IBM RS/6000 - Manual (Page 86)

Figure 22. Boot Image Layout on PCI-Based RS/6000 Systems
When the system is booted, the firmware loads the first sector containing the
Master Partition Header, which includes the AIX boot record and the partition table.
The Partition Entry Offset and the Length of Partition In Sectors, extracted from the
first partition entry in the table, indicates where to find the next partition to be
loaded and the size of the partition, respectively. This partition contains the
Software ROS code.
After Software ROS takes control and performs the appropriate initialization, it
locates the AIX Version 4 boot image and loads it at address zero as required by
AIX.
Figure 23 shows the structure of the RSPC-style boot image. Areas Reserved 1
and Reserved 2 are reserved for padding purposes.
Figure 23. Structure of the Boot Image on PCI-Based RS/6000 Systems
62 Introduction to PCI-Based RS/6000 Servers