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Fishing for Chips: Part 1

When Apple introduced the Power Macintosh back in 1994, it pulled off an engineering feat that’s rarely been equalled in the computing industry: Apple successfully migrated an operating system and the vast majority of existing applications from the 68000 family of processors to RISC-based PowerPC processors. For those of you who are unfamiliar with the jargon, 68000-based Macs are often called "68K" Macs, and RISC stands for Reduced Instruction Set Computing.

More than two years after their introduction, however, understanding the relative merits of PowerPC processors can be confusing, and Apple has further muddied the situation through its use of cryptic model names. What’s the difference between a PowerPC 601 and a 603? How much does clock speed matter? What’s a Level 2 cache? And what does any of this say about the difference between a Performa 5400 and a Power Mac 7600?

Answers to questions like these are hard to find, and are all but absent from materials Apple and other Mac systems manufacturers make available. Further, news sources (TidBITS included) rarely explain these terms, since we have much to do just to keep up with the latest releases. So, with that in mind, what follows is an overview of PowerPC processors and some of the terms and technology associated with them. Next issue, I’ll cover real-world aspects of PowerPCs, including emulators, system software, and performance tuning.

Worth the RISC? All PowerPC processors are software compatible, so as long has you have a PowerPC chip in your Macintosh, you can run any PowerPC-native Macintosh software. PowerPC-based Macintoshes can also run older software written for 68K Macs, but in emulation mode, which tends to be a little slower than what you’d expect from machines touted as blazingly fast. 68K Macs, however, cannot run software written solely for the PowerPC.

This doesn’t mean 68K Macs suddenly become useless; most of these machines will be useful for years to come. I certainly plan to continue using mine. In a way, this is a problem for Apple and other software developers, since this long life span means plenty of people will use 68K Macs for years into the future, and these people will want to be able to upgrade their software in order to take advantage of new features.

But, the writing is on the wall. As time goes on, current system and application software will increasingly only work with the PowerPC. It’s unlikely that System 8 will be available for 68K Macs, although certain technologies will probably be broken out and made available for older machines. Similarly, software will be optimized for better performance on more recent PowerPC processors, so more recent processors have potential benefits.

Of Clocks & Cache — I’ll just take a moment to define some terms commonly used to describe PowerPC-based Macintoshes:

  • Clock speed: Clock speed measures how fast a processor processes instructions, and clock speeds are measured in megahertz (MHz); 1 MHz is one million operations per second. Current clock speeds on PowerPC-based Macs range from 50 to 180 MHz, and you can expect 200+ MHz models soon. Before you get excited about a Mac carrying out millions of operations per second, note that – unfortunately – this doesn’t mean millions of menu commands per second! An operation is a tiny thing – moving data into a memory location, moving data out of a memory location, or performing a logical transformation. Choosing a menu item requires untold thousands of operations. Similarly, one assembly-language instruction can conceivably consume hundreds of operations – particularly if it’s emulated.

  • Level 1 Cache: A Level 1 cache is a bit of high speed memory built into PowerPC processor. The processor can cache frequently-needed data here and access it rapidly, saving it the trouble of requesting data from RAM or disk. Level 1 caches vary among PowerPC designs, but loosely speaking, PowerPCs have between 16K and 32K of Level 1 cache. Because the cache is built into the processor, you can’t upgrade it separately from the processor.

  • Level 2 Cache: A Level 2 cache works much like a Level 1 cache, but it is separate from the processor and you can upgrade it. Some Macs ship with no Level 2 cache, though most currently ship with a 256K Level 2 cache, and you can often upgrade to 512K or 1 MB. Results vary, but increasing Level 2 cache can improve performance somewhere between 5 and 30 percent, with best results for processor-intensive functions common to science, engineering, or high-end graphics applications. For many users, increasing the Level 2 cache is an inexpensive way to improve the performance of their Macs.

A problem with Level 2 caches is figuring out how much you have – the About This Macintosh dialog doesn’t report such information, and it’s tough to figure out unless you know what your Mac model shipped with or you feel like opening your Mac and reading cryptic numbers on the cache module. Newer Technologies has a free tool that reports on a Power Mac’s Level 1 and Level 2 caches (up to 1 MB). Its results have been accurate on machines I’ve tested.

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  • Clock Multipliers (or Bus Divider Ratio): A clock multiplier allows a processor to run faster than a computer’s bus oscillator, and it’s one way recent machines have achieved such astoundingly high clock speeds. As an example, the PowerTower 180 sports a PowerPC 604 running at 180 MHz. Power Computing did this by using the 3x clock multiplier built into the PowerPC 604 in combination with a 60 MHz bus speed on the PowerTower motherboard. Similarly, Apple’s Power Mac 9500/150 runs at 150 MHz, three times the unit’s 50 MHz bus speed. Different PowerPC chips have different clock multipliers available; for instance, the Performa 6300 uses the PowerPC 603e’s 2.5x multiplier to get to 100 MHz using a 40 MHz bus, speed. The upcoming PowerPC 603e-200 and 604e also have 4x, 5x, and 6x multipliers.

Current PowerPCs — Here’s a brief outline of the PowerPC processor family as it relates to the Macintosh.

  • PowerPC 601: The 601 has the honor of having been the first PowerPC processor available, and it’s at the heart of many systems from Apple, IBM, Power Computing, Radius, and other vendors. Mac systems based on the 601 range from 60 to 120 MHz. Development of the 601 has basically ceased in favor of newer processors; however, 601-based systems are certainly still viable today.

  • PowerPC 603: The 603 is intended to be a low-power version of the 601, aimed at laptops and other devices where power consumption and heat are significant design factors. The PowerPC 603 typically uses between one-quarter and one-third the power of a PowerPC 601 running at the same clock speed. The 603 is also supposed to be equivalent in performance to a 601 at the same clock speed. However, that didn’t prove to be the case in Apple’s early 603-based 5200 and 6200 series LCs and Performas, or prototype PowerPC-based PowerBooks, mostly due to the 603’s small Level 1 cache. A 75 MHz 603 delivered roughly the same real-world performance as a 60 MHz 601.

  • PowerPC 603e: The PowerPC 603e (also known as the 603+) is basically a 603 with a larger cache and higher clock speed, and is equivalent in performance to a PowerPC 601 at the same clock speed. Most 603-based Mac systems shipping today (including desktop units and PowerBooks) use the 603e chip. Machines based on the 603e should be around for some time, and their speed and performance should continue to improve. Right now, shipping 603e systems peak at 120 MHz.

  • PowerPC 604: At the moment, the PowerPC 604 chip comes at the high end of the line, with configurations currently shipping at speeds of 120 to 180 MHz. The PowerPC 604 is intended for high-end workstations and servers, and a PowerPC 604 is, roughly speaking, about 50 to 75 percent faster than a 601 running at the same speed, making it the chip of choice for users with processor-intensive tasks. It also consumes two to three times the power of a 601, so don’t expect to see a 604 in a laptop or hand-held device.

  • PowerPC 602: The 602 is a lower-end chip intended for set-top boxes and similar devices. I don’t know of any Macintosh-related projects using the 602, but 3D0 plans to use it in a 64-bit game console codenamed M2.


Future PowerPCs — The PowerPC shows no signs of slowing down in terms of developments of faster processors. Future processors should include the PowerPC 603e-200, which is essentially a 200 MHz version of the PowerPC 603e, sporting that processor’s low power requirements and higher clock multipliers. If you’d rather think about improvements to the 604, think about the PowerPC 604e, an enhanced version of the 604, offering higher speeds (166, 180, and 200 MHz, to start with), larger clock multipliers, and increased processor cache size. Quantities of the 604e are shipping right now, and you can expect to see high-speed 604e-based machines from Apple, Power Computing, and other vendors later in 1996.

If you think the 604 is fast, the forthcoming PowerPC 620 is the first 64-bit PowerPC implementation, and it’s an even higher-performance processor designed for very high-end systems. The PowerPC 620 uses the same basic design process as the 604e. Although the 620 has been delayed more than a year by problems with technology and reported staffing problems, I expect to see 620-based machines available from Apple and other vendors by early 1997, and some manufacturers have versions of the 620 in-hand now, reportedly running at 200 MHz. The 620 is geared toward multi-processor implementations and transaction processing, and could support up to a whopping 128 MB of Level 2 cache.

IBM and Motorola are currently the sole providers of PowerPC chips, but a little company in San Jose could change that. IBM has granted Exponential Technology a licence to develop PowerPC-compatible processors. Headed by CEO Rick Shriner, a former Apple vice president, and other industry veterans, Exponential plans to use BiCMOS technology to form its processors’ core logic, while using more conventional CMOS for on-chip memory – sort of the reverse of the way Pentium chips are manufactured. Although Exponential hasn’t made specific speed claims, it anticipates achieving twice the performance of today’s microprocessors, which would put their processors in the 300 to 400 MHz range. Exponential still has to prove the feasibility of its technology, but the company has significant financial backing from Apple and other investors, and it claims that its chips will be ready in early 1997.


Stay Tuned — Next issue, I’ll talk about emulators, system software, real-world performance, and how to use this information when buying a Power Mac. Please note that we’ll be taking a brief vacation for the Fourth of July and there will be no issue next week.

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