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Under The Hood: Windows CE CPUs and Chipsets

Microsoft's new Windows CE operating system creates new opportunities, and new challenges, for processor manufacturers. The seven handheld personal computers (HPCs) announced at Comdex were based on three different CPUs, none of which had ever been used for a Microsoft operating system before. Why did all of these vendors choose RISC chips, forsaking the Intel x86 CISC architecture supported by all previous Windows operating systems? The answer has as much to do with economics as technology.

For years, Intel has been almost exclusively focused on developing x86 processors for personal computers. The company's closest approach to the HPC market is the 486GXSF, a 486-class chip with no integrated peripherals and a typical power consumption of over 0.5W at 33 MHz, yielding a performance of only about 12 million instructions per second (MIPS). This is quite low compared with RISC-based processors.

Other companies offer x86-compatible parts for handheld computers, such as AMD's Elan and National's NS486, but none can match the price/performance ratios of the three processors chosen for the first HPCs.

Hitachi SH7708
The most popular choice is Hitachi's SH7708, used in the Casio Cassiopeia, Compaq PC Companion, HP Palmtop PC, LG GP40M, and Hitachi's own Handheld PC. The SH7708 is a 32-bit RISC chip first announced in early 1995, an upgrade of the SH7604 found in the Sega Saturn video game. The SH7708 offers good performance, scaling up to 100 MHz although the initial HPCs all run at 40 MHz. The SH7708 has a unified 8K primary cache, shared between instructions and data. The cache is four-way set associative, reducing the likelihood of cache thrashing in a multitasking environment like WindowsCE.

The SH7708's instruction set is unusual among 32-bit RISC processors. Instead of 32-bit instructions like other RISCs or the variable-length instructions of CISC processors like the x86 family, Hitachi's SH-3 architecture uses fixed-length 16-bit instructions. The smaller instructions produce smaller compiled programs, but compromise many of the advantages of RISC. For example, SH-3 processors have only 16 registers and don't support three-operand arithmetic operations, forcing more memory transactions as data is moved in and out of the smaller register set.

To accelerate certain signal-processing functions like audio decompression, the SH7708 has a hardware multiply-accumulate (MAC) unit. This block of logic can multiply two 16-bit operands stored in memory, forming a 32-bit product, then add the product to a special 42-bit accumulator, all with a single instruction. Because the MAC function references memory, it is relatively slow. The normal multiply instructions use the same 16x16-bit multiplier unit; the chip does not support 32x32-bit multiplies except through software emulation. While the earlier SH7604 had a hardware divide unit, the SH7708 does not, so 32x32-bit divide operations take 37 cycles.

Still, the SH7708 offers a peak efficiency of one MIPS per MHz, and comes at a good price probably under $20 in large quantities. Hitachi sells millions of SH-3 parts every year, helping to keep manufacturing costs low. Power consumption is also low, around 250 mW when running at full speed. All HPCs are equipped with effective power-management hardware and software, so the average power drain from any of these CPUs will be much lower than the typical figures provided by the chip vendors.

All current SH7708-based HPCs share the same basic architecture. The SH7708 connects directly to ROM, RAM, and an application-specific IC (ASIC) that includes the LCD controller, PC Card interface, and other essential peripherals. Since the SH7708 only has 26 address lines, these HPCs are limited to a total of 64Mb of direct-addressed memory, but this is more than adequate for handheld devices.

The remaining two HPCs come from NEC and Philips, and both of these vendors provide their own processors. Both are members of the MIPS family of RISC microprocessors used in a wide range of systems from the Nintendo-64 video game to SGI supercomputers.

NECVR4101
NEC's MobilePro HPC is driven by the VR4101, a 33-MHz, 33-MIPS device with a 32-bit processor core and relatively small primary caches: just 2K for instructions and 1K for data. These caches are direct-mapped, reducing efficiency compared to the set-associative caches of the SH7708. The VR4101 also has several integrated peripherals, including a ROM/DRAM controller, five-channel DMA controller, real-time clock, and interfaces to external audio, IrDA (infrared), keyboard, touch-screen, and serial devices.

The VR4101's small, non-associative caches will have an adverse effect on the part's performance, but NEC may have chosen to give up large caches in exchange for the on-chip peripherals. The VR4101 also has a MAC function, but unlike the SH7708, the VR4101 performs a 16x16-bit multiply plus addition into a 64-bit accumulator in only one cycle. This gives the VR4101 enough signal-processing power to handle demanding applications like software modem implementations. The MobilePro doesn't take advantage of this capability, but it is likely that future products will.

Despite its small size, only 6.5 x 6.5 mm, the VR4101 is a true 64-bit processor. It has 64-bit registers and supports 64-bit addressing, just like high-end MIPS processors. This goes beyond Microsoft's basic requirements. MIPS processors for Windows CE need only be compatible with the 32-bit mode of the R4000 architecture.

The VR4101 provides all this performance at a typical power consumption of only 250 mW at 33 MHz. Since NEC's Computer Systems Division buys the VR4101 from NEC Electronics, it's difficult to say what they're paying for it, but the retail price of the VR4101 is under $25. NEC's internal price is probably much lower.

Figure 1 shows the system block diagram of the MobilePro, illustrating the relationship between the VR4101 and the ASIC that includes the LCD controller and other peripherals not built into the CPU itself.

Philips TwoChipPIC
Philips also uses its own MIPS processor in its Velo 1, but took a different approach, favoring even higher integration than the VR4101. Philips designed almost all of the necessary logic for an HPC into a two-chip set it calls the TwoChipPIC (for Personal Intelligent Communicator). The chip set consists of the PR31500 microcontroller and the UCB1100 analog interface chip.

The PR31500 includes a 32-bit MIPS core, 4K of instruction cache, 1K of data cache, and many peripherals. In addition to all the basic features of the VR4101, the PR31500 has an LCD interface that supports color and gray-scale displays up to 1024x1024 pixels in size. It also sports a PCMCIA interface, a Concentration Highway Interface (CHI) port (a very high-speed interface for ISDN or wireless communications at up to 4 Mbit/s), an internal keyboard controller, and 39 bidirectional I/O pins.

As seen in Figure 2, the Velo HPC contains little more than the TwoChipPIC chip set, memory, and some interface logic. This simple organization is a consequence of the sophistication of Philips' processor design.

Philips licensed a version of Toshiba's R3900 MIPS RISC processor core for the PR31500. The company describes the core as an R3000-class processor, but it is compatible with the R4000 instruction set. The core runs at 37 MHz, achieving roughly 37 MIPS, the best performance of all HPC processors. The caches are larger than the VR4101's but still smaller than the SH7708's. The registers are only 32 bits wide, unlike the VR4101; this reduces the size and complexity of the core, and Windows CE does not use the 64-bit mode of the VR4101 anyway.

The UCB1100 is connected to the PR31500 via a four-wire 15-MHz serial bus. The UCB1100 provides a 12-bit audio codec and a 14-bit modem codec, a touchscreen interface, and a 10-bit A/D converter for measuring battery voltages and other analog inputs. The UCB1100 can be connected to a microphone and headphones with no external components, and only a 600-ohm transformer is required for a telephone-line interface. The Velo's voice-memo feature was made possible by the UCB1100's audio codec.

A third "component" of the TwoChipPIC is a complete V.32bis software fax/modem implementation developed for Philips by General Magic. The Velo is unique among the first HPCs in having a built-in modem, made possible by the high integration of the TwoChipPIC and the General Magic software. The modem code uses the PR31500's MAC instruction, which operates much like the VR4101's MAC implementation.

Together, the two devices in the chip set consume less than 400 mW under normal conditions, excellent performance considering the broad range of functions built into the devices. The price is also very competitive, less than $39 - probably less than the combination of the SH7708 or VR4101 plus the ASIC(s) required by those processors.

All of the early HPCs offer similar features and performance; even the Velo is really only slightly faster. Exact comparisons are impossible until we have reasonable benchmarks for Windows CE. Future HPCs are likely to provide much more differentiation, since Microsoft has announced that Windows CE will also be supported on three other CPU architectures: x86, PowerPC, and ARM.

It's likely that x86-based WindowsCE products will be limited to embedded applications like office equipment. There is no advantage to x86 compatibility under Windows CE, since there is no way to run existing Windows 95 or Windows NT software in the much simpler Windows CE environment. However, there may be a market for Windows CE versions of conventional x86 subnotebooks and pen computers. Within the next year or two, we may see Windows CE appear as an option on these products.

The PowerPC option probably presages the development of PowerPC-based Windows CE devices. Without specific interest, Microsoft is unlikely to go to the trouble of porting the OS. Microsoft is not supporting the high-end PowerPC processors used in Apple's Macintosh computers, however. Instead, it is working with Motorola's embedded PowerPC parts. Currently, Windows CE is running on the PowerPC MPC 821 and MPC 823, and the MPC 801 is not far behind. Motorola has a strong history of providing integrated peripherals with its 68000 and PowerPC embedded processors, and already supports many different operating systems on its various CPUs. Access to the Windows CE market is likely to help expand Motorola's market even further.

It is also likely that we will soon see Windows CE products using Advanced RISC Machines' ARM processor architecture. The best of these devices, Digital's SA-110 StrongARM processor, is also the heart of Apple's new Newton MessagePad 2000. At 161.9 MHz and a price below $34 for the version Apple uses, it offers dramatically better price/performance than any other Windows CE processor, and this will help to open up new markets for high-performance mobile computing devices. The ARM architecture is licensed to a total of 16 microprocessor vendors, and at least 11 operating systems run on the ARM platform.

The first seven Windows CE products are just the tip of the iceberg-expect to see many more announcements over the next year, based on a wide array of processors. With Microsoft supporting five completely separate processor families, Windows CE can bring the power of personal computing to a wide range of consumer-electronics products.

- Peter Glaskokski