Apple G3 manual Benefits to the user

Dedicated processor bus RAM

Memory bus

Learning to look beyond megahertz

The performance enhancements of the PowerPC G3 processor significantly reduce the usefulness of clock speed in attempting to compare com- puter 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.

Processor	card
CPU	Level	2 Cache
	IIIIIIIIIIIIIIIIIIIIIIIIIII

System bus

PCI slots

Level 2 backside cache. 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 transac- tions 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 it 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.

Large level 1 (on-chip) data and instruction caches. In comparison with the 8K on-chip caches incorporated into the design of the original PowerPC 603, the PowerPC G3 processor includes 32K of instruction cache and 32K of data cache, for a total of 64K level 1 cache. These relatively large on-chip caches support—and add to—the overall performance gains offered by the PowerPC G3 processor.

New manufacturing process. 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 perfor- mance not only to desktop systems, but even to notebook computers—using the same processor.

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 requiring 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

The “why”

Backside Cache

The backside cache design allows the CPU to access the cache directly at speeds that vary proportionally to the CPU speed

The rationale for Apple’s introduction of the PowerPC G3 processor is strik- ingly simple. It’s the same concept that led us to embark on the PowerPC effort initially: At Apple, we are committed to developing and supporting processor technology that can offer our entire range of users truly outstanding perfor- mance—so they can spend less time dealing with the mechanics of computing and more time exploring their creative potential.

Apple Computer, Inc. 1 Infinite Loop Cupertino, CA 95014 (408) 996-1010 www.apple.com

© 1997 Apple Computer, Inc. All rights reserved. Apple, the Apple logo, Mac, Macintosh, PowerBook, and Power Macintosh are trademarks of Apple Computer, Inc., registered in the U.S.A. and other countries. Adobe is a trademark of Adobe Systems Incorporated. PowerPC is a trademark of International Business Machines Corporation, used under license therefrom.

November 1997. Product specifications are subject to change without notice. Printed in the U.S.A.

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