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Lexra turns to multithreading MIPS processors

Posted: 28 Aug 2002 ?? ?Print Version ?Bookmark and Share

Keywords:ip core? processor? silicon? network processor? lexra?

Many companies whose sole aim was to provide IP cores have either bitten the dust or are about to do so. Others are reinventing themselves by doing things the old-fashioned way: designing and selling their own silicon.

One former IP company that has taken that route is processor company Lexra Inc. Late last year, Lexra settled a legal dispute with MIPS Technologies Inc. when it agreed to stop selling its MIPS-compatible processor cores, which MIPS claimed violated MIPS patents, in exchange for taking a MIPS license. MIPS also paid Lexra for its cores and architectures.

Since then, the 35-person company has been using the $6 million in settlement money it received from MIPS to stay afloat and develop what could be the first multithreaded processor based on the MIPS instruction set. It hopes to introduce the processor next year.

To be sure, there is nothing new about multithreading; it has served as an architectural underpinning for network processors in the market for several years now. But most of those processors are based on proprietary architectures, and each is therefore tied to a single vendor's development tools.

Standard makes it easier

Linley Gwennap, founder and principal analyst of The Linley Group, sees ample opportunity for a company to sell processors based on a well-known instruction set architecture that can bridge the control and data plane. "When you get up to tens of thousands of lines of code for firewalls and intrusion detection, it gets to be unwieldy in assembly code," Gwennap said. "Having a standard instruction set like MIPS makes it much easier."

In the parlance of processor design, Lexra's pending LX4580 is known as a fine-grained multithreaded processor because it works on a different thread - four of them, in this case - every cycle.

"If there is a cache miss, you pull the thread out of the round robin, so the CPU stays busy. As long as you have two threads active, you keep the CPU 100 percent active," said Prasad Sabada, vice president of marketing for Lexra.

Coarse-grain multithreaded CPUs, by contrast, work on a single thread until a context switch occurs, such as during a cache miss. Both are employed by networking processors to hide the memory latency delays by keeping the CPU active when it fetches memory from external DRAM.

Lexra's rules

One of the more important aspects of the four-context instruction issue is what Lexra calls a context even-issue rule, which essentially allows it to eliminate a pair of critical paths common in CPU architectures. That, in turn, permits higher clock speeds and makes the approach less routing-dependent. Under this scheme, Lexra has also been able to get rid of branch predictions, which normally cause two-cycle penalties.

What's more, Lexra's hardware multithreading approach does not rely on extensions to the MIPS32 instruction set architecture extensions, so it will not require new development tools or OS changes, according to Lexra. There will be some separate user-defined instructions for packet processing, Sabada said.

It is possible that other MIPS licensees are considering multithreaded architectures, but Gwennap said he is not aware of any other specific plans.

MIPS Technologies itself has not come up with any multithreading extensions that its licensees could put to use, and Sabada said MIPS has no claim to Lexra's hardware multithreading technology.

To be sure, Lexra still has to produce working processor silicon. The company expects to tape out its first LX4580 by November or December and added it will be ready to ship samples running at 500MHz starting in mid-2003. The chips will be manufactured using Taiwan Semiconductor Manufacturing Co.'s 0.135m process technology.

The chip was created using a conventional ASIC design methodology, and the design has been proved in an FPGA that is being used to power the company's Web server. "This is not a full-custom chip; it is synthesizable. So if it runs in an FPGA, it will run. We are not getting 500MHz by handcrafting every transistor," said Lexra's president and CEO Charlie Cheng.

The toughest part may be starting over again. Lexra was able to make some inroads largely because it was an attractive alternative to MIPS Technologies, but it has not proved itself as a chip supplier. "From a customer standpoint, they are starting from scratch, which ideally you do not want to be doing," Gwennap said.

- Anthony Cataldo

EE Times





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