Digital 21264 Sets New Standard: 10/28/96

Brushing aside pretenders to the performance throne, Digital’s Jim Keller demonstrated at last week’s Microprocessor Forum why Alpha continues to be king. The company is pushing microarchitecture to new levels, introducing innovations in branch prediction, datapath design, and system interfaces as part of the forthcoming 21264. These features will not only propel the 21264 to unprecedented levels of performance, they will also set standards for future designs. Previous Alpha processors, while delivering leadership performance, have generally pushed hardest on the “speed” button while relying on simpler CPU cores than their competitors. This tradition goes back to the design of the original instruction set, which ruthlessly eliminated any instruction that might reduce clock speed. The 21264 does not disappoint on clock speed: at 500 MHz, it is faster than any other announced CMOS microprocessor and twice as fast as most. Yet the new chip adopts an aggressively out-of-order CPU core that exceeds the capabilities of current chips. The 21264 adds a high-bandwidth system interface that channels up to 5.3 Gbytes/s of cache data and 2.6 Gbytes/s of main-memory data into the processor, feeding its demanding CPU core. Digital’s Dick Sites laid the groundwork for this interface through his research into performance bottlenecks (see 101004.PDF), summarized in his rallying cry, “It’s the bandwidth, stupid!” While other processors do well on programs, including many of the SPEC95 benchmarks, that fit into large L2 caches, the 21264 will excel even on code that frequently accesses main memory. Which is not to say the 21264 does poorly on SPEC95: Keller projects scores in excess of 30 on SPECint95 and 50 on SPECfp95 (base). These scores more than double those of any existing microprocessor. One caveat: the chip has not yet taped out, and Digital projects system shipments no sooner than 4Q97. Even with more than a year to catch up, however, Digital’s competitors have nothing on the books that will come close to the 21264’s performance in that timeframe.