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The PowerPC G3

Processor: Taking

the Macintosh to

the Next Level

Nearly six years ago, Apple, IBM, and Motorola joined forces to create a new processor
technology that would bring the performance advantages of the RISC (Reduced Instruction
Set Computing) architecture—at that time limited to costly workstations—to personal
computers. The result of this initiative was the development of PowerPC processor tech-
nology, which Apple debuted in 1994 with the introduction of the Power Macintosh line.

The initial Power Macintosh models were based on the first implementation of the

new chip technology: the PowerPC 601, which was intended for use in high-end personal
computers. In early 1995, Apple introduced products based on the PowerPC processor
technology’s second generation—the PowerPC 603, which utilized a chip design optimized
for use in low-end to midrange desktop systems and portables. This was quickly followed
by the introduction of the second-generation high-end PowerPC processor: the PowerPC
604. Since then, both IBM and Motorola have made enhancements to the PowerPC 603 and
604 (now the 603e and 604e), and these enhanced chips have been used in subsequent
Apple systems.

The emergence of the PowerPC G3 processor marks the third
phase in the development of this 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:
• The PowerPC G3 is the first processor specifically optimized for

the Mac OS.

• It incorporates an innovative backside cache design that speeds

access to level 2 cache.

• It contains large (32K) on-chip level 1 data and instruction

caches, for a total of 64K level 1 cache.

• It’s produced using an industry-leading 0.25-micron manufac-

turing process.

These four innovations share one very important characteristic: the capacity to provide
significant performance gains. Following is a more detailed breakdown of the advantages
offered by each.

Because the earlier PowerPC processor models were essen-

tially developed simultaneously with Apple’s Power Macintosh line, there was no opportu-
nity to optimize these chips’ performance for running Mac OS–based applications. But that
was more than half a decade ago. Today, the Power Macintosh line is well established and
Mac OS–based software abounds, placing the developers of the PowerPC G3 processor in
the unique position of having the luxury to consider—and optimize—chip design in light
of actual software performance.

• Addition of a second integer ALU (arithmetic

and logic unit), which allows the processor
to execute two successive integer operations
in parallel

• The ability to fetch four instructions per cycle

from the cache

• A “hardware tablewalk” feature, which allows

the CPU to access virtual page tables directly

• Adoption of a dynamic prediction method for

improving the efficiency of branch handling