They proposed a fundamentally different approach to processor design. Patterson and Ditzel argued that CISC CPUs were still attempting to solve code bloat problems that had never quite materialized. Meanwhile, the cost of memory was steadily decreasing, making an emphasis on code size less important. Implementing complicated instruction set architectures, with support for a large number of rarely used instructions, consumed die space and lowered maximum achievable clock speeds. They often had to be implemented across multiple chips, because the VLSI (Very Large Scale Integration) techniques of the time period couldn’t pack all the necessary components into a single package. CISC instruction set architectures typically offered a wide range of specialized instructions.īy the late 1970s, CISC CPUs had a number of drawbacks.

As a design philosophy, CISC attempted to improve performance by minimizing the number of instructions a CPU had to execute in order to perform a given task. They emphasized code density and some instructions performed multiple operations in sequence on a variable. So-called CISC designs, including the original 8086, were designed to deal with the high cost of memory by moving complexity into hardware. This perceived need for a new approach to CPU design came about as the bottlenecks limiting CPU performance changed. They offered the term “CISC” - Complex Instruction Set Computer - to describe many of the various CPU architectures already in existence that did not follow the tenets of RISC. RISC is a term coined by David Patterson and David Ditzel in their 1981 seminal paper “ The Case for a Reduced Instruction Set Computer.” The two men proposed a new approach to semiconductor design based on observed trends in the late 1970s and the scaling problems encountered by then-current CPUs. Image by David Bauer, CC BY-SA 2.0 A Simplified History of the Parts People Agree On

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