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When faster isn't so fast: The Intel PXA250 XScale Processor.
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At the heart of any pda, desktop computer, or mainframe, is the CPU, or central processing unit. It is the main engine for a computer, executing the countless number of instructions that make up the programs which make computers an ever more essential tool in modern life.
CPU's come in a wide variety of designs. While they all do essentially the same things, cpu's have been specifically designed to the environment in which they operate. For the pda, the cpu is designed for a small space, limited hardware resources, and low power consumption.
PDA's run several different types of cpu, such as MIPS, SH3, x86, ARM, DragonBall, OMAP, XScale, etc. Each of these uses its own instruction set and programs written for them will not usually run on another cpu type. The current cpu of choice for the PocketPC, and the one running inside the Dell Axim, is the Intel XScale PXA250. Its imminent, and hopefully eminent, successor, is the PXA255.
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The PXA250 XScale processor has for the most part been a major disappointment. Running at 300 or 400Mhz, the XScale replaces the 206Mhz Intel StrongArm processor found in the earlier Pocket PC's. From the beginning, Intel touted the XScale as being able to run faster with a lower power consumption than the StrongArm.
In practice, this has not been the case. In spite of being 50-95% faster in terms of MegaHertz, few applications saw an increase in performance and quite a few others actually decreased in performance.
The XScale was designed around an enhanced version of the ARM5 instruction set. The ARM5 is a superset of the ARM4 instruction set used by the StrongArm processor which was further enhanced by Intel, so programs compiled under the ARM4 instruction set generally run without error on the XScale.
But, because of design compromises in the XScale, the ARM4 instructions cannot not take advantage of the new features of the XScale architecture and actually run slower than on the StrongArm CPU. The only thing that has redeemed the XScale thus far has been its much greater clock speed. The problem is further magnified by the fact that Intel and Microsoft apparently did not care to resolve their differences between the CPU and the operating system.
Both companies had logical arguments for their positions. Intel wanted to incorporate new power management features and multimedia performance in the new chip architecture which required new compiler instructions. Microsoft did not want to abandon current users of the Pocket PC on the StrongArm and could not justify the cost and complexity of supporting two versions of the OS optizimized for two different instruction sets.
Unfortunately, that leaves the consumer caught in the middle and there is no indication that the next version of either the CPU or OS will resolve the issue. Several application developers have produced XScale enhanced versions of their programs, including audio and video players such as PocketTV(mpeg) and PocketMVP(mp3/avi). Performance over the non-optimized versions is substantial and impressive, but actually revealed another weakness of the PXA250: it is memory bandwith constrained.
The bus connects the CPU to the memory and other components the CPU accesses. The CPU has a memory reserve that resides on the microprocessor chip itself and is called the cache. The cache holds frequently used instructions and data for processing the work waiting to be completed by the CPU.
Because the cache resides on the same chip as the microprocessor, calls between the CPU and the cache are very fast. When the CPU needs additional instructions or data not contained in the cache, it retrieves them from the system memory via the bus. System memory takes longer to access than the cache.
The StrongArm processor runs at 206Mhz and it uses a 103Mhz bus. The XScale processor runs at 400Mhz, but it uses only a 100Mhz bus. For applications which are primarily CPU intensive where most of the required data and instructions can reside in the cache, the slower bus is not substantially limiting.
However, for applications which are Memory intensive, the bus speed becomes the bottleneck for program performance. So, even applications which have been optimized for the ARM5 instruction set, will have limited performance on the XScale due to the bus speed, a problem which the software developers have no real way to mitigate.
Enter the PXA255 and it's 200Mhz bus.
Beginning this month, all new Pocket PC's will use the new XScale chip, including Dell's X3 and X7 Axims. The PXA255 should allow pda's with the XScale, for the first time, to demonstrate substantial improvements in performance over the StrongArm. The added performance will magnify the advantages of the Pocket PC over its Palm OS pda rivals in multi-tasking and multi-media.
Additionally, the PXA255 has enhancements to its power management and utilization. This will allow better CPU clock scaling for balancing performance and battery life. A new deep sleep function will also allow a greater amount of time between recharges for periods of limited pda usage.
For the average pda user, the new CPU will mean smoother video and audio playback, faster application switching and startups, and a longer battery life.
For the technophile, it will add even more fun. Programs such as PocketHackMaster, which allow CPU overclocking up to speeds of 500Mhz in the Dell Axim, have provided disproportionately modest performance gains due to the memory bus limitation. The new 200Mhz bus should relieve much of this congestion and let the bold (or reckless as the case may be) push the envelope of pda computing.
You might ask if it's possible to use so much processing power in a handheld unit. The answer is that if there isn't now, there will be! Oh yes....there will be!
UPDATE: See BRIGHTHAND.COM's ARTICLE on the new PXA255 chip - http://www.brighthand.com/article/PXA255_Is_Noticeably_Faster_than_Previous_Chip
LINKS:
PocketHackMaster
Intel XScale PXA250
Pocket TV
PocketMVP
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