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March 2002 Release
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Appendix E
PowerPC
®
6xx/7xx Compatibility
This appendix outlines the programming model differences between the 40x family and
the 6xx/7xx family of PowerPC processors. The PowerPC 6xx/7xx family complies with
the original PowerPC architecture designed for desktop applications. The PowerPC 40x
family complies with the PowerPC embedded-environment architecture designed for
embedded applications. The information contained in this appendix is useful to system
programmers porting software from one family to another.
The two architectures are compatible at the user instruction-set architecture (UISA) level
but differ at the level of the virtual-environment architecture (VEA) and operating-
environment architecture (OEA). The PowerPC embedded-environment architecture
optimizes the VEA and OEA to meet the unique requirements of embedded applications.
These optimizations include changes in memory management, cache management,
exceptions, timer resources, and others. Many of these optimizations are reflected by the
different special-purpose registers (SPRs) supported by the families.
Porting software between implementations is usually limited to the operating-system
kernel and other privileged-mode software. Applications usually require no modification.
Software porting can be simplified through the use of structured programming methods
that localize program modules requiring modification. For example, if all access to the time
base are performed using a single function, only that function needs to be modified when
porting software to another PowerPC processor.
More information on the PowerPC architecture can be found in the
PowerPC
Microprocessor Family: The Programming Environments.
Refer to implementation-specific
documentation for more information on initialization and configuration, performance
considerations, special-purpose registers, and other software-visible details that can vary
from processor to processor.
Registers
summarizes the registers supported by the PowerPC 40x family that are not
supported by the PowerPC 6xx/7xx family.
summarizes the registers supported
by the PowerPC 6xx/7xx family that are not supported by the PowerPC 40x family. Not all
registers shown for a particular family are supported by all members within that family.
Table E-1:
40x Registers Not Supported by 6xx/7xx Processors
Name
Description
Purpose
SPRG4–7
SPR general-purpose registers 4–7
Software defined
USPRG0
User SPR general-purpose register 0
CCR0
Core-configuration register
Processor configuration
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