PowerPC G3 Processor: Description

This article describes the PowerPC G3 microprocessor.
This article has been archived and is no longer updated by Apple.
The emergence of the PowerPC G3 processor marks the third generation in the development of advanced processor technology. The PowerPC G3 builds on many of the features pioneered by the PowerPC 603 and 604. However, this 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 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.

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 process
or rapidly and efficiently. In fact, as clock speeds increase, so does the performance value offered by the backside cache design.

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 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.
Published Date: Feb 20, 2012