Superscalar instruction execution in the 21164 Alpha microprocessor

n September 1994, Digital Equipment Corporation introduced the 21164 Alpha microprocessor. This processor exceedU ed the performance level o f existing Alpha microprocessors by over 50 percent and delivered exceptional Performance on computing-intensive applications such as large database manipulation, scientific and technical simulation, CAD, and powerful user interfaces. The first microprocessor to achieve over 300/500 SPECint92/SPECfp92 in a uniprocessor system, the 21164 has been shipping to customers since January 1995. This superscalar (quad-issue) processor is the second completely new implementation of Alpha.'~j (Digital Semiconductor introduced the 200-MHz 21064 in 1992 and the 275-MHz 21064A in 1994.) Designed in 0.5-Fm CMOS technology, the 21164 currently achieves an estimated 345/505 SPECint92/SPECfp92 performance when operating at 300 MHz. Figure 1 (next page) shows a photomicrograph of the 16.5x18.1-mm' chip, which contains 9.3 million transistors. Key to the Alpha 21 164 architectural performance are its four-way superscalar instruction issue; low latencies in functional units; highthroughput, nonblocking memory subsystem with low-latency primary caches; and large second-level, on-chip write-back cache.' ' The chip operates 10 percent faster than the previous 21064 implementationi would if scaled