Power on the Move

Everyone talks of style and features in mobile devices, but have you wondered what really makes them tick? The heart of any mobile phone is its processor, and it's becoming more powerful by the day

Friday, February 01, 2008

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Life minus a mobile phone is simply unimaginable. We all need mobiles, not only to talk and SMS, but also to store numbers, set reminders, alarms, surf websites, listen to music, click pictures, and so on. Gone are the days when mobile phones were meant only for phone calls. Today, nobody buys a mobile phone just for phone calls! In fact there would hardly be a model available that would offer just that. Even low cost mobiles today come loaded with features. All this implies that the processors inside mobiles need to be extremely smart to handle complex applications and multitask, without compromising on call quality. With increasing demand, the mobile phone architecture has also undergone major modifications.

Unlike its big daddies-the desktop and server processors, a mobile phone processor needs to be cost effective and also be able to dish out tons of functionality, without consuming too much power. As an example, let's consider the scenario, where the entire processing power of a cell phone is being consumed by a certain application and the cell phone receives an SMS. You need more power to run the SMS, and if simultaneously the phone starts ringing, then it's curtains for the processor.

So, a processor should be perfectly capable of performing multiple tasks at all times. This is especially true in Windows based mobiles, as the Windows OS would itself stress the processor to a great extent. So, like desktop and server processors, even a mobile processor has to generate great computational power. We shall now discuss what these processors need to have to enable them to deliver so much compute power. We also provide you with a small snippet of all application platforms that are available in the market and which enable developers to develop applications that can stress the processor to its limits.

The Nokia N95 is based on the ARM II family of processors and comes loaded with maps, office suites, browsers, etc

Mobile processors and RISC
Most current mobile processors are based on the RISC (Reduced Instruction Set Computer) platform. So it's important to understand RISC in order to get an in-depth knowledge of mobile processors. RISC is a CPU design that emphasizes on simple instructions that do less but still provide higher performance, if this simplicity can be utilized to make instructions execute fast. It was devised in 1970s when it was felt that compilers at that time weren't able to take full advantage of features that would ease coding. It was decided that such functions could be carried out by a sequence of simpler instructions, if this could enable implementations which are simple enough to cope with high frequencies and small enough to allow many registers.

The main goal was to make instructions so simple that they could easily be pipelined to achieve a single clock throughput at higher frequencies. Some of the well known RISC families are DEC Alpha, ARC, ARM, AVR, MIPS, PA-RISC, Power Architecture and SPRAC. Around 75% of the embedded 32-bit RISC CPUs are from ARM, based on different generations of ARM design architecture. The CPU is a 32-bit RISC processor architecture that is widely used in a number of embedded systems. The best thing about ARM CPUs is that they are very power efficient, making them an ideal choice for mobile devices where small battery sizes make power consumption a critical factor.

Mobile processor architecture
ARM design saw light of the day in 1983 as a development project at Acron Computers Ltd. Over the years many samples were designed. The first real production system was ARM2, which was probably the simplest and most useful 32-bit microprocessor with only 30,000 transistors. It was considered the simplest 'coz it didn't have any microcode.

Nokia's N Gage is based on the ARM 9E family, majorly around the ARM v5TEJ architecture

The ARM architecture includes some of the RISC features such as no support for misaligned memory access, load and store architecture, orthogonal instruction set, large 16x32 bit register file and mostly single cycle execution. In 1980s, Apple Computers started working with Acron on a new version of ARM core. Thus ARM6 was born and Apple used the ARM6-based ARM10 as the base for their Apple Newton PDA in 1991. Even though the ARM architecture has changed over the ages, its core has remained largely the same size.

ARM2 had 30,000 transistors while ARM6 grew only to 35,000. The common architecture supported on smartphones, PDAs and other handheld devices is ARMv4, even though Apple eMate300 was based on the ARM7 family of ARMv3 architecture. Nokia N-gage phone, which was a rave for its design, was based on ARMv5TE architecture with ARM946E-S core design having enhanced DSP instructions and very tightly coupled memories.

The Sony Ericsson K and W series phones are based on ARM9E family built majorly around the ARMv5TEJ architecture. Along with enhanced DSP instructions this architecture also has Jazelle DBX, which is meant to execute Java bite code. It typically functions at 220 MIPS at 200 MHz. Even Siemens and BenQ (X65 series and newer) are also based on the same architecture and belong to the same ARM series.

One would notice each family having various architecture, which will have different core designs and instruction sets functioning at different MIPS. HTC Universal, Blackberry 8700, Blackberry Pearl (8100) all belong to the XScale architecture designed on ARMv5TE architecture but have different cores.

HTC Universal is based on PXA27x core and has application processor instructions whereas Blackberry is based on the PXA900 core and doesn't have application processor instructions. Recently Nokia launched their latest N95 model with 'This is what the computer has become' tag line, implying they wanted to emphasize various tasks that you can do with the Nokia N95.
The N95, N93 and N800 are based on the latest ARM11 family. Even the Apple iPhone is based on the same family of processors. It's just that the core design is different along with a couple of instructions. The next generation of this family is Cortex and will feature various architecture and core designs along with advanced instruction sets.

The enhanced instruction set is what distinguishes Apple's iPhone from others, and it is again based on ARM II family of processors

Mobile Extreme Convergence (MXC) architecture
The MXC architecture promises to simplify smart wireless devices. It separates the two main domains of a cell phone: a modern core that communicates with the base station and an application core that powers user experience. The benefit of separating the two cores is that now a designer can create new applications as quickly as they're required, as they don't disturb the modern core. An open operating system approach lets software developers deploy applications across a broad range of devices. The key advantage with MXC is the efficient shared memory system which provides the flexibility to partition tasks between MCU and DSP. It also has the advantage of lower power design and enhanced security architectures such as SIM lock, program and data integrity and hardware features to support digital rights management.

Instruction sets in ARM
An instruction set enables a processor to perform different tasks. It also helps to differentiate each processor within a particular family from the other in terms of overall functionality. So it is important to know what exactly the instruction sets do.

Thumb: It is a compressed instruction set which uses 16-bit instruction encoding but still processes 32-bit data. Here, smaller opcodes have less functionality.

DSP Enhancement Instructions: These have mainly been incorporated to improve ARM architecture for digital signal processing and multimedia applications. One can easily identify the architecture with such an instruction set by looking for 'E' at the end of the name of the architecture as in ARMv5TE.

Jazelle: Jazelle DBX (Direct Bytecode eXecution) allows ARM architecture to execute Java byte code in hardware as another execution state alongside the existing ARM and Thumb state. It is mainly used by mobile phone makers to increase the execution of Java ME games and applications.

Thumb2: Announced in 2003, it extends the limited 16-bit instruction set of Thumb with additional 32-bit instructions, mainly to increase the breadth of the instruction set. All ARMv7 chips support Thumb2.

Advanced SIMD: Marketed as NEON, this is a combined 64 and 128-bit SIMD (Single Instruction Multiple Data) instruction set that provides standardized acceleration for media and signal processing applications.

VFP: It provides low cost single precision and double precision floating point computation. It also supports execution of short vector instructions, which is useful in graphics and signal processing applications by reducing code size and increasing throughput.

Security Extension (TrustZone): It provides a low cost alternative to adding an additional dedicated security core to an SoC, by providing two virtual processors backed by hardware based access control.

Conclusion
The onslaught of smart phones has led to clamours for processors with multitasking capabilities. The market promises to evolve in the coming months, throwing challenges in front of the developer and the chip manufacturer to come up with more enhanced solutions. The processors within a mobile phone will witness major upgrades with newer instruction sets and core designs been planned. So, it will be interesting to see how far the processor can be stressed before it reaches its saturation point.

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