IBM RS/6000 - Manual (Page 88)

Figure 24. AIX Version 4 Boot Image Creation
When the bosboot command is started, it creates a RAM file system and calls the
mkboot command to create the boot image as is normal for an AIX system. Upon
detection of a RSPC system, the bosboot command calls the mkmboot routine to
encapsulate the AIX Version 4 boot image into the RSPC-style boot image.
4.4 Understanding the Firmware
The firmware is the first code to execute when the machine is powered-on. This
code resides on the hardware, and it is coded in the factory.
The firmware uses the console and the speaker as means to communicate the
results of certain tests. By default, the firmware uses the serial port S1 to display
logos, messages and errors. In case a supported graphics adapter is installed in
the machine, the firmware will redirect the output to this adapter instead of the
serial port.
The firmware is organized into three major sections:
Boot Block Receives control at power-on and is able to perform a soft firmware
recovery from diskette when necessary
64 Introduction to PCI-Based RS/6000 Servers