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Chapter 3. Optionally configure the BIOS

Some BIOSs provide support for serial consoles. If your computer's BIOS is one of these you should investigate the extent of the support provided. Depending upon the extent of serial console support you may not need to explicitly configure the boot loader to use the serial port.

The contributors to this HOWTO have encountered the following styles of BIOS support for serial consoles.

Redirection of textual VGA output to the serial port

The BIOS takes the interrupt 0x10 "video" requests used to write to the screen and sends the characters that would have appeared on the screen to the serial port. Characters recieved from the serial port are used to supply characters to BIOS interrupt 0x16 "read key" requests.

Any 16-bit application which uses the BIOS functions for outputing text to the screen and reading from the keyboard is redirected to the serial port. This includes the BIOS itself, the boot loader, and 16-bit operating systems (such as MS-DOS).

When a 32-bit operating system (such as Linux, BSD or Windows NT/2000/XP) loads the 16-bit BIOS is no longer accessible and the BIOS can no longer be used for input and output. The 32-bit operating system loads its own device drivers for this purpose. These device drivers then need to provide the redirection of console I/O to the serial port.

If your BIOS uses this technique then you should:

  1. Configure the BIOS to redirect keyboard input and video output to the serial port.

  2. Do not configure the boot loader, as the BIOS will redirect this 16-bit application's input and output to the serial port.

  3. Configure Linux to use the serial port as a console, as Linux is a 32-bit operating system.

BIOS configuration and power on self-test uses the serial port

These BIOSs use the serial port for configuration and the power-on self-test, but do not redirect the interrupt 0x10 "video" requests interrupt 0x16 "read key" requests to the serial port.

Some BIOSs which usually redirect all keyboard and video output to the serial port can be configured in only to redirect BIOS input and output. Look for a BIOS configuration option similar to Cease redirection after boot.

If your BIOS uses this technique or you choose to set Cease redirection after boot then you should:

  1. Configure the BIOS to send its output to the serial port.

  2. Configure the boot loader to use the serial port.

  3. Configure Linux to use the serila port as the console, as Linux is a 32-bit operating system.

Redirection of graphical VGA output to the serial port

Some graphical 32-bit operating systems do not provide their own facilities to send console output to the serial port. Some BIOSs attempt to overcome this shortcoming, using a propietary serial protocol to send graphical output to a remote serial client.

As these machines cannot be connected to from a standard terminal emulator this facility is best left unconfigured when using the Linux operating system.

  1. Configure the BIOS not to send output to the serial port.

  2. Configure the boot loader to use the serial port.

  3. Configure Linux to use the serial port as the console.

No serial port facilities

The BIOS cannot be accessed from the serial port, so power-on self-test messages cannot be seen.

Note that BIOS may still be able to be configured remotely using the /dev/nvram device. This takes some care.

  1. Configure the boot loader to use the serial port.

  2. Configure Linux to use the serial port as the console.

If you need to configure the boot loader to use the serial port then continue to Chapter 4. Otherwise go directly to Chapter 5 to configure the kernel; this is done by configuring the boot loader to pass boot parameters to the Linux kernel.