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Eyeing the next wave in RISC computing

Some critics say RISC's time has passed. Sun Microsystems' David Yen has another idea.

Despite what has been written (and rewritten) about the potential obsolescence of platforms based on RISC processors, they keep on going and going.

Indeed, RISC processors have never been more critical to driving innovation in computer design than they are today.

Design innovation isn't driven by the economics of large volume, the agendas of microprocessor companies or the plans of companies building commodity computers. Innovative designs come from companies with expertise in building entire systems from scratch--from chip to crossplane to code. These companies work closely with end users to anticipate new demands, and they utilize RISC-based technology to create the leading-edge systems needed to meet those demands.

In the 1980s, RISC (reduced instruction set computing), changed the rules of computing. The premise of RISC was that earlier CISC (complex instruction set computing) processors used only about 20 percent of the instructions they implemented. By comparison, RISC processors required far fewer transistors, making them cheaper to manufacture.

It is time for RISC designs to once again show the way to a new era in computing.

Further, because of their streamlined design, RISC processors could execute far more instructions every second. The result was an unmatchable price-performance advantage for RISC over older CISC-style designs, with RISC designs coming to dominate workstation, server and high-end embedded markets.

In the early '90s, RISC processors pioneered another advance, becoming the first processors to leap to 64 bits. That provided a huge increase in address space required for applications and data sets that were fast approaching the limits of 32-bit processing power.

About a decade after the first commercial RISC processors appeared, the manufacturers of CISC processors began to adapt the principles of RISC design to their own products. Today, CISC processors such as the Pentium 4 and Opteron translate their complex instructions into sequences of simple RISC-style instructions, then rely on a powerful "under the hood" RISC-style engine to execute the resulting "micro-ops." Similarly, about a decade after RISC processors had shown the way to 64-bit processing power, the first 64-bit x86 architecture processors appeared.

Now the computer industry faces a new set of challenges. Clock frequency gains have declined precipitously. Memory access times have risen from a few clock cycles to hundreds of cycles. Power budgets and associated thermal problems are rapidly escalating out of control. In short, it's time for RISC designs to once again show the way to a new era in computing.

At Sun, we've termed this brave new future of RISC processor design radical chip multithreading (CMT). Radical CMT processors differ from today's dual-core and symmetric multithreaded designs in much the same way a horseless carriage differs from a Formula One racer. That is to say, radical CMT designs aren't awkward adaptations of pre-existing technology to a new and unforeseen use. They are designs crafted from the ground up to employ available resources as efficiently as possible in pursuit of their throughput goal. We're not talking about executing two to four threads in parallel, but 30 or more threads simultaneously.

We're not talking about executing two to four threads in parallel, but 30 or more threads simultaneously.

More specifically, radical CMT designs will be based on new core designs, tailored to provide exactly the right level of per-thread performance needed for a targeted class of applications. This design then will be replicated eight or more times on a single die, along with appropriate mechanisms to ensure the cores operate together effectively. Each core also will be designed to gracefully switch between threads, so that when one thread stalls on a cache miss or branch misprediction, another can keep the core's execution resources productively occupied.

Although radical CMT, like previous waves of innovation pioneered by RISC processors, eventually will spread to commodity processors, once again this process is likely to take a decade or so. By the time commodity processors catch up, RISC processors will be ready to move on again, pioneering the next critical wave of innovation in processor design.

Technology is a process, not a result. For RISC, this means an endless migration toward the next big industry challenges that wait just over the horizon.