IBM RS/6000 - Manual (Page 77)

Chapter 4. Boot Support and Firmware
The PCI RS/6000 Entry Server systems are based on the PowerPC Reference
Platform Specification (PReP); thus, in contrast to the microchannel-based RS/6000
systems systems, their hardware is not bound to a specific operating system.
In order to separate the hardware from the software, an abstraction layer, defined
in PReP, is required. The abstraction layer is called Software ROS (Read-Only
Storage) and includes the following two components:
BTAS (Boot Time Abstraction Software) Abstracts the hardware that a platform's
boot program (for example, firmware) uses at boot time. It also
abstracts the hardware that the operating system loader uses to load an
operating system.
RTAS (Run-Time Abstraction Software) A collection of data and software that
abstracts hardware from the operating system kernel
For more information on the PReP-specific abstraction layer, refer to the Managing
AIX V4 on PCI-Based RS/6000 Workstations redbook, or to the PowerPC
Reference Platform Specification.
Figure 16. Boot Structure on PCI-Based RS/6000 Systems and Microchannel-Based
RS/6000 Systems
Figure 16 shows the components involved in booting a PCI-based RS/6000 system
compared to the microchannel-based RS/6000 systems.
Since on the microchannel-based RS/6000 systems the only operating system
supported is AIX, the firmware (also called system ROS on those machines) builds
the IPL control block required by the AIX boot image using the structure defined by
AIX.
On PCI-based RS/6000 systems, the firmware initializes the hardware in a generic
way to allow any PReP-compatible operating system to boot on the system. The
hardware initialization data is passed to the abstraction layer software via residual
 Copyright IBM Corp. 1996 53