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The PowerPC G3 Processor

Information and Background
Power Macintosh G3

PowerBook G3

The PowerPC G3 Processor:
Taking Macintosh to the Next Level
(PDF Version)

Power Macintosh G3:
Innovative Product Design
(PDF Version)

PowerPC Microprocessor Roadmap (PDF Version)

IBM: All About the PowerPC

Motorola: PowerPC

PowerPC G3 -- Taking Macintosh to the Next Level
The emergence of the PowerPC G3 processor marks the third generation in the development of advanced processor technology. Touted by Microprocessor Report (February 17, 1997) as "an outstanding combination of high performance and low cost," the PowerPC G3 builds on many of the features pioneered by the PowerPC 603 and 604. However this innovative chip differs from the earlier implementations of PowerPC processor technology in several significant ways, all designed to further increase performance:

  • Mac OS optimization
    The PowerPC G3 is the first processor specifically optimized for the Mac OS. Optimization designs were based on many popular Macintosh applications to ensure minimal branching and the shortest data transfer routes -- resulting in a very efficient processor.

  • Innovative level 2 backside cache design
    The level 2 cache is the link between level 1 on-chip cache and main memory. By relocating the level 2 cache from the logic board to the processor card (hence the name "backside"), the CPU can access level 2 cache directly using a dedicated bus specifically designed for this purpose. This enables the more effective use of level 2 cache, because even relatively large amounts of data can be stored and accessed rapidly and efficiently. As processor speed increases, so does the performance value of the backside cache. This design significantly speeds access to the level 2 cache and dramatically increases the overall system performance.

  • Large level 1 (on-chip) data and instruction caches
    Level 1 cache is a small amount of high speed memory built into the processor and in the PowerPC G3 this cache has been doubled from 32k to 64k. With 32k dedicated to instructions and 32k dedicated to data, the processor can store frequently requested data in the cache and access it more rapidly.

  • State-of-the-art manufacturing process technology
    Finally, the industry-leading 0.25-micron process used to produce the PowerPC G3 processors does more than merely boost performance; it also enables the creation of smaller, cooler processors with extremely low power requirements. In essence, it represents a brand-new approach to chip design, one that brings workstation-class performance not only to desktop systems, but even to notebook computers -- using the same processor.

Backside Cache -- Unleashing Processor Performance
By far the biggest boost to performance that the PowerPC G3 offers can be credited to its incorporation of an approach to level 2 cache memory known as backside cache. This approach effectively bypasses limitations on the speed at which transactions between the processor and the level 2 cache can occur. Earlier PowerPC processors used the system bus to access both the level 2 cache memory and the main memory, which could result in conflicts. For example, under the previous approach, at processor clock speeds above 200 megahertz, the CPU would often stall as it waited for data to arrive from the level 2 cache. To prevent such slowdowns, the PowerPC G3 processor features a new dedicated bus that handles only the CPU/cache transactions. This bus can operate at higher speeds than the system bus -- speeds that relate incrementally to the clock speed of the processor. This enables the more effective use of level 2 cache, because even the relatively large amounts of data they can store can be accessed by the processor rapidly and efficiently. In fact, as clock speeds increase, so does the performance value offered by the backside cache design.

Backside Cache Diagram

Learning to Look Beyond Megahertz
The performance enhancements of the PowerPC G3 processor significantly reduce the usefulness of clock speed in attempting to compare computer performance. Apple systems based on this processor consistently outperform systems with higher clock speeds -- in fact, they also outperform Pentium II- based systems. Some examples follow.

  • A 250-megahertz Macintosh PowerBook G3 is faster than a 266-megahertz Pentium II desktop.*

  • A 233-megahertz Power Macintosh G3 is faster than the Power Macintosh 6500/300 and the Power Macintosh 8600/300.**

  • A 266-megahertz Power Macintosh G3 provides performance that is on average 30 percent faster than that of a comparable 266-megahertz Pentium II system.*

All of which means that when you're looking for the computer that's right for you, it's more important than ever to consider overall product design -- megahertz alone does not tell the whole story.

* Based on Apple internal tests running 15 separate Adobe Photoshop filters.
** Based on Apple internal testing using MacBench 4.0 processor performance scores. Actual performance on applications may vary. MacBench is a subsystem-level benchmark that measures the relative performance of Mac OS-based systems.

Benefits to the User
As the PowerPC G3 processor becomes central to Apple system designs, increasing numbers of Macintosh users will enjoy these benefits:

  • Significant performance gains, which enhance the power available to handle such resource-intensive tasks as video editing, Internet authoring, and Windows emulation through software alone, rather than more costly and complex hardware add-ons.

  • Even more affordable higher performance -- in particular, providing a radical improvement in the value proposition offered by our entry-level and midrange systems.

  • The ability to purchase a notebook system that can truly offer the performance of a desktop computer.

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