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Introduction

Hardware Overview

LinuxBIOS Overview

Serial output and the Super I/O

The Super I/O is a chip found on most of today's mainboards which is — among other things — responsible for the serial ports of the mainboard (e.g. COM1, COM2). This chip usually the first thing you'll want to support, as it's required to get serial debugging output from the mainboard (via a null-modem cable and the proper software, e.g. minicom or CuteCom).

Winbond W83977EF Super I/O.
ITE IT8705F Super I/O.

The steps for adding support for a new Super I/O chip are:

Add a directory src/superio/vendor/device (e.g. src/superio/winbond/w83627ehg).
In that directory, add a file device_early_serial.c (e.g. w83627ehg_early_serial.c). This file will be responsible to setup a serial port on the mainboard so that you can get serial debugging output. This will work even before the RAM is initialized, thus is useful/required for debugging the RAM initialization process.
In this file you now declare a function device_enable_serial() which enables the requested serial port. Example:

static void w83627ehg_enable_serial(device_t dev, unsigned int iobase)
{
       pnp_enter_ext_func_mode(dev);
       pnp_set_logical_device(dev);
       pnp_set_enable(dev, 0);
       pnp_set_iobase(dev, PNP_IDX_IO0, iobase);
       pnp_set_enable(dev, 1);
       pnp_exit_ext_func_mode(dev);
}

...

Northbridge

RAM init

Resources:

Understanding DDR Serial Presence Detect (SPD) Table
Micron 512 MB SDRAM Datasheet (PDF) -- contains some helpful explanations

Southbridge

Mainboard

IRQ Table

Creating a new Target

Miscellaneous Tips

minicom

Minicom is not just a serial terminal. It was written long before the internet existed and electronic communication was only possible with a modem to a mailbox-computer. Minicom is written with the ncurses library and provides its magic via a text interface. Other than logging, it provides z-modem up- and download-capability.

CuteCom

This is an easy to use serial-terminal-program which is even able to write all communication into a log-file. It needs a computer with installed Qt-libs.

Developer Manual