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A Matter of Fax

The Chip

The latest generation tile based renderer has been named Kyro II and is based off the PowerVR Series 3 technology. You may recall, last June we looked at the original Kyro processor. Let's see what the second incarnation of this chip entails.

First off, the Kyro II is not actually a completely new chip at all. The only difference between the original Kyro and the Kyro II seems to lie in clock speed, as both are PowerVR Series 3 based. Interesting enough, the Kyro II boast 15 million transistors, up from the 12 million on the original Kyro. When we asked STMicroelectroncis why the Kyro II features more transistors than the original Kyro, they responded by saying that the additional transistors were added to allow the Kyro II to hit a higher clock speed. Exactly how more transistors results in a higher clock speed, STMicroelectronics could not explain. Our best guess is that they've lengthened the pipeline and reduced the amount of work that is done in each step of the pipeline, much like Intel did with the Pentium 4. This is only a guess, however, and we have no way of confirming or denying that this is the case.

Regardless of how they reached a higher clock speed, there is no question that the Kyro II beats the pants off the original Kyro when it comes to clock speed. Up from a 125 MHz core, the Kyro II now features a 175 MHz clock speed. Since the core clock and the memory clock of the PowerVR Series 3 chips are synchronous, the memory clock is 175 MHz as well. It is interesting to note that originally the Kyro II was speced at a 166 MHz core and memory clock but tests conducted with board manufacturer Hercules/Guillemot showed that the chips could hit 175 MHz reliably.

The way that STMicroelectronics could assure a higher clock speed was by implementing a process shrink. The original Kyro chip was based on a 0.25 micron fabrication process, which resulted in limited clock speed. The new Kyro II is produced on one of STMicrolectronic's 0.18 micron fabs, not only shrinking die size but also increasing maximum clock speed while simultaneously decreasing power consumption. In fact, the new Kyro II running at 175 MHz dissipates only 4 watts of heat. Compare this to the 0.18 micron GeForce2 GTS with its 25 million transistors and 8 to 9 watts heat dissipation and one can see where simpler is better.

Like the original Kyro, the Kyro II features a 128-bit SDR memory bus that can support 16MB, 32MB, or 64MB configurations. Although SDR memory is typically associated with slow performance, this is not the case with tile based renderers. Since the memory bandwidth required to render a complex scene with a tile based renderer is drastically less than that required on an immediate mode rendering system, extreme amounts of memory bandwidth are not required. Therefore, it is likely that the Kyro II in its current configuration would actually not benefit at all by moving to a more spacious DDR memory bus.

Also like the Kyro, the Kyro II features two pixel pipelines capable of processing a single texture unit per clock. Once again, the Kyro II does not need additional pipelines in order to keep up with a card with more pipelines and texture units per clock because of the card's efficiency. There is no question, however, that adding additional texture units per pipe or increasing the pipelines will result in a substantial increase in performance, as memory bandwidth will not be holding the card back.

Missing from the Kyro II feature set is a T&L; engine. Claiming that the current generation of CPUs are far superior at T&L; calculations than any graphics part can be, STMicroelectronics choose to leave T&L; off the Kyro II. Perhaps STMicroelectronics is referring to the fact that the non-programmable T&L; we've seen on GeForce2 and Radeon cards has not been taken much advantage of, but the programmable unit of the GeForce3 will be much more useful and much more powerful. Whether or not this will hurt the Kyro, we will see in our CPU scaling tests, but it certainly would not hurt to put a T&L; engine on the chip. Look for the next PowerVR chip to feature such an engine.

Tile Based Rendering: PowerVR's Solution The Chip, Continued

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