The G3 has had a great run on Mac computers, but there comes a time to put even the best technologies out to pasture" />



Leopard Set To Lower The Curtain On The Power PC G3

13147 There comes a tipping point in the life of most technologies that they shift from being current (or reasonably current) to antiquated. For example, I can't think of any automobiles, at least ones marketed in the developed world, that are still equipped with a carburetor. Electronic Fuel Injection has rendered carburetors obsolete, although my carburetor-equipped 1989 Toyota Corolla still runs great.

Some PC users still use floppy disks, and quite a few PC and Mac users still have CRT displays, but both of those technologies are now widely considered obsolete. Apple hasn't equipped any of its systems with floppy drives since 1998, and the last CRT Mac was the eMac that was discontinued last year.

When Apple switched to the IBM/Motorola Power PC central processing unit in 1994, even the mightiest of the foregoing Motorola 680x0 series microprocessors became old hat overnight, although the transition was handled exceptionally smoothly.

The next such revolution was the introduction of the Power PC 750 family of CPUs, more familiarly known as "G3" in November, 1997, which made the most powerful of the previous generation 603 and 604 Power PC CPUs yesterday's news.

I remember a couple of weeks later, the publisher of a small local newspaper called me up to ask if I thought buying one of the leftover 604 Macs would be a good idea. I quickly persuaded her to go with one of the first-generation beige G3 desktops.

The G3 had a relatively long run, thanks to a series of revisions, and it wasn't rendered obsolete overnight when the next-generation G4 family of Power PC chips was introduced in the late summer of 1999, not quite two years after the G3's debut. In fact, Apple continued selling G3-based systems right up until the G3 dual USB iBook was replaced by the G4 iBook in October, 2003. Many MAc-users are still getting great service from their G3 machines, including me with my 700 MHz G3 iBook.

However, it looks like the sun is about to set on the G3 as a contemporary processor chip when OS X 10.5 Leopard is released in October, almost 10 years after the G3 first made its bow. If the scuttlebutt is correct, the minimum hardware specification for Leopard support will be:
• An Intel processor or a PowerPC G4 or G5 processor
• A DVD drive
• Built-in FireWire
• At least 256 MB of RAM for a PowerPC based Mac and 512 MB for an Intel-based Mac (additional RAM is recommended for development purposes)
• A built-in display or a display connected to an Apple-supplied video card supported by your computer
• At least 6 GB of disk space available, or 8 GB if you install the developer tools

If that turns out to be the case, whatever the ultimate 10.4.x revision ends up at will be the last Mac OS X version to support the G3 processor, at least officially. Ryan Rempel will no doubt come up with an XPostFacto workaround hack to persuade Leopard to install on G3 machines.

Whatever, 10 years of OS support is more than respectable, and even those last 900 MHz PPC 750 FX iBooks will be four years old when Leopard pounces. It's time to put the G3 out to pasture and move on. Or not. OS 10.4 Tiger will still be a kick-ass OS for years to come yet.

I'm extremely gratified by how well OSD 10.4.9 runs on both my G3 iBook and a cherry 500 MHz G3 Pismo PowerBook I just picked up. It's fast and smooth and perfectly acceptable with few exceptions. Quartz Extreme is even supported by the 16 MB VRAM and RADEON 7500 GPU in the iBook.

On the other hand, I'm very curious as to whether Leopard will installable (without reference to XPostFacto) on my 550 MHz G4 upgraded Pismo PowerBook.

In any event, I regard the G3 microprocessor fondly. Even the first-generation PowerPC 750 233 MHz chip in my old WallStreet PowerBook remains an amazingly capable performer, especially running Classic Mac OS 9.2.2, after nearly 10 years of service.

A review of the PowerPC G3's illustrious decade is in order.

It really began with the formation of an alliance formed in 1991 between Apple Computer, IBM and Motorola to develop a new personal computer microprocessor based on IBM's Reduced Instruction Set Computing (RISC) mainframe POWER architecture that could challenge Intel's dominant X86 Complex Instruction Set Computing (CISC) CPU's for market dominance, Apple introduced the first PowerPC based Macs in 1994 powered by 66 MHz PPC 601 CPUs, The 603 and 604 Power PC variants followed over the next four years with increased clock speeds, eventually reaching 300 MHz with the 603e and 400 MHz for the 604ev, although the fastest 604 Apple ever used in a computer was 350 MHz in the 1997 Power Mac 9600/350.

The first generation PowerPC G3 CPU, officially known as the PowerPC 750, was the first PPC chip designed to run on a 66 MHz bus, and introduced to the public in the original beige Power Mac G3 desktops in November 1997. The "beige" G3 was initially offered in 233, 266, and 300 MHz versions, and was also the first Power Mac to use an IDE bus for hard drives. Also released in November, 1997 was the original "Kanga" PowerBook G3, which was essentially a PowerBook 3400c with a 250 MHz G3 motherboard. Indeed, it is possible to upgrade a PowerBook 3400 to G3 power by grafting in one of these motherboards.

The G3 CPU was really the first PPC CPU that was optimized for use with PowerPC based personal computers. The earlier PowerPC 601, 603, and 604 RISC processors had been originally designed for compatibility with IBM's mainframe POWER architecture. The PowerPC G3 was the first processor specifically optimized for the Mac OS.

The PowerPC 740/750 design (codename Arthur) was conceived as an evolutionary replacement for the PowerPC 603e chip that was used in many Apple desktop and laptop computers in the mid-1990s.

The optimization efficiencies incorporated in those first 233 MHz G3s were so great that it could hold its own against a 300-350 MHz PPC 604e CPU, so Apple almost immediately discontinued the older CPU except in multiprocessor models (the G3 has very limited dual processor support), and planned development of a successor to 604 was scrapped. The G3 750 both outperformed Intel's contemporaneous Pentium IIs while drawing far less power, and was smaller in size to boot.

The PPC 750 sported two 32 KB level 1 caches, one for instructions, the other for data, but the real; genius of the G3 processor was its support for a level 2 (L2) cache of 256 KB, 512 KB, or 1 MB running at an intermediate speed between the 66 MHz system bus and the CPU's core frequency. The Level 2 cache of the 750 increased performance by approximately 30% in most situations. The original PPC 750 processor was offered in clock speeds up to 366 MHz. One of Apple's less successful ideas was leaving the L2 cache out of the entry level WallStreet PowerBook G3 model, which hobbled that machine's performance substantially.

PowerPC 745/755

In 1998, Motorola upgraded the G3 chip, shrinking the die size to 51 mm2, which was facilitated by a new aluminum based fabrication at .22 ?m. Clock speeds for the PPC 745/755 microprocessor supported a 100 MHz system bus and reached clock speeds up to 600 MHz. The 755 was the last Motorola G3 processor, and was used in the 600 MHz iBook.

PowerPC 750CX

After the PPC 755, Motorola abandoned of the G3 architecture for its G4 chip, but IBM continued G3 development, with its PowerPc 750CX (codename Sidewinder), which was released in 2000 and was the first G3 CPU to incorporate an on-chip L2 cache instead of a separate cache running at a fraction of the CPU's full speed. This both enhanced performance and reduced both power consumption and complexity. The fact that the the onboard cache was only 256 KB in size was mitigated somewhat by it running at full CPU speed. The 750CX was used in the last-iteration (Paris) clamshell iBooks, which at 366 MHz and 466 MHz clock speeds.

PowerPC 750CXe




IBM's next upgrade of the G3 chip was the 750CXe (codename Anaconda), introduced in 2001 and featuring 256 KiB on-die L2 cache. Running at 400 MHz, the 750CXe drew less than four watts, and had had 20 million transistors including its L2 cache. Its die size was 43 mm2 facilitated by a 0.18 ?m copper process, and it also featured improved floating-point performance over the 750CX. The PPC 750CXe supported up to a 133 MHz system bus and reached speeds as high as 700 MHz. It was first used in the 500 and 600 MHz iMacs in February 2001, and also found its way into several models of the iBook.

PowerPC 750FX




The 750FX (code-named Sahara) debuted in 2002 and was available in clock speeds up to 900 MHz. The 750FX's bus speed climbed to 166 MHz and the size of the on-die L2 cache was doubled to 512 KiB. There were also several improvements to the memory subsystem: an enhanced and faster (200 MHz) 60x bus controller, a wider L2 cache bus, the ability to lock parts of the L2 cache. The 750Fx was manufactured using a 0.13 ?m copper based fabrication with Low-K dielectric and Silicon on insulator technology, incorporating 39 million transistors, a die size of 35 mm2 and consuming less than four watts at 800 MHz under typical loads. The 750Fx was the ultimate G3 microprocessor used in Apple computers, the last on being the 900 MHz iBook of 2003. My 700 MHz iBook has a 750FX processor, and it's been a surprisingly capable performer, and currently supports OS X 10.4.9 Tiger very nicely.


PowerPC 750GX

The 750GX (code-named Gobi), rolled out in in 2004 was the latest and most powerful G3 processor from IBM, but it never was used in any Apple computers. The 750GX has a on-die L2 cache of 1 MiB, top frequency of 1.1 GHz, support bus speeds up to 200 MHz among other enhancements compared to 750FX.

Apple offered this summary of the G3's advantages when it first came out:

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.


However, as revolutionary as it was, the G3 750 family of microprocessors had its shortcomings, particularly lack of multi-processor support and SIMD capabilities and a relatively weak floating point unit, which are reasons why Motorola decided to break away from G3 development and concentrate on its 74xx G4 range of Power PC processors. While this column is about the G3, it is germane to briefly discuss differences between the G3 and G4 architecture.

Apple and IBM were involved in the PPC 7400's design and engineering in close cooperation with Motorola at its Somerset design center, but IBM decided that a Vector Processing Unit wasn't necessary. The biggest distinction of G4 architecture compared with G3 is the said vector processing unit (called "AltiVec" by Motorola, with Apple referring to it as the "Velocity Engine").

AltiVec enables the 7400 microprocessor to do four-way single precision (32-bit) floating point math, or 16-way 8-bit, 8-way 16-bit or four-way 32-bit integer math in a single cycle. The vector processing unit is also superscalar, and can do two vector operations at the same time.

Additionally, the 7400 has enhanced support for symmetric multiprocessing (SMP), and also supports two bus technologies, the 60x bus which it shares with the PowerPC G3, and the more advanced MPX bus.

So while the G4 is still a reasonably formidable and capable microprocessor, the G3 is definitely showing its age, and it's there comes a time to put even a great technology out to pasture. I don't think anyone should feel too hard done by if Apple, as expected, drops support for the G3 from the Leopard OS release, and even if Leopard can be persuaded to install on G3 hardware vis third-party hijacking, few G3-powered Macs will have really adequate video support to run Leopard satisfactorily.

My plan is to continue running OS 10.4.10 (or whatever the final update of Tiger is) on my remaining G3 machines for as long as they're useful. I will, however, be interested to see if Leopard will install on my G4 upgraded Pismo PowerBook. Again, even if it will, I'm skeptical as to whether the Pismo's puny ATO RAGE Mobility 128 GPU with 8MB of VRAM will be up to the task of supporting Leopard's graphics demands. My guess it that it will be painfully lazy at best. In the meantime, Tiger is still a very decent performer on the 550 MHz G4, and indeed on the 500 MHz G3 Pismo I just picked up.


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Charles W. Moore



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