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The CPU market is going in a definite direction. We know almost all about Intel and AMD plans, so the CPU market can be called predicable. But there is one thing we can not find out from roadmaps: the actual performance of the further CPUs. Not even the manufacturer is able to estimate the benchmark performance of a CPU, they can tell us a lot about the theoretical performance which is similar to the brute performance measured in number of operations per second, but for the benchmark numbers they need to have a sample CPU. The marketing team embosses the processor's advantages, but forgets to tell us about its disadvantages. Any architectural change or core improvement implies a list of disadvantages: weak compatibility, low performance with old software, new motherboards, new compilators, etc. Currently there are three x86 CPU manufacturers on the market: Intel, AMD and VIA. The first two companies are well known and do not need further presentations, but VIA is not known for their processors. A CPU is more than a chipset and implies a much more complicate designing and manufacturing process and even if VIA surprised with good chipsets this doesn't mean that their processors have inherited the same features. The VIA C3 processing line hasn't succeeded to impress many people due to its weaker performance. But VIA claimed that the low frequency processors have other areas of appliance and the public should consider them as well. Today we will try to find these areas and we will discuss frankly about the legitimacy of choosing the lowest frequency CPU on the market.
Introduction
VIA didn't enter the CPU market with no experience, in fact the C3 processor is based on a combination of Centaur and Cyrix technology. The CPU tries to impose a different approach to x86 architecture, but I can not call this approach new. It is known that x86 processors performance depends mostly on several instructions. VIA C3 optimizes the execution time of the most frequent instructions and limits the hardware for the less likely used instructions. These less used instructions are implemented using the microcode, a solution adopted by many manufacturers that target to thin clients market. There are different opinions about the use of microcode to translate instructions on desktop processors, but the use of microcode is generally needed to ensure compatibility even in high performance CPUs. For example 64bit processors will need to include a translation logic to ensure compatibility with 32bit processors. Generally every CPU uses a microcode stored in a very fast ROM memory to form the CPU language. If you are familiar with Transmeta Crusoe CPUs you probably know that Crusoe uses a Code Morphing Internal CPU software to translate instructions into a standard type of instructions called VLIW. The CPU core is optimized to work with this so called processor language so that external instructions variety has a very small impact on performance.
Transmeta targets to mobile market and may be a concurrent to VIA
C3, but from the technological point of view C3 is somehow between
any other x86 CPU from AMD or Intel and a Transmeta Crusoe. When
a instruction translation occurs, the CPU loses time so the performance
may not be as high as the performance of a traditional CPU running
at the same frequency. However, there is one big advantage, the
use of a relatively low number of transistors. Practically VIA C3
wants to provide a good performance while keeping the heat level
very low.
From the engineering point of view, VIA C3 is a very simple CPU.
Many advanced optimization techniques like out of order execution
are not implemented because they require a lot of onchip logic.
Instead VIA C3 implements an advanced memory bus managing technique:
two 4 way 64Kb caches, two 8 way 128 entries TLB, two four entry
page directory caches, advanced prefetching, etc. I do not want
to talk about cache because this is known by most readers, but about
TLB function and features. The Translation Lookaside Buffers role
in a CPU is to store the most recently used page-directories and
page-table entries. If you want just think that TLB is like a level
0 cache. TLB speeds up the data availability because the memory
access is not required for every operation. TLB plays a major role
in system performance and VIA C3 TLB is indeed impressive. In fact
Pentium 4 has also a 128 entries TLB, but it's 4 way associative,
which means that it's less efficient.
The C3 pipeline facilitates higher frequencies because it's twelve
staged, a little bit larger than Pentium 3 pipeline, but not huge
and potentially inefficient
like Pentium 4 pipeline. Let's take a look to the pipeline.
VIA C3 Pipeline
Take a look below to VIA C3 processor's pipeline structure:
The four main
functional groups of VIA C3 are: I-fetch, Decode and Translate,
Execution and Data Cache. I-fetch component brings
instruction data from I-cache or from system bus. The Decode and
Translate unit prepares instructions for internal execution. Early
in this step branches are identified, predicted and targets are
prefetched.
The Execution core issues execute and retire instructions. All these
steps can perform asynchronously and independently. You can see
in the above diagram how important are buffers in the VIA CPU architecture.
The Data Cache
loads execution instructions from bus, cache and other internal
parts. As you can see VIA C3 executes instructions in order, one
per clock and the pipeline misses are minimized due to multiple
buffer store levels.
The C3 branch prediction is quite advanced, it includes two level
of prediction, simple or history based. Most conditional branches
are simple to predict because these are based on the dynamic execution
state, but there are other branches which usually are not predicted
accurately due to their nature. For such kind of branches C3 uses
a branch history mechanism which is based on previous branch behavior.
There are three prediction tables that are checked to see if agree
on the different type of branch prediction. In practice, more than
20% from the total number of instructions are calls, jumps or returns
so an accurate branch predictor mechanism is very important. On
paper VIA C3 seems to have a very accurate predictor.
The processor is very particular and I can talk about it in several
pages, but I do not want to go very deep inside its architecture
because datasheets are free for everyone who wants to find out more.
But I have to emphasize a particular characteristic of the C3 processor.
If you remember our previous
article you know that Pentium 4 ALU runs at twice the speed
of the CPU. Well, on C3 the FPU runs on half of CPU speed. Looking
at the FPU design we discovered that the floating-point operation
of the C3 might not be very efficient. VIA counted on the less usage
of FPU instructions on the market they target and they didn't want
to insist on FPU because it can eat a lot of transistors.
The support for multimedia instructions is rather modest, C3 supports
only 3D Now! and MMX instructions which cannot bring a huge performance
improvement.
Product close up
After we discussed the architecture of the VIA C3 it is time to go to work and review the product. A C3 doesn't look more special than any other CPU on the market. Because it has to fit on socket 370 the CPU format is familiar. Well, if you look at the picture below you will notice that there is nothing special about the VIA C3 CPU.
VIA
C3 processor 866Mhz product picture
Actually we
reviewed for the first time a CPU made in 0.13um technology:
VIA C3 866Mhz. The situation will change very soon with Pentium
4 0.13um CPU release, but until then C3 is unique :). The
product we reviewed was shipped in a nice looking box, together
with a small cooler. The cooler remembered me about the old days,
when you didn't need a noisy monster to cool down your system. VIA
targets the processor to embedded systems and applications which
require a powerful CPU but a low thermal dissipation. In fact VIA
officials claim that a C3 is able to work perfectly without a fan.
Of course that we will investigate this affirmation.
Take a look below to the box picture, I think it deserves attention
because it's quite interesting.
Inside the box you can find installation advice, but especially if you got an old motherboard precaution is required. Let's install VIA C3 on a motherboard and find out more.
Installation
VIA C3 is advertised
to offer socket 370 compatibility, but the motherboard support can
be a little bit nasty. Well, VIA C3 can't work on BX motherboards,
it works on several Intel 810 and 815 motherboards and can fit most
VIA 693 motherboards. Many manufacturers have BIOS updates for their
motherboards, but in order to update the BIOS you need a running
system. In most cases the BIOS update only recognizes the CPU or
fixes problems, doesn't offer compatibility. So that you can boot
and update BIOS with the standard procedure.
The biggest installation problem can be the voltage required by
the CPU, which can be too low for many motherboards. For example
the CPU we tested required 1.35V to operate. A 10%-15% higher voltage
should not raise problems, but above this value problems may occur.
The minimum voltage allowed by VIA C3 VID pins combinations is 1.30V,
so you won't find VIA C3 processors which require a lower voltage.
We tested VIA C3 with a Shuttle
FlexATX motherboard reviewed at PC Hardware early this year
(on 1 January 2001!). This Shuttle
board is very good to be used with VIA C3 because it's extremely
small and provides a good stability in most applications. The board
is based on 810E, but it's still competitive to the market VIA C3
targets. Take a look to the board + CPU picture below.
Shuttle FE22 integrates video, audio and LAN and I don't think you will need to use the PCI slot. Recently I have seen in a shop a very interesting case (something about the height of a 1U rack server) which didn't seem to be able to accommodate a PCI card. In fact I think you needed low profile memory and heatsink to install a mobo into that case. Unfortunately it was manufactured by a company which was unknown to me and I can't remember its name right now. For such a low profile system Shuttle FE22 + VIA C3 are a perfect combination.
Test System / Stability
Surprisingly, after we installed the CPU into the socket the motherboard posted from the first attempt with a one year old BIOS. The CPU physical installation doesn't raise problems, because the fan is old style and can be attached to the motherboard very easily. Shuttle FE22 doesn't have onboard jumpers and it doesn't allow overclocking. Too bad, but not catastrophically because I don't thing you will buy VIA C3 to overclock. For such purpose a Duron is a much better choice.
Take a look above to a picture of the POST screen which was taken shortly after we powered up the system for the first time. The motherboard identified the CPU as Cyrix III and worked without a BIOS update. Take a look to the test system used for testing:
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VIA C3 passed the Linux compatibility tests. Although the motherboard was not benchmarked under Linux, we installed Linux Mandrake 8.1 and it worked perfectly. In several hours we encountered no stability problems or other issues which could raise problems. VIA C3 is not a CPU to be used on small servers and for desktop computers Linux may not be the easiest to use operating system. It is known that users with advanced needs use Linux, but these people also require a high power CPU.
We ran stability tests for over 100 hours, although it would have been quite hard to address problems. The cause of the fault is more likely the motherboard, not the CPU. Fortunately the system was extremely stable, in 106 hours we encountered only one lookup in the first hours due to a subtle memory incompatibility. First we used a Siemens 256 Mb memory module that appeared to have several compatibility problems. After we replaced it with a Crucial PC133 module we encountered no hang. After about eighty hours we removed the on top fan and left the CPU only passive cooled. No hang occurred after the fan removal. As far as I have understood from companies which sell processors and other hardware VIA C3 is very reliable, the RMA is extremely low.
Performance
We tested the system using Windows 2000 SP2 because it's more reliable. Users with limited needs should consider Windows ME because it uses less hardware resources. For comparison I have chosen a Celeron 700Mhz, running at a 66Mhz bus. Unfortunately that's the only CPU we had in lab which was close to the VIA C3 frequency. We had to use another heatsink with Intel Celeron because the VIA retail cooler was not enough.
In a Business environment, the VIA C3 866Mhz performance is very close to Celeron. Looking just at the Business score VIA C3 becomes a very good choice.
Content Creation 2001 contains applications which require much more FPU power. The difference between the two solutions becomes larger, but I think that the wider VIA C3 memory bus plays an important role in this performance.
Quake III Arena is an old game, but our system is limited by the onboard video performance. VIA C3 may not be the best choice for 3D games which use intensively the FPU.
We encoded a small movie for five times and averaged the results. Looking above you can see that VIA C3 loses a lot in this test. Fortunately the solution is powerful enough to software decode DVD movies in real time and there are no many situations when you got to encode DVD or MP3 files.
The CPU performance is not extraordinary, in fact it was not able to do better than a Celeron 700Mhz in none of the tests. We didn't test the CPU overclocked because the motherboard didn't allow that, but I think that it can easily reach 1Ghz. As I predicted the CPU power is limited by the weak FPU and this fact is obvious in targeted benchmarks. For everyday applications VIA C3 is good, for hardcore gamers no.
Conclusions
It's hard to
reach a conclusion regarding VIA C3. There are many factors involved
in a fair conclusion. The CPU has a simple design which facilities
its use in small devices, not necessary notebooks or mobile devices.
If you want to compare VIA C3 with a Pentium 4 is like comparing
Mercedes Smart with a Mercedes SLK. Both are cars, both can drive
you were you want, but in order to buy or use both cars require
a certain amount of sacrifice from you. With C3 I can see clearly
one point where you make a sacrifice: performance. Performance is
the essence of computing and it is obvious that a CPU which is not
able to deliver what's expected loses its adepts. In our case there
are other factors which should be considered because VIA C3 is definitely
not a CPU targeted to performance market. VIA tried to make C3 suitable
for end users, not only for manufacturers which came up with integrated
devices or other stuff for home computing like integrated Internet
boxes or smart multifunctional machines. The end user's attraction
consists in its socket 370 compatibility (a land left at VIA disposal
after Intel leaving the 370 market), price and decent performance
in the low market segment.
Probably that you want to find out if we do recommend C3. The answer
is quite simple: depends upon many factors. If you want to buy a
new desktop system we recommend VIA C3 only if you are looking for
a highly integrated, miniature sized computer with a FlexATX motherboard.
A Duron or a Celeron are a better choice on a regular computer.
If you have an old Intel i810 , i815 or VIA mobo and want to upgrade,
VIA C3 is a good choice, although there are areas where its performance
can not be compared with a Celeron running two hundred MHz lower.
If you buy a small device which uses VIA C3, it's OK, there is no
need why you should worry. As simple as it seems the CPU has enough
power to sustain a real time DVD decoding process. My conclusion is simple: What do you want to do with VIA C3? Do you have where to use it? If so, it's a choice.
