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[ Go to September 1997 Table of Contents ]
WinLab Reviews
-- by Jonathan Blackwood
It's happened again. Just when you got a grip on the bewildering array of microprocessor choices, out come two new chips from the Lone Star State: Cyrix Corp.'s 6x86MX processor (formerly called M2) and new player Centaur Technology's IDT-C6 (see sidebar "C6 Promises New Value"). We recently benchmarked an early engineering sample of the Cyrix processor at the company's headquarters in Richardson, Texas. It's still too early to benchmark Centaur's C6, designed at its facility in Austin, Texas. Both the 6x86MX and the IDT-C6 are MMX-enabled, Socket 7 processors that will fit handily into existing motherboards with current chipsets. It's been an eventful year for chip introductions, and things promise to get even more interesting. Intel's first MMX-enabled processor, the P55C Pentium with MMX, just debuted last January. AMD followed up in April with its launch of the K6, a sixth-generation design that, like the P55C, fits into Socket 7 motherboards. The K6 was the first x86 processor on the market to hit 233MHz-don't be surprised if AMD launches 266MHz and 300MHz versions later this year. In June, Intel rolled out its hot new Pentium II, which uses Slot One technology instead of Socket 7. They started at 233MHz and 266MHz, and will soon be available in a 300MHz version. July brought the news of Intel's 233MHz P55C Pentium. Now it's Cyrix's turn to roil the waters with the 6x86MX. Cyrix 6x86MX The 6x86MX runs fast as blazes when running typical business applications (see chart), and not so fast when running our Wintune benchmarks. But ask yourself which you run more often: business applications or synthetic benchmarks? As with previous Cyrix processors (and AMD's K5), the 6x86MX uses a PR (Power Rating) to indicate the performance of a given processor relative to an Intel Pentium processor of a certain clock speed. For example, though the 6x86MX chip we examined had an actual clock speed of 187.5MHz (about 80 percent of the clock speed of a 233MHz P55C), it was rated by Cyrix as a "PR233" processor. Sure enough, our macro tests showed the 6x86MX offers application performance equivalent to that of Intel processors running at 233MHz. But our other Wintune benchmarks rated the 6x86MX at about 80 percent of the power of the 233MHz P55C. Cyrix reports that its advanced, sixth-generation design is inherently more efficient at the sort of processing required by real applications. Cyrix's engineers attribute the difference to the 6x86MX chip's greater efficiency with data dependencies, which enables it to execute more instructions in parallel. This is precisely the sort of behavior that shows up in real-world applications, and precisely not the sort of instructions likely to be found in synthetic benchmarks. In addition, the 6x86MX accesses its level 1 cache as though it were a register, which likewise improves performance on business applications. And that level 1 cache on the 6x86MX is 64KB, twice that of the Pentium II or P55C. The accompanying chart compares different systems running various 233MHz processors to the Cyrix 6x86MX PR233. The Cyrix provided very respectable application performance. In addition to being the cheapest processor of the lot, it will run in most new Socket 7 (Pentium) motherboards. Keep in mind that comparing differently configured systems is a risky business. The 233MHz K6 system shown in the chart has a full 1MB of level 2 cache and a SCSI caching controller for its Ultra-Wide SCSI hard disk. The AST Pentium II system uses a totally different motherboard design and chipset. Different graphics adapters and drivers add to the complexity. If we were trying to compare desktop PCs, grouping systems with different components would be justifiable. When you're determining precise differences in performance among microprocessors though, you're on more questionable turf. We provide the information here because it represents the best data we currently have available. In the coming months we'll take a look at these Socket 7 chips in a single system with a motherboard that will support any of them. How'd they do that? How does Cyrix manage to get 233MHz-level performance out of only 187.5MHz? The short answer is that Cyrix's engineers quadrupled the internal cache to 64KB, tripled the translation lookaside buffer (TLB)-both improvements in comparison to the company's previous 6x86 processor-and increased the clock frequency scalability beyond 200MHz. There's also a scratchpad RAM feature that can hold instructions a single line at a time. Like other sixth-generation processors such as the Pentium Pro, Pentium II and K6, the 6x86MX has a "superpipelined" architecture, register renaming, out-of-order completion, branch prediction and speculative execution. These technologies allow code to be processed in parallel, with multiple pathways processing instructions simultaneously. The 6x86MX, like the P55C, uses dual voltages: 2.8 volts for its core, and 3.3V for its I/O bus. It operates on 60MHz, 66MHz and (uniquely) 75MHz buses. Its clock multipliers of 2.5, 3.0 and 3.5 will ultimately carry the chip to internal clock speeds of 233MHz with a PR rating of 266 on a 66MHz motherboard. The increasing competition in the microprocessor arena will likely produce better products and lower prices. That can only be good for the consumer. Cyrix's newest processor raises the ante, offering the application performance of the best Intel and AMD 233MHz processors, but at a cost that's substantially lower than Intel's and marginally lower than AMD's. If you want Pentium II-level performance in complete systems that will cost less than $1,500, check out the 6x86MX.
SIDEBAR: C6 Promises New Value Glenn Henry is either tilting at windmills, or he's ready to become the king of discount microprocessors. The founder of Centaur Technology is launching an effort to build a simpler, smaller, cheaper Intel Pentium-compatible processor. Dubbed the IDT-C6, it will sell in lots of 1,000 for much less than $200-perhaps as low as $100-it will fit into standard Socket 7 motherboards, and it will support the full Intel MMX instruction set. The processors will be manufactured at the Oregon and California wafer fabrication plants run by Centaur's parent company, Integrated Device Technology (IDT) Centaur's approach is to keep it simple. Its processor uses a relatively uncomplicated design that runs at high internal clock speeds, uses large, on-chip caches, and eschews such sixth-generation concepts as superscalar execution, out-of-order instruction execution, reorder buffers and nonblocking caches. There are no fractional clock multipliers that could result in a 233MHz clock speed, for example, rather than a 240MHz speed. To strip down to the basics in an x86 processor, the company is drawing largely on the experience of Henry-who is a former IBM fellow and former senior vice president and chief technology officer at Dell-in RISC processor design. The IDT-C6 is optimized for the most basic x86 instructions (load, store, branch, register-to-register) that comprise 90 percent of code anyway: There is a five-stage pipeline execution core. Instructions are issued one at a time in program order, and executed and retired in order. Cache misses stall the pipeline until data is available to complete the instruction. This is very basic design, but Centaur compensates with high internal clock frequency, large on-chip caches, TLBs-and lots of fine-tuning. The result is a smaller (88mm2, using a 0.35micron process), lower-power chip that produces higher yields on a typical silicon wafer, which means lower cost. Henry is quite candid about the fact that there are some operations for which the C6 will be slower-perhaps substantially slower-than its competitors from Intel, AMD and Cyrix. But the trade-offs, he believes, are worth it to achieve impressive performance for the vast majority of applications at very low cost. Henry envisions 200MHz P55C-level performance in systems costing less than $1,000. Intel has traditionally kept its margins high by manufacturing its ever-more-complex chips in very expensive FABs, whose cost alone provided a barrier to entry. AMD met the challenge head-on, with state-of-the-art plants in California, Texas and Germany. Cyrix chose to pursue the role of "FABless" chip maker, contracting with IBM and SGS Thomson to produce its designs. Now, IDT will be manufacturing a low-cost chip in wafer FAB plants converted from memory production. If the company succeeds, it will lower the barriers to entry in the microprocessor business, which will result in more competition, lower prices-and lower margins, not just for Intel, but for AMD and Cyrix as well. Yes, things are starting to get interesting ...
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