PowerPC - Design Features

Design Features

The PowerPC is designed along RISC principles, and allows for a superscalar implementation. Versions of the design exist in both 32-bit and 64-bit implementations. Starting with the basic POWER specification, the PowerPC added:

• Support for operation in both big-endian and little-endian modes; the PowerPC can switch from one mode to the other at run-time (see below). This feature is not supported in the PowerPC 970. This was the reason Virtual PC took so long to be made functional on 970-based Macintosh computers.
• Single-precision forms of some floating point instructions, in addition to double-precision forms
• Additional floating point instructions at the behest of Apple
• A complete 64-bit specification that is backward compatible with the 32-bit mode
• A fused multiply–add
• A paged memory management architecture which is used extensively in server and PC systems.
• Addition of a new memory management architecture called Book-E, replacing the conventional paged memory management architecture for embedded applications. Book-E is application software compatible with existing PowerPC implementations, but needs minor changes to the operating system.

Some instructions present in the POWER instruction set were deemed too complex and were removed in the PowerPC architecture. Some of the removed instructions could be emulated by the operating system if necessary. The removed instructions are:

• Conditional moves
• Load and store instructions for the quad-precision floating-point data type
• String instructions.