IBM RS/6000 - Manual (Page 26)

The PowerPC Microprocessor Common Hardware Reference Platform, previously
known as the Common Hardware Reference Platform (CHRP), is a superset of the
PReP Specification. Although IBM has not yet announced any system based on
this new specification, it is becoming very popular, and many companies have
published their intentions to develop CHRP-compliant systems. For this reason, we
include a description of the CHRP specification in this chapter.
1.1 Rationale for the PowerPC Reference Platform Specification
Computer systems today span a wide range of environments, from hand-held
portables to room-size mainframes. The largest percentage of systems are based
on the IBM PC/AT, Apple Macintosh or a variety of workstation-level RISC
architectures.
These machines cover the needs of personal productivity, entry engineering design,
entry commercial data management, information analysis, and database, file, and
application servers. Today, despite their high levels of performance and
functionality, existing architectures limit the system designer's ability to add
innovative new features without jeopardizing operating systems and applications.
These limitations restrict the use of hardware and software enhancements which
promise improved user interfaces, faster system performance and broader
operating environments. Many times, system designers must carry obsolete
hardware structures to maintain compatibility.
Figure 1. Old Compatibility Model. Software communicates directly with hardware.
Figure 1 shows the old compatibility model, where the software communicates
directly with the hardware. Changes applied to the hardware require changes to
software and vice-versa, as shown in Figure 2.
Figure 2. Innovation Within The Old Compatibility Model
2 Introduction to PCI-Based RS/6000 Servers