Mac Musings

Will the Cell Processor Appear in Future Macs?

Daniel Knight - 2005.03.09

Yesterday we looked at the potential of Freescale's new single- and dual-core 8641 (G4) CPUs in future Macs. Today let's look at the other processor being hyped for use in future Macs, the Cell processor, which could have 10x the capability of current PC processors.

The Cell processor is a member of the PowerPC family that was jointly developed by Sony, IBM, and Toshiba. Sony's goal was to design a new kind of processor optimized for the next generation Playstation, but the Cell processor has implications for the entire computer industry.

What Is It?

The Cell processor uses a different model from today's CPU. Instead of being designed to function as the single "brain" (or sometimes one of two brains) of a computer, the Cell is designed from the ground up to work with distributed processing as well. It's not an afterthought.

In fact, the heart of the Cell is distributed processing. Each Cell processor has a dual-threaded core processing unit and eight single-threaded attached processing units (APUs), and the Playstation 3 is expected to incorporate four Cell processors.

APUs are independent vector processors, and their work is distributed and coordinated by the core processing unit. Each APU has 256 KB of dedicated memory, 128 128-bit registers, four FPUs, and four integer units.

That 256 KB is not cache memory - it's dedicated memory that the APUs can access, eliminating many of the headaches of cache coherency associated with traditional multiprocessor designs. In brief, each APU acts like a separate computer that does its job and reports its results back to the core processor. It's sort of like the way SETI@home works, but at much higher speeds and all within the same chip.

And it gets better. The APUs can be configured to stream a process, which means that when one APU has completed its calculations, it hands off the data to the next APU to perform the next step in the process. These steps can also be distributed to two or more APUs.

That's a lot of power, and it's going to run at high clock speeds. As Nicholas Blachford observes, "The speed difference is so great that nothing short of a complete overhaul of the x86 architecture will be able to bring it even close performance wise."

There's already talk about Cell-based supercomputers, as each APU itself is capable of 32 gigaflops - multiply that by eight (256 gigaflops) per Cell processor, and that by the number of Cells in a computer or on a network.

And here's the kicker, which I've deliberately avoided mentioning until now: The Cell processor will run at speeds over 4 GHz (4.5-4.6 GHz is often mentioned as a top speed for the current design). That's faster than anything Intel or AMD offer, and nearly twice the CPU speed of the fastest G5.

It also appears that the Cell core (but not the APUs) supports VMX (a.k.a. AltiVec), the one feature that makes the G4 and G5 far superior to the G3 for graphics, video, and sound work.

Finally, the Cell will be relatively energy efficient, according to PCWorld, and should work with air-cooled designs. Steve Jobs has got to love that!

Cell Drawbacks

The Cell processor is a member of the PowerPC family, and although IBM claims it can run programs written for the PowerPC, that doesn't mean they'll run the same way. While the Cell is very powerful and very efficient, there are differences that will have to be taken into account when optimizing software for the new processor.

That's nothing new for Apple. Mac software had teething pains when Apple added the 68020 CPU to its line, when it began using 32-bit addressing, because of the bigger cache on the 68040 CPU, moving to PowerPC architecture in the first place, and adding the G5 to the line. Further, there were areas where code tweaking made the G4 and G5 processors even more superior to their predecessors than they had been with un-tweaked code.

Another drawback is that ALUs are at their best doing single-precision math using vectors. Doing double-precision math and non-vector math involves more overhead - then again, there is AltiVec on the core processor....

Overall, it sounds like the Cell core unit is pretty much a very fast G4.

Finally, there's the whole issue of memory speed. The Cell needs very, very, very fast memory, and the two architectures supported run at 3.2 GHz (XDR) and 6.4 GHz (FlexIO). That's probably going to mean very expensive memory.

A Cell in Apple's Future?

The Cell is designed to be a cheap processor. Cheap enough that Sony can put four into a Playstation 3 and not charge a thousand dollars for it.

In fact, US$500 is a commonly predicted price point for the next generation Playstation - time will tell.

Regardless, Sony expects to sell a lot of them, and with four Cells per Playstation 3, that's probably tens of millions of Cells per year just for gamers. (Playstation 2 has sold about 60 million units to date.)

With IBM already claiming the Cell processor can run current PowerPC software, it's not hard to imagine Apple adopting it for future CPUs. A single 4.0 GHz Cell processor in an iBook or Mac mini would undoubtedly run circles around today's 1.25-1.33 GHz entry-level Macs, and a quad processors Power Mac at 4.0 GHz should handily outperform today's 2.5 GHz Power Mac G5.

In fact, if "the mother of all thermal challenges" stymies Apple long enough, it's conceivable that when the PowerBook moves from a G4 CPU it will instead adopt the Cell.

All speculation, of course, but the Cell opens up a bunch of possibilities for Apple - not to mention for the rest of the personal computer industry to move away from costly Intel and AMD CPUs and the malware magnet known as Microsoft Windows.

Update: Sony's Playstation 3 has an overall performance score comparable to the 1.6 GHz single-processor Power Mac G5 under Geekbench 2006. CPU scores range from half that of the Power Mac to 20% higher, and memory performance is up to 300% better.

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