Skip to Main Content
Very long instruction word (VLIW) refers to a computer architecture and algorithms that take advantage of large amounts of instruction level parallelism (ILP). Joseph A. (Josh) Fisher, a former Yale professor and a Hewlett-Packard Senior Fellow, introduced VLIW architecture in the early 1980s. The insights underlying Fisher's invention of VLIW came to him when he was a graduate student at New York University's Courant Institute in the late 1970s. He was microcoding a clone of the Control Data Corporation (CDC) 6600 computer. To maximize the performance of the clone, called PUMA, he used a standard trick of writing microcode with many concurrent operations. In doing so, he realized he could get even more concurrency and performance by moving operations speculatively above branches. His study of this motion, and how it violated the laws of computer architecture of the day, led to his invention of the trace scheduling compiler algorithm. He wrote his Ph.D. thesis about trace scheduling. He later became a professor at Yale University, where he started the Extremely Long Instruction (ELI) project, which developed the architecture for the trace scheduling compiler.