The PowerPC architecture consists of the following layers, and adherence to the PowerPCarchitecture can be measured in terms of which of the following levels of the architectureis implemented:• PowerPC user instruction set architecture—Defines the base user-level instruction set, user-level registers, data types, floating-point exception model, memory models for a uniprocessor environment, and programming model for uniprocessor environment. Chapter 1. Overview 1-11• PowerPC virtual environment architecture—Describes the memory model for a multiprocessor environment, defines cache control instructions, and describes other aspects of virtual environments. Implementations that conform to the PowerPC virtual environment architecture also adhere to the PowerPC user instruction set architecture, but may not necessarily adhere to the PowerPC operating environment architecture.• PowerPC operating environment architecture—Defines the memory management model, supervisor-level registers, synchronization requirements, and the exception model. Implementations that conform to the PowerPC operating environment architecture also adhere to the PowerPC user instruction set architecture and the PowerPC virtual environment architecture definition.Note that while the 601 is said to adhere to the PowerPC architecture at all three levels, itdiverges in aspects of its implementation to a greater extent than should be expected ofsubsequent PowerPC processors. Many of the differences result from the fact that the 601design provides compatibility with an existing architecture standard (POWER), whileproviding a reliable platform for hardware and software development compatible withsubsequent PowerPC processors. Note that except for the POWER instructions and the RTC implementation, the differencesbetween the 601 and the PowerPC architecture are primarily differences in the operatingenvironment architecture.The PowerPC architecture allows a wide range of designs for such features as cache andsystem interface implementations.
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