Communications of the ACM

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design, but ran them five times as fast. The advantage of the approach was so great, Patterson says, that processors designed that way could outperform those from talented designers using older methods. “The Berkeley RISC and Stanford MIPS processors designed by grad students were probably better microprocessors than what Intel could build,” he says. In fact, Patterson’s first prototype, which had 44,000 transistors, outperformed a 100,000-transistor device made the conventional way.

MIPS—Microprocessor without Interlocked Pipeline Stages—was Hennessy’s RISC architecture, which sped up processing by using instructions loaded from the memory into a register, which could be accessed faster. Hennessy founded a semiconductor design company, MIPS Computer Systems, in 1984 to commercialize the technology, spurred in part by doubts from the computer industry that the approach would work in the real world. Around the same time, Bill Joy of Sun Microsystems in Santa Clara, CA, became interested in RISC, and brought Patterson on as a consultant.

Both projects were successful, which
won over the skeptics. “When people
start making money, it’s hard to argue
However, two young professors had a
different suggestion. “John and I come
along and say absolutely the opposite.
Not only should we not make it more
complicated, we should make it even
simpler,” says David Patterson, who at
the time was an assistant professor of
computer science at the University of
California, Berkeley. “We weren’t just
criticizing the trend. We were making
an argument that people thought was
dangerous, and was just going to make
software fail more.”
That “dangerous” idea promoted
by Patterson and John Hennessy, then
an assistant professor of computer sci-
ence at Stanford University, was the Re-
duced Instruction Set Computer (RISC)
architecture, which relied on a simpler
collection of general functions the pro-
cessor would perform, shrinking the
number of transistors needed to carry
out a task. Today, 99% of the more than

16 billion microprocessors produced each year are based on RISC architecture, powering smartphones, tablets, and the Internet of Things, and earning Hennessy and Patterson this year’s 2017 ACM A.M. Turing Award.

It wasn’t that the number of instructions was necessarily smaller, Hennessy says; it was that they were less complex. “We said it’s not how big the program is, it’s how fast the program runs,” he says. Those early machines had perhaps 25% more instructions under RISC

Rewarded for RISC

ACM A.M. Turing Award recipients David Patterson and John Hennessy developed the “dangerous” idea that software should be simpler so it can be executed more quickly, which evolved into the Reduced Instruction Set Computer architecture.