Apple G3 - page 93

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 perfor mance 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 perfor mance—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”

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.

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.

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

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.

L02588A

Backside

Cache

The backside cache design allows the CPU to access the cache

directly at speeds that vary proportionally to the CPU speed

IIIIIIIIIIIIIIIIIIIIIIIIIII

Dedicated processor bus

Memory bus

System bus

CPU

Level 2 Cache

Processor card

RAM

PCI slots