Internal Organization of the Alpha 21164, a 300-MHz 64-bit Quad-issue CMOS RISC Microprocessor
A new CMOS microprocessor, the Alpha 21164, reaches 1,200 mips/600 MFLOPS (peak performance). This new implementation of the Alpha architecture achieves SPECint92/SPECfp92 performance of 345/505 (estimated). At these performance levels, the Alpha 21164 has delivered the highest performance of any commercially available microprocessor in the world as of January 1995. It contains a quad-issue, superscalar instruction unit; two 64-bit integer execution pipelines; two 64-bit floating-point execution pipelines; and a high-performance memory subsystem with multiprocessor-coherent write-back caches. The Alpha 21164 microprocessor is now a product of Digital Semiconductor. The chip is the second completely new microprocessor to implement the Alpha instruction set architecture. It was designed in Digital's 0.5-micrometer (um) complementary metal-oxide semiconductor (CMOS) process. First silicon was powered on in February 1994; the part has been commercially available since January 1995. At SPECint92/SPECfp92 ratings of 345/505 (estimated), the Alpha 21164 achieved new heights of performance. The performance of this new implementation results from aggressive circuit design using the latest 0.5-um CMOS technology and significant architectural improvements over the first Alpha implementation. The chip is designed to operate at 300 MHz, an operating frequency 10 percent faster than the previous implementation (the DECchip 21064 chip) would have if it were scaled into the new 0.5-um CMOS technology. Relative to the previous implementation, the key improvements in machine organization are a doubling of the superscalar dimension to four-way superscalar instruction issue; reduction of many operational latencies, including the latency in the primary data cache; a memory subsystem that does not block other operations after a cache miss; and a large, on-chip, second-level, write-back cache.