I hope that ALi 1683 was the last single-channel chipset for Pentium 4. We are expecting to see only dual-channel solutions and a transition to chipsets for DDRII. First solutions for DDRII were announced a long time ago but the DDR products from VIA (PT880) and SiS (655FX/TX) haven't hit the retail market yet. However, today we have a chance to study the most uninteresting one SiS655FX.
This is actually a transition from SiS655 to SiS655TX: the FX and TX have actually no difference in the architecture, the latter has only a more efficient memory controller. Whether it's true and what is the nature of the advancement will be discussed in the future review of SiS655TX. Right now I must say that this reminds the difference between i875P and i865PE. Have a look at the block diagram of the today's hero:
The difference from the SiS655 is expected: it's the support of
the accelerated FSB (800 MHz) and a new south bridge. It's typical
of FX versions of SiS chipsets (this company numbers its products
in a strict series), and the south bridge (and it's typical of all
modern chipsets) is pin-compatible with their predecessors and, probably,
their followers. The SiS964 possesses the full functionality: the
number of USB 2.0 ports is increased up to 8, support of 2 SATA with
RAID (0, 1, JBOD) and other features which can be easily found in
specs of any chipset. The FireWire controller is not supported (south
bridges didn't match this function in SiS model).
However, there's one interesting moment in the SiS655FX. You might remember the confusing dual-channel mode of SiS655: 1x128 and 2x64. We didn't clarify all the aspects of functioning of these modes, but we found out that the former can be used only if two/four identical memory modules are installed (it's typical of modern dual-channel solutions, like Intel 875P/865PE). The other suits also asymmetrical pairs and even three(!) modules without losing the advantage of the dual-channel memory performance. In the review of the more promising SiS655TX we will focus on this aspect, and right now I must say that such solution is very flexible and relieves you from spending time on looking for a matching pair of modules for the dual-channel mode in the Intel based boards. SiS655FX supports up to 4 GB (4 modules) of unbuffered DDR266/333/400 memory without ECC.
What I can say about the SiS655FX chipset before testing its performance is that this is a dual-channel chipset and it must be faster than most competitors, but a very poor performance of its single-channel memory SiS648FX doesn't let us hope that it can beat the i875P/i865PE.
OS and drivers:
Among two SiS655FX boards we chose the fastest one for comparison. We also combined the scores of all single-channel chipsets for Pentium 4 with a FSB 800 MHz and added the scores of the leading dual-channel Intel 875P chipset, one of the fastest boards based on it. Only i865PE didn't get into our summary table - we could see that it goes on par with i875P, but it can also be much slower depending on a certain board model (and even its BIOS version). At the moment we have no more dual-channel chipsets announced. Finally, we also tested our hero together with the dual-channel DDR333 (the case of lacking the high-quality DDR400 or its too high price) and with the single-channel DDR400 (the case of one module).
In the tests of the maximum read memory bandwidth all single-channel chipsets managed to get approximately the same speed of ~2850 MB/s (which is ~90% of the theoretical maximum). Certainly, the dual-channel modes of SiS655FX look preferable in this synthetic test, but they are still far behind i875P.
Intel's top product has the highest write memory rate, but we should applaud the i848P in first turn because it yields only to SiS655FX+2xDDR400. The low mode of SiS655FX is not impressive at all - it hardly differs even from SiS648FX.
This difference is noticeable only in the memory latency test where SiS648FX set an antirecord - obviously, the two FX chipsets have certain internal differences, and one is not just a crippled version of another. The i875P/i848P demonstrate fantastic scores. The latency of SiS655FX in the optimal mode doesn't show any noticeable advantage over the older single-channel competitors.
Well, in the low-level memory benchmarks the tested chipset naturally outscores its competitors in reading and writing but demonstrates quite average latency. It means that the scores of i875P are considered as reference without giving SiS655FX the first place. Now let's see what the real tests show.
Archiving depends a lot on the latency the chipset reaches, that is why it's not surprising that Intel's single-channel product takes the second place. The high read/write speed of SiS655FX is sufficient neither to make up for its lag from i848P, nor to outdo VIA PT800 though this parameters is equal for these chipsets. This is true not for all correlations which is well demonstrated by ALi's chipset which has decent latency and quite bad writing resulted in the last place shared with SiS648FX (which has the opposite combination of these two factors).
The final rendering in 3ds max with the Brazil Rendering System is not that critical to the memory speed, even the outsider ALi 1683 falls behind the permanent leader by just 6.5%, SiS655FX - by less than 2.5%.
Adobe Photoshop is a more demanding consumer, here the difference between the best and worst performers (ignoring an unexplainable failure of ALi) makes 11.5%, and SiS655FX finally takes the second place with only a 6% gap from i875P.
In the games the difference between the chipsets is not that big and I can say that SiS655FX shares the second position with i848P, while falling behind the i875P by 69% and outscoring the closest follower (VIA PT800) by 23%.
Let's combine some comparison lines into one table (SiS655FX runs in 2xDDR400 mode if another mode is not indicated):
So, SiS655FX is the company's top chipset for Pentium 4 (before SiS655TX). What can it offer?
The SiS655FX based boards can be recommended for everyone who can't afford i875P/i865PE based solutions but who need a high-performance system. Compare the prices of the competitive products to make a right choice. If you don't need a new board that fast you'd better wait for the review of VIA PT880 and (especially) SiS655TX.
Sergei Pikalov (firstname.lastname@example.org)
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