IBM RS/6000 - Manual (Page 79)

Different partitions could hold different operating systems or different versions of
AIX. The user could be able to choose the operating system to boot, as in
“dualboot” on OS/2 and DOS.
The problem with this implementation is that both the Master boot record and the
AIX boot record are required to start on disk block 0. To satisfy this requirement,
the boot record on PCI-based RS/6000 systems is a combination of both, as shown
in Figure 19.
Figure 19. Hardfile Layout on Microchannel-Based RS/6000 Systems
Note
Partitioned disks are not supported in the current release of AIX. Currently, AIX
requires all disks used by AIX to be dedicated disks.
4.2 Boot Components Specifically for AIX 4 on PCI-Based RS/6000
Systems
The firmware on microchannel-based RS/6000 systems is sophisticated enough to:
Find the boot device
Build the IPL control Block required by the AIX boot image
Load the AIX boot image from the boot device
Pass control to the boot image
On PCI-based RS/6000 systems, however, the firmware is simple and generic,
independent of the operating system; therefore it does not build the IPL control
block for AIX, but:
Finds the boot device
Locates the operating system bootstrap code
Passes control to the bootstrap code
regardless which operating system is booting.
In order to create the IPL control block and perform the initialization required by the
AIX boot image, a new, intermediary software component has to be added to the
boot process on PCI-based RS/6000 systems, the Software ROS.
The boot process on PCI-based RS/6000 systems includes the following three
major components:
Chapter 4. Boot Support and Firmware 55