XScale ClarificationBy Peter Naulls. Published: 18th Apr 2003, 10:48:00 | Permalink | Printable
RiscStation Spin Doctor busy againThere seems to still be a fair amount of confusion about XScale processors. This is reasonable enough, it's still a relatively new thing. However, this has been recently compounded by confusing statements made by RiscStation's spokesperson. We'd like to take this opportunity to clarify some points, in our usual, lucid no-nonsense style.
Thanks to David Ruck and Hedley Simons for clarifying and checking some points.
If you've got any further questions, drop us a line, and we'll see what we can do about answering them.
Disclaimer: I am not a hardware engineer.
"XScale is a machine on a chip like the 7500FE"
Doubly incorrect. Firstly, the term "XScale" in general terms refers to Intel's range of processors which supersede the StrongARM. This family contains quite a number of members. In technical terms, "XScale" refers to the processor core, or the bit that actually handles the ARM instructions.
The 7500FE by contrast, is a specific chip (although it does have several versions), which is found in several RISC OS machines including the A7000 and RiscStation machines.
The specific XScale used in Castle's Iyonix machine is the Intel 80321 I/O Processor, also referred to as the 80321 or IOP321. This chip contains a great deal of functionality which makes it a good choice for a desktop RISC OS machine, including a PCI controller and I2C bus.
The IOP321 doesn't have a video or audio controller, nor any functionality to talk directly to the outside world. In the Iyonix, video and sound are provided by PCI cards, and a separate Southbridge chip contains functionality for IDE, serial, USB and power management. The Iyonix also contains some other logic to help simulate parts of the RiscPC hardware. This is some way from a machine on a chip solution.
By contrast, the 7500FE does contain most of the functionality around which you could build a computer, including video, audio, serial ports (but no PCI). This has made it a popular choice for RISC OS machines in the past.
"A machine with seperate chips will run faster than an integrated one like the IOP321"
This is a red herring. The machine's maximum speed is governed by one thing - its processor speed. Access to other devices depends upon how things are put together, and the existence of any bottlenecks (such as the infamous slow RiscPC bus). In fact, a machine with an integrated chipset will tend to run faster due to less latency between devices. Finally, the overall speed of the machine is governed by the speeds and architecture of specific devices (whether integrated or not).
(I have intentionally not mentioned DMA here for simplicity)
"The XScale doesn't give clock for clock performance compared to the StrongARM"
Careful. It's true that the XScale processors clocked at 400Mhz in some early iPAQs didn't perform any better than comparable StrongARM devices. In searching for the answer, people suggested that the issue was the compiler not optimising for XScale. It's also true that you can get some improvement by optimising correctly, but most of the reason came down to architectural issues.
By contrast, the 600Mhz IOP321 in the Iyonix has no such issues, as quickly becomes evident if you use a machine for a short while. Benchmarks show the relative performance to be within a few percent.
"Why didn't Castle choose the faster 733Mhz XScale?"
Although 733Mhz IOP200 is clocked ~20% faster, it only has a 100Mhz bus compared to the 200Mhz one found in the IOP321, and doesn't contain as much functionality suitable for a desktop machine.
"What about the 1Ghz ARMs?"
It's not clear when these might be available. It's also been claimed that the 733Mhz IOP200 could be overclocked to 1Ghz, but I can't confirm the veracity of this.
"Will the Iyonix processor be upgradeable?"
Only if Intel produce a pin-compatible IOP321 processor. Even then, it will be a specialist job. Anything else will require a new motherboard, which may be desireable anyway, if Castle make improvements in other areas.
"Any futher questions?"
Contact me at firstname.lastname@example.org.
ARM 7500FE Data Sheet (PDF)
XScale Product Brief (PDF)
Intel IOP321 I/O Processor
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