Pentium 5 Trumps PowerPC G5

After seeing the column I wrote about Window XS on Low End Mac, my publisher asked if I'd like to become a contributor to Low End PC. With Apple (or, more precisely, IDG) pulling press passes faster than Intel bumps the speed of the Pentium 4, I jumped at the opportunity.

In the Mac world, there have been rampant PowerPC G5 rumors for some time, including my own wild musings on why it hasn't yet been released. Well, according to our usually unreliable sources, Apple is prepared to announce a Power Mac G5 at Macworld Expo later this month. Not only will this give Apple a numeric edge (G5 vs. P4), but it's also going to put an end once and for all to the "MHz Myth."

The MHz Myth

If you're an Intel user, you may never have heard of the so-called MHz Myth. Intel certainly hopes that's the case, since the theory behind the MHz Myth is that clock speed, which is measured in MHz or GHz, is the only meaningful indicator of a CPU's performance. That's like saying that RPM is the only meaningful indicator of your car engine's performance, but MHz/GHz is an easy number to market to the public.

If you're a power user, an AMD user, or a Mac user, you've heard all sorts of claims to counter the MHz Myth. In fact, AMD has gone so far in their denial as to label Athlon CPUs in "equivalent" MHz ratings. In reality, their Athlon 2200 fall short of running at 2,200 MHz.

Apple has taken a more direct approach, comparing their top-end models with top-end Windows computers using obscure, professional, production level applications such as Photoshop to demonstrate the superiority of the G4 processor and its Velocity Engine.

The PowerPC

No longer, if our unreliable sources are to be believed. Apple, Motorola, and IBM have learned a few tricks from Intel, and the next version of the PowerPC processor, G5, is a joint effort explicitly designed to offer the same kind of GHz ratings as the Pentium 4.

(Please ignore all those mindless claims that the Pentium 4 is less efficient MHz for MHz than the Pentium III. Ignore the benchmarks. Ignore real world tests. Remember, GHz is everything.)

The PowerPC G5 is designed so that parts of the CPU actually run at speeds of up to 3 GHz today (well, by the Macworld Expo anyway) and can easily be scaled to 4 and even 5 GHz by the end of 2003. Apple believes that will give them a real marketing edge over Intel.

Pentium 5

Driven more by marketing than engineering, Intel has rushed development of the Pentium 5 and promises it will be shipping in limited quantities before the Macworld Expo. Like the G5, parts of the P5 will operate at much higher speeds than today's CPUs. Using the latest technology, that fastest P5 will be rated at 4 GHz - and Intel promises to reach 5 GHz by the end of the year.

How did Intel pull this off? Well, they used the same kind of reasoning that Apple, IBM, and Motorola did when designing the G5. They realized that consumers are more impressed with hype (read: high GHz ratings) than performance (read: real world benchmarks), so they designed a CPU that doesn't work at a single clock speed.

That said, Intel has learned its lesson, and the core of the P5 will have the same kind of efficiency as the PIII, not the GHz-boosting, performance-robbing design of the P4. Here's how things will work:

1. The P5 will initially ship at speeds of 2, 2.5, and 3 GHz. This speed is based on an accelerated clock circuit running at this GHz rating.
2. The core of the P5 will ship at 1, 1.25, or 1.5 GHz. This core will have shorter pipes and reduced latency like the Pentium III. However, it will also have more processing units, making the P5 significantly more powerful on a MHz for MHz basis than the PIII.
3. The P5 will have a 1 MB level 2 cache running at full core speed and an integrated 4 MB level 3 cache operating at 500 MHz. Both caches will include a 256-bit bus.
4. The P5 will incorporate a new 250 MHz DDR data bus, offering the equivalent of a 500 MHz system bus.

The end result will be a CPU that marketing can legitimately promote as a 3 GHz processor. From the perspective of the Intel board, that's crucial. After all, their goal is turning a profit.

Fortunately engineering has had a lot more input into this design and been forced to make far less compromises than they did with Pentium 4. Even though the fastest core "only" runs at 1.5 GHz, because of design improvements, this will easily outperform a 3 GHz Pentium 4.

The Competition

PowerPC

As I've already noted, IBM, Motorola, and Apple have teamed up to produce a G5 processor using similar tricks. In the case of the G5, the CPU will be available in 1.6, 2.0, and 2.4 GHz ratings. Based on the kind of efficiencies Apple has demonstrated with G3 and G4 the over the years, the 2.4 GHz G5 would be expected to offer at least twice the performance of the 2.4 GHz Pentium 4.

But that's not going to be the case. As with the P5, only parts of the G5 run at these rated speeds. But in this case, it's not just a clock circuit running at an inflated GHz rate to inflate the marketability of the new CPU. Motorola and IBM engineers have found a way to make the branch prediction unit run at twice the clock speed of the CPU's core.

The core CPU will run at half rated speed, just like the P4. As with the G3, Apple and its partners have profiled billions of lines of code - placing a special emphasis on Mac OS X and X-native applications - to create a CPU tweaked for real world code, not just theory and benchmarks.

They've also devised a new Velocity Engine that handles twice as much data, making the G5 twice as good for ripping music files, encoding digital video, running Photoshop filters, and displaying DVDs.

The G5 will sit on a 200 MHz bus, include a 2 MB level 2 cache running at core speed, and support an 8 MB level 3 cache running at 400 MHz - twice bus speed.

Still, while Apple thinks the G5 will catch them up in the GHz war, it won't have the marketing edge of Intel's 3 GHz Pentium 5. So what if the G5 includes two cores per CPU....

AMD

AMD's solution to the MHz Myth has been to market their Athlon CPUs using an equivalent MHz rating. They will continue to do this, and are introducing a new series of CPUs, the Athlon GHZ series, later this month. Rather than rate processors by clock speed, the model numbers - such a GHZ 3.0 - indicate relative performance.

Technical details are sketchy, but it looks like the GHZ series will use a 533 MHz DDR bus, include a 1 MB level 2 cache running at the CPU's real clock speed (estimated at two-thirds of the equivalency rating, or 2.0 GHz for the GHZ 3.0 model), and support a bus speed L3 cache of up to 4 MB that sits between the CPU and the motherboard.

And the Winner Is....

From a marketing standpoint, it looks like both the 3 GHz Pentium 5 and Athlon GHZ 3.0 will have the edge, although the increased GHz rating for the G5 will make it much easier for Apple to market their clock-speed-trailing models against the clock speed champions.

In all cases, the consumer - savvy or not - will be the winner. Despite seemingly inflated GHz ratings, the Pentium 5, Athlon GHZ, and PowerPC G5 will all offer significant real world improvements over their predecessors.

Sources at Intel, AMD, Apple, Motorola, and IBM refused to comment on this idle speculation.