165 lines
7.9 KiB
Plaintext
165 lines
7.9 KiB
Plaintext
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SERIAL CONSOLE USAGE NOTES
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Written by
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Bill Paul <wpaul@ctr.columbia.edu>
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The FreeBSD boot block can now be used to boot FreeBSD on a system with
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only a dumb terminal on a serial port (COM1) as a console. This feature
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is provided for the benefit of people who wish to install FreeBSD on
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dedicated file/compute/terminal server machines that have no keyboard
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(or monitor) attached, just as is possible with Sun workstations and
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servers. People who don't need this extra functionality shouldn't notice
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the changes at all (unless I've screwed something up horribly).
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Note that 'options COMCONSOLE' can still be used to force the kernel to
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boot in 'serial console' mode regardless of what boot options you use.
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To boot FreeBSD in serial console mode, you must do the following:
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- UNPLUG YOUR KEYBOARD. Most PC systems probe for the keyboard during the
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Power-On Self-Test (POST) and will generate an error if the keyboard
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isn't detected. Additionally, many machines will pause the boot process
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and wait for you to reattach the keyboard and press a key before
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proceeding any further. If your computer complains about the lack of a
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keyboard but boots anyway, then you don't have to do anything special.
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(One machine with a PHOENIX BIOS that I have here merely says 'Keyboard
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failed' then continues to boot normally.) If your machine complains
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loudly about the lack of a keyboard and won't continue to boot until you
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plug it back in, you'll have to go into your CMOS configuration menu and
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change the 'Keyboard' setting to 'Not installed' in order to bypass the
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keyboard probe.
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NOTE #1:
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Setting the keyboard to 'Not installed' in the CMOS configuration
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does *NOT* mean that you won't be able to use your keyboard. All this
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does is tell the BIOS not to probe for a keyboard at power-on so that
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it won't bitch and moan if the keyboard isn't plugged in. You can leave the
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keyboard plugged in even with this flag set to 'Not installed' and the
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keyboard will still work. I repeat: changing the CMOS 'keyboard' setting
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to 'Not installed' only disables the BIOS's keyboard probe; it does
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*NOT* actually disable the keyboard.
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NOTE #2:
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If your system has a PS/2 mouse, chances are very good that you will
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need to unplug your mouse as well as your keyboard. This is because
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PS/2 mice share some hardware with the keyboard, and leaving the mouse
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plugged in can fool the keyboard probe into thinking the keyboard is
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still there. I have access to a Gateway 2000 Pentium 90Mhz system with
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an AMI BIOS that behaves this way. In general this is not a problem
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since the mouse isn't much good without the keyboard anyway.
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- PLUG A DUMB TERMINAL INTO COM1. If you don't have a dumb terminal, you
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can use an old PC/XT with a modem program, or the serial port on
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another UNIX box. If you don't have a COM1, get one. At this time,
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there is no way to select a port other than COM1 without recompiling
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both the kernel and the boot blocks. If you're already using COM1 for
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another device, you'll have to temporarily remove that device and
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install a new boot block and kernel once you get FreeBSD up and running.
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(It is assumed that COM1 will be available on a file/compute/terminal
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server anyway; if you really need COM1 for something else (and you can't
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switch that something else to COM2), then you probably shouldn't even
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be bothering with all this in the first place.)
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NOTE #1:
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The serial port settings are hardcoded to 9600 baud, 8 bits, no parity,
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1 stop bit.
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NOTE #2:
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In addition to a serial cable, you will need a null modem adapter
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in order to connect the terminal to the PC's serial port. If you don't
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have one, go to Radio Shack and buy one: they're cheap.
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NOTE #3:
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If you wish to drop into the kernel debugger from the serial console
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(useful for remote diagnostics, but also dangerous if you generate a
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spurious BREAK on the serial port!) then you should compile your kernel
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with the following options:
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options BREAK_TO_DEBUGGER
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options DDB
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- BOOT THE MACHINE. The boot block will probe for a keyboard on your
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system. If it fails to find one, you'll see a prompt appear on the
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terminal that looks something like this:
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No keyboard found.
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>> FreeBSD BOOT @ 0x10000: 640/7168 k of memory
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Usage: [[[0:][fd](0,a)]/kernel][-abcCdhrsv]
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Use 1:sd(0,a)kernel to boot sd0 if it is BIOS drive 1
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Use ? for file list or press Enter for defaults
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Boot:
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This is identical to the prompt that normally appears on the VGA console,
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except for the 'No keyboard found' message that indicates a keyboard
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couldn't be detected. (If a keyboard is detected, the boot prompt will
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appear on the VGA display as usual.)
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From here you can boot the system (or let it autoboot by itself) just
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like you can from the VGA console and the kernel will automatically
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use COM1 as the console device. No recompilation or 'options COMCONSOLE'
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is required. This is done by passing a special flag to the kernel in
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the 'boothowto' word. (The curious can refer to <sys/reboot.h> and the
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sio driver sources for details.)
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- You will notice that there's a new boot flag: -h. You can use this to
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force the kernel to switch console devices. For instance, if you boot
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from the VGA console, you can use -h to force the kernel to use the
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serial port as its console device. Alternatively, if you boot from
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the serial port, you can use the -h to force the kernel to use the VGA
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display as the console instead. (Can you say 'toggle' boys and girls?
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I knew you could. :)
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Should you wish to force booting off a serial console no matter if
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there's a keyboard connected or not, you can also uncomment the line
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with the ``FORCE_COMCONSOLE'' definition in the Makefile. Remake and
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reinstall your bootblocks, and finally relabel your disk (disklabel -B)
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to pick up those boot blocks.
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CAVEATS:
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- The idea here is to allow people to set up dedicated servers that require
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no graphics hardware or attached keyboards. Unfortunately, while (most?)
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every system will let you boot without a keyboard, there are quite a few
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that will not let you boot without a graphics adapter. Machines with
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AMI BIOSes can be configured to boot with no graphics adapter installed
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simply by changing the 'graphics adapter' setting in the CMOS configuration
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to 'Not installed.' However, many machines do not support this option
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and will refuse to boot if you have no display hardware in the system. With
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these machines, you'll have to leave some kind of graphics card plugged in,
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(even if it's just a junky mono board) although you won't have to attach
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a monitor into it. You might also try installing an AMI BIOS. :)
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- Using a port other than COM1 as the console requires some recompiling.
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Again, it's usually assumed that COM1 will be available for use as a
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console device on a dedicated file/compute/terminal server, so hopefully
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you'll never need to do this. But if you feel you must change the console
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to a different port, here's how:
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o Get the kerndist kernel source package.
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o Edit /sys/i386/boot/biosboot/Makefile and set COMCONSOLE to the
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address of the port you want to use (0x3F8, 0x2F8, 0x3E8 or
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0x2E8). Only COM1 through COM4 can be used; multiport serial
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cards will not work. No interrupt setting is needed.
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o Create a custom kernel configuration file and add the following
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lines:
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options "CONADDR=0x3F8"
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options "CONUNIT=0"
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Set CONADDR to the same address that you selected for COMCONSOLE
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in the bootbios Makefile. Set CONUNIT to the unit number of the
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serial port that this address corresponds to (0 = sio0, 1 = sio1,
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etc). This implies that the serial port you want to use must be
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configured into the kernel in the normal way first. I'm not
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going to list all the possible combinations here; just use your
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head and you should be okay.
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o Recompile both the boot blocks and the kernel.
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o Install the boot blocks with the disklabel command and boot
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from the new kernel.
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$Id: README.serial,v 1.5 1996/04/13 11:57:18 jkh Exp $
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