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VIA's C3 'Ezra' processor

Look ma, no fan!

VIA'S RECENT SUCCESSES in the chipset market have put VIA products at the heart of loads of high-performance PCs. While VIA chipsets have earned accolades for their performance, VIA's approach to processors is quite different. Unlike VIA's KT333 and P4X266A chipsets, VIA's C3 processor plays in a whole other ballpark: value PCs, notebooks, rack and blade servers, and digital appliances. This different market focus gives VIA a very different set of priorities with its C3 processor. Low power consumption and minimal heat output are the orders of the day.

For the C3's target markets, this approach may make sense. The question is: how fast is fast enough? For mainstream applications, 2GHz is a waste; users may in fact be better off with a chip that consumes less power, and produces less heat, all while being "fast enough."

So has VIA succeeded in lowering the power requirements and heat dissipation to the point where the C3 becomes compelling? Is the C3's performance "good enough"—and for what? Are there worthwhile C3 applications, even for the enthusiast? Read on to find out.

VIA's microprocessor solution
The subject of our attention today, VIA's C3 800 "A" MHz processor is based on the relatively new Ezra core, designed by VIA's Centaur group in Austin, Texas. Ezra is the third in a series of C3 processors, succeeding the Samuel 2 and Samuel 1 cores. Ezra C3s, fabbed by TSMC on an advanced, 0.13-micron process, have been available since September. C3s currently top out at 933MHz.

So what does this Ezra core have to offer? Let's check out some specifics.

  • A small, simple die — The Ezra die measures a scant 52mm2, tiny by today's standards. Ezra is also simple by today's standards, part of a conscious design decision on VIA's part to keep the processor easy to manufacture. Design simplicity should make it easy for the C3 to reach high speeds, and do so with minimal power requirements.

    VIA estimates that over 90% of processor instruction execution time is due to only a small number of basic x86 instructions. To keep the Ezra core simple, these simple instructions are performed in hardware, while the rest are executed in microcode. This arrangement isn't supposed to have a major effect on real world performance, but we'll see what happens in the benchmarks.

  • Low power requirements — The 800MHz C3 runs on only 1.35 volts, less voltage than any other Socket 370 processor. This low voltage translates to, under load, about half the wattage of a Celeron 800MHz. However, the wattage numbers VIA quotes compare the C3 to Intel's older Celeron core. Newer, Tualatin-based Celerons enjoy a 0.13 micron die and voltages under 1.5V, which should lead to lower power consumption.

    An added benefit of Ezra's low voltage is its modest cooling requirements. The 800MHz C3 requires only a passive heat sink, which barely gets warm under load. Even in cramped cases with poor airflow like Shuttle's SV24, no fan is required.

  • A fairly deep pipeline — Ezra's ALU pipeline is 12 stages deep, and runs at the full processor speed. Because of the impact on die area and power requirements, the C3 doesn't do out-of-order instruction execution. Again, VIA claims this fact should have little impact on performance, but we'll let the benchmarks tell that story.

    The floating point unit on Ezra runs at only half the processor speed, or 400MHz in the case of our 800MHz C3. VIA even includes this warning in their C3 literature:

    The VIA C3 processor has not been designed or optimized for high-end graphics intensive applications that usually rely heavily on floating point calculations. VIA do not believe that performance in these applications in an important factor for the VIA C3 processor's target markets.
    The Ezra's "half-pumped" FPU doesn't bode well for performance to begin with, and this explicit warning leaves little room for optimism.

  • On-chip caches — Ezra's L1 cache is 4-way associative, split between two 64K on-chip caches. There's also 64K of L2 cache, which is exclusive, so it doesn't replicate the contents of the L1 data cache. All caches run at full processor speed, and VIA has even built in some aggressive data prefetch logic similar to what you'll find in Intel's Tualatin and AMD's Palomino processors.

  • A 133MHz front-side bus — Because the C3 is VIA's only chip, they don't have to worry about product differentiation. This means that, unlike the Celeron, the C3 runs on a 133MHz front side bus. A faster front side bus should give VIA an edge over a comparably clocked Celeron, at least in one department.

  • MMX and 3DNow! instructions — Just for kicks, VIA borrows from both Intel and AMD camps with support for MMX and 3DNow! instructions. Unfortunately, that's where it ends; neither SSE, nor 3DNow! beyond what you'll find in the K6-II are supported.

  • A variety of packages — Since the C3 is targeted at everything from set top boxes to desktops, it comes in an array of packages including CPGA, PPGA, EBGA, and uPGA. To date, I've personally seen C3s in notebooks, Socket 370 desktops, and even soldered right onto VIA's Eden platform.
VIA has made a number of sacrifices with the Ezra core in order to get a small, simple die with low power requirements. Although VIA admits that the C3's FPU isn't designed for high-end applications, they claim that moving less-used x86 instructions to microcode and the lack of out of order execution shouldn't hurt performance too much. The rationale here is that, for the C3's target markets, any performance hit will be offset by the value of the C3's low power consumption and cooling requirements. For ultra-dense rack servers, set top boxes, notebooks, and mainstream PCs in small form factors, the trade-off might be worth it.