Before and after this months unveiling of the PowerPC-based “Cell” processor, the Mac industry buzzed with predictions that the forthcoming chip would be adopted by Apple Computer as its next-generation computing platform.
While the specs of the new chip are impressive, especially with its integrated support for virtualization and speedy video performance, analysts said differences between the Cell and the current PowerPC architectures will make any transition an unlikely prospect for the next few years.
Besides, use of the Cell chip in a Mac would make sense only if Apple Computer Inc. could get its Mac OS X to run faster than it does on a PowerPC—a task that may prove more difficult than it sounds.
Formally introduced at the International Solid State Circuits conference in San Francisco, the 64-bit Cell processor is a joint project of IBM, Sony Corp. and Toshiba Corp., and will first be used in Sonys PlayStation 3 box.
The group claims that the Cells potential goes beyond gaming, however, extending to everything from cell phones and televisions to high-end Linux workstations. IBM and Sony expect to have prototypes of Cell-based content-creation workstations available in the fourth quarter.
about the Cells future in gaming boxes and development.
Speculation that Apple will adopt the high-performance Cell was fueled by a Feb. 15 report from Merrill Lynch & Co. Inc. that predicted a Cell-based multimedia workstation from Apple. At the same time, the company raised its Apple stock price objective from $85 to $102 per share, causing Apples stock to jump 4.5 percent in one day.
IBM and Apple declined to comment on Apples possible use of the Cell, but IBM spokesman Glen Brandow emphasized the compatibility between the Cell and the PowerPC.
“User applications for PowerPC will run on the Power Processor Element on Cell,” Brandow said. “The Power Architecture remains compliant with applications written for previous, 32-bit, PowerPC processors. The Linux operating system for Cell extends the work for Linux on Power that already exists.”
At the rollout event, IBM Fellow Jim Kahle said the Cell supports multiple operating systems, and that a version of Linux is being tested in the lab.
However, some analysts said they think IBMs claims of PowerPC compatibility may be an attempt to persuade Apple to at least consider a porting project to the Cell.
“IBM Micro[electronics] may be trying to entice Apple to get involved,” said Kevin Krewell, editor in chief of In-Stat MDRs Microprocessor Report, based in San Jose, Calif. “It would be fairly easy to recompile Mac OS X to run on Cell, but optimizing the OS would be a lot of work.”
The optimization difficulties result from the fact the Cell is more like a cousin to PowerPC than a successor, he said. Although both processors are based on IBMs Power architecture, also used in the Power4 and Power5 lines, each processor family uses a different version of the Power architecture.
“Broadband Processor Architecture [BPA] is the formal name for the architecture of the Cell family of processors,” Brandow said. “BPA extends the Power architecture with coherent off-load accelerators and real-time management functions.”
BPA is designed to process large amounts of information and move it in and out of the processor, and is flexible enough to be reconfigured in future implementations, but is different than the PowerPC architecture.
This is why IBM stresses PowerPC compatibility with the Power Processor Element—the Cells Power-based core. However, Krewell said he thinks the difficulties in optimizing Mac OS X for the Cell arise from differences between the cores of the PowerPC and Cell.
“Even though it runs at over 4GHz, the Cell is has much simpler core than the G5,” Krewell said. “Apple might not be happy with the simpler core. The higher clock rate may not buy them much.”
For instance, the Cell processor does not perform out-of-order execution, a feature found in most processors designed in the past decade. This type of execution shuffles instructions, executing them without regard to the order in which they were programmed. Cells core also issues fewer instructions.
“The core in the G5 can issue eight instructions at one time,” Krewell said. “The Cells core only issues two.”
This simpler core design works well in the Cell architecture, but because Mac OS X is built for the more complex PowerPC core, it would require significant modifications in order to see any performance gains over todays Macs, he said.
Multiple Cores and I
/O Differences”> The Cell makes up for its simpler Power core by including eight “synergistic processing units” that can work on different tasks in parallel. These multiple cores help the processor run at the high clock rates and with floating point operations that can reach 256 gigaflops (billion floating-point operations per second).
However, the Cell isnt IBMs only multicore processor. The Power5 architecture is multicore, as is the forthcoming PowerPC 970MP, which may be Apples choice to receive its PowerPC G6 branding. The chip contains two Power cores that can operate independently and simultaneously.
In some reports, the Cell is described as having nine cores, leaving the impression that the Cell is an advanced version over the two-core 970MP. The Cell processors single Power core controls the eight synergistic processing units in a master/slave relationship. In the Cell, the Power-based core is the taskmaster, feeding subtasks to the synergistic processing units.
On the other hand, the two cores in the 970MP are Power-based, each acting separately to handle tasks and operations.
The result of these differences is that the 970MP and Cell excel in different types of tasks. For instance, IBM said the Cells multiple cores will let it run multiple operating systems simultaneously using software virtualization techniques. Each operating system will be run natively, not in emulation.
The Cell also will offer outstanding performance for 2-D and 3-D graphics and video—10 times the performance of traditional PC processors, IBM executives said in their Cell briefing.
The PowerPC architecture, currently expressed in 970 family, on the other hand, is designed to run the varied tasks required in a personal computer. Krewell said the first generation of Cell processors appears ill-suited to running a desktop or a mobile PC.
“As a general-purpose processor, the Cell might run 10 times slower than what IBM is claiming,” he said.
Meanwhile, the Cells bus, which is designed by Rambus Inc., is also application-specific, IBM said.
“The Cells bus is actually a little more limited because its not designed to be a general-purpose bus,” Krewell said. “The Cell [was] optimized for Rambus I/O, which might not be appealing for a mainstream box.”
The Rambus specs are impressive. For memory, the Cell uses Rambus XDR (extreme data rate), which can provide a total bandwidth of 25.6GB per second. For I/O, the Cell uses FlexIO to pass information outside of the processor.
According to Rambus, FlexIO has a maximum bandwidth of 76.8GBps, giving the Cells bus a theoretical bandwidth total of more than 100 GBps bandwidth. No PC processor on the market comes close to that performance.
This bandwidth, together with the potential for high gigaflops speed, could make the Cell a graphics powerhouse—which is why Sony is betting the future of the PlayStation platform on it.
Assuming the Rambus architecture could be made useful as a general-purpose bus in a PC, the bandwidth probably would saturate todays Mac hardware. The change would require Apple to redesign its bridging controller to interface with the Rambus technology. While this hardware design would not be as difficult as porting the Mac OS X to the new hardware, it would be another hurdle to overcome.
Future Cells, Future Macs
The challenges that Apple might face with the Cell processor apply to the first-generation version announced by the Cell consortium. For future evolutions of the technology, all bets are off, analysts said.
The Cells architecture is flexible enough to accommodate changes that would make the Cell more attractive to Apple. IBMs current overtures to Apple might be aimed at future evolutions of Cell technologies.
“There certainly could be another version of the Cell in the future with a different Power core and a different I/O,” Krewell said. This redesigned Cell would mean less work for Apple to get the Mac OS X running efficiently on it than on todays Cell.
On the other hand, it may be just as likely that IBM could move Cell technology into a PowerPC processor, which might make a processor transition even smoother for Apple.
“The Cells Broadband Processor Architecture could be rejiggered to be used in other processors, including the PowerPC,” Krewell said.
Neither of these scenarios could occur overnight. Krewell estimated that it would take IBM at least two years, possibly longer, to come up with a Mac-friendly Cell or a “cellified” PowerPC.
So, what of Merrill Lynchs Cell-based Mac multimedia workstation? There is another possibility, the analyst suggested.
“The Cell is good for ray tracing, graphics and video editing [applications],” Krewell said. “Its possible that Apple would do a media PC with the Cell as a second media processor. The Cell could be very useful for this.”
With the Cell as a coprocessor, Apple would not have to port and optimize all of Mac OS X. It could offload certain graphics, video and other functions to the Cell, which also would reduce the utilization of the main processor.
The Quartz Extreme routines in the current “Panther” version of Mac OS X, as well as the forthcoming Core Image and Core Video Xcode components in the “Tiger” version, offload tasks to a range of graphics coprocessors.
Still, Apple might have to wait for a future generation of the Cell in order to make this work, Krewell said.
As a coprocessor, the Cells core and I/O would require less developmental resources from Apple. However, that savings would bring a cost on the hardware side of the system. Certainly, two processors are more expensive than one.
However, Krewell said that the first-generation Cell might cost more than the CPU used in todays dual-PowerPC G5 Power Macintosh models. Since Sony will sell more PlayStation 3s than Apple can move Power Macs, Apple wont receive the same unit pricing as Sony.
The Cells size also makes it more expensive than the G5. If todays Cell were used, the coprocessor would have four times as many transistors as the main processor—234 million in the Cell and 58 million in the G5. For many applications, this level is more than is needed or justifiable for a coprocessor.
One way to eliminate transistors, and therefore to lower cost, in a future Cell would be to use fewer synergistic processing units. Die size and costs could be further lowered if IBM manufactured the chips using a 65nm process instead of the 90nm process of todays Cell and PowerPC processors.
Of course, Apple is now as much about handheld devices as it is about desktop computers. IBM pitches the Cells scalability to a variety of devices, pointing to Sonys gaming platform, Toshibas televisions and IBMs workstations, as well as handhelds and phones. Most would agree that the iPod just doesnt need the power of a Cell, but what about an expanded iPod that incorporates other functions, such as gaming?
Once again, this plan would require a smaller, cheaper implementation of the Cell. Estimates of Cell power usage run from 30 to 80 watts.
“IBM says the Cell is air-cooled, which probably means it needs a fan,” Krewell said. “They could crank it down to the 3GHz range, but [the Cell] makes no sense in a handheld at this point.”
Two things seem certain: For the near term, Cell technology wont affect Apple products. And for the longer term, its nearly impossible to tell.