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PDA Processors, Part 1: ARM Wrestling By Dan Costa
From the premiere PalmPilot to recent times, Motorola dominated the handheld market with its DragonBall processors -- variations on the venerable 68000 CISC architecture used in pre-PowerPC Apple Mac systems. First released in 1995, according to IDC, the DragonBall is found in more than 75 percent of PDAs worldwide, thanks to its use in all Palms (and Handspring Visor and Sony Clie compatibles) until just last year.
Motorola is currently shipping its fourth- and fifth-generation PDA processors, the DragonBall Super VZ and DragonBall MXL/MX1, respectively. The former doubles the speed -- from 33MHz to 66MHz -- and adds a host of system-on-chip functionality -- ranging from DMA and USB to MultiMedia Card/Secure Digital and Sony Memory Stick flash-card controllers -- to its 68K-based predecessor, the DragonBall VZ. Sony chose the DragonBall Super VZ for the Clie PEG-NR70V, first of its digital camera- and keyboard-equipped Palm OS handhelds.
Motorola's fifth generation, however, jumps from CISC to RISC and joins the ARM camp. The DragonBall MX (Media Extensions) chips combine an ARM9 core with highly integrated system functions, running at 140MHz to 200MHz. Performance is comparable to that of Intel's StrongARM processors (more on them in a moment).
When developing its Palm OS 5 response to Microsoft's surging Pocket PC platform, Palm chose to optimize the operating system for ARM-based processors. When the company released its first OS 5 handheld, the Tungsten T that debuted last October, it put the Texas Instruments OMAP1510 under the hood.
Part of TI's Open Mobile Application Processor initiative, the OMAP1510 is a dual-core processor that combines a 175MHz, TI-enhanced ARM925 CPU for general OS tasks with a 200MHz TI DSP (digital signal processor) core for demanding multimedia and speech applications. The chip can run business applications, such as displaying Word, Excel, and PowerPoint files, faster and with higher resolution than the DragonBalls that powered previous Palms.
There are still plenty of non-ARM Palm devices available; the forthcoming Tungsten W, for example, is a keyboard-equipped smart phone that runs the older Palm OS 4.1.1 on a 33MHz DragonBall VZ. But most future Palm devices, at least on the high end, will be built around the ARM architecture.
If backers like European smart-phone leader Symbian and TI weren't enough to make a company confident, ARM must have popped a few buttons when mighty Intel signed up. StrongARM -- developed jointly by ARM and Digital Equipment Corp., the latter later acquired by Intel -- and XScale are Intel's brands for ARM-architecture processors, used in what the chip giant calls PCA or Personal Internet Client Architecture products for PDA, wireless, or networking applications.
In terms of architectural generations, says ARM's Cormie, "StrongARM performance is approximately equivalent to ARM9, while XScale performance is approximately equivalent to ARM11" (although XScale has some differences, such as a seven- rather than eight-stage integer pipeline and what Intel calls Media Processing Technologies rather than ARM's SIMD instructions).
The newer XScale, Cormie adds, is gradually replacing the ARMv4-based StrongARM in Intel's product portfolio. One reason is clock speed that blows away the older design: Intel's fastest StrongARM processor, the SA-1110, runs at 206MHz, which is still on the high side of the handheld spectrum, but the XScale PXA250 is available at up to 400MHz.
As users of the first XScale Pocket PC handhelds have discovered, however, you shouldn't expect the newer chip to deliver double the performance. Not only is clock speed no more a perfect predictor of real-world results in the handheld space than it is on the desktop, but most PDA buses can't move data fast enough to keep the XScale satisfied. It would help if Microsoft would write a version of Pocket PC 2002 specially optimized for the XScale instead of the ARMv4 code base, but that doesn't seem likely -- Microsoft doesn't want to orphan the many StrongARM iPaq owners.
Intel is working hard to get application developers to write fresh code for XScale, optimizing applications by writing for Integrated Performance Primitives (IPPs) and Graphics Performance Primitives (GPPs) that bunch data into libraries which can be quickly read and processed by the XScale CPU. This kind of batch processing could greatly improve bandwidth-intensive operations such as playing video clips or displaying 3D animations.
That begs the question: Why do you need to support video and 3D graphics on a handheld? Intel is clearly betting that PDAs and smart phones will do more than just carry names and numbers in the future, and it's not alone in making that bet. Next week, we'll examine how soaring subminiature CPU speeds will be put to use -- and how AMD plans to use a little Alchemy and a custom-built MIPS core to start a new chip war in the handheld space.
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