)
- You will notice that there's a new boot flag: -h. You can use this to
force the kernel to switch console devices. For instance, if you boot
from the VGA console, you can use -h to force the kernel to use the
serial port as its console device. Alternatively, if you boot from
the serial port, you can use the -h to force the kernel to use the VGA
display as the console instead. (Can you say 'toggle' boys and girls?
I knew you could. :)
Should you wish to force booting off a serial console no matter if
there's a keyboard connected or not, you can also uncomment the line
with the ``FORCE_COMCONSOLE'' definition in the Makefile. Remake and
reinstall your bootblocks, and finally relabel your disk (disklabel -B)
to pick up those boot blocks.
CAVEATS:
- The idea here is to allow people to set up dedicated servers that require
no graphics hardware or attached keyboards. Unfortunately, while (most?)
every system will let you boot without a keyboard, there are quite a few
that will not let you boot without a graphics adapter. Machines with
AMI BIOSes can be configured to boot with no graphics adapter installed
simply by changing the 'graphics adapter' setting in the CMOS configuration
to 'Not installed.' However, many machines do not support this option
and will refuse to boot if you have no display hardware in the system. With
these machines, you'll have to leave some kind of graphics card plugged in,
(even if it's just a junky mono board) although you won't have to attach
a monitor into it. You might also try installing an AMI BIOS. :)
- Using a port other than COM1 as the console requires some recompiling.
Again, it's usually assumed that COM1 will be available for use as a
console device on a dedicated file/compute/terminal server, so hopefully
you'll never need to do this. But if you feel you must change the console
to a different port, here's how:
o Get the kerndist kernel source package.
o Edit /sys/i386/boot/biosboot/Makefile and set COMCONSOLE to the
address of the port you want to use (0x3F8, 0x2F8, 0x3E8 or
0x2E8). Only COM1 through COM4 can be used; multiport serial
cards will not work. No interrupt setting is needed.
o Create a custom kernel configuration file and add the following
lines:
options "CONADDR=0x3F8"
options "CONUNIT=0"
Set CONADDR to the same address that you selected for COMCONSOLE
in the bootbios Makefile. Set CONUNIT to the unit number of the
serial port that this address corresponds to (0 = sio0, 1 = sio1,
etc). This implies that the serial port you want to use must be
configured into the kernel in the normal way first. I'm not
going to list all the possible combinations here; just use your
head and you should be okay.
o Recompile both the boot blocks and the kernel.
o Install the boot blocks with the disklabel command and boot
from the new kernel.
$Id$
