History of General Purpose CPUs - Early 1980s: The Lessons of RISC

Early 1980s: The Lessons of RISC

In the early 1980s, researchers at UC Berkeley and IBM both discovered that most computer language compilers and interpreters used only a small subset of the instructions of a CISC. Much of the power of the CPU was simply being ignored in real-world use. They realized that by making the computer simpler and less orthogonal, they could make it faster and less expensive at the same time.

At the same time, CPU calculation became faster in relation to the time for necessary memory accesses. Designers also experimented with using large sets of internal registers. The idea was to cache intermediate results in the registers under the control of the compiler. This also reduced the number of addressing modes and orthogonality.

The computer designs based on this theory were called Reduced Instruction Set Computers, or RISC. RISCs generally had larger numbers of registers, accessed by simpler instructions, with a few instructions specifically to load and store data to memory. The result was a very simple core CPU running at very high speed, supporting the exact sorts of operations the compilers were using anyway.

A common variation on the RISC design employs the Harvard architecture, as opposed to the Von Neumann or Stored Program architecture common to most other designs. In a Harvard Architecture machine, the program and data occupy separate memory devices and can be accessed simultaneously. In Von Neumann machines the data and programs are mixed in a single memory device, requiring sequential accessing which produces the so-called "Von Neumann bottleneck."

One downside to the RISC design has been that the programs that run on them tend to be larger. This is because compilers have to generate longer sequences of the simpler instructions to accomplish the same results. Since these instructions need to be loaded from memory anyway, the larger code size offsets some of the RISC design's fast memory handling.

Recently, engineers have found ways to compress the reduced instruction sets so they fit in even smaller memory systems than CISCs. Examples of such compression schemes include the ARM's "Thumb" instruction set. In applications that do not need to run older binary software, compressed RISCs are coming to dominate sales.

Another approach to RISCs was the MISC, "niladic" or "zero-operand" instruction set. This approach realized that the majority of space in an instruction was to identify the operands of the instruction. These machines placed the operands on a push-down (last-in, first out) stack. The instruction set was supplemented with a few instructions to fetch and store memory. Most used simple caching to provide extremely fast RISC machines, with very compact code. Another benefit was that the interrupt latencies were extremely small, smaller than most CISC machines (a rare trait in RISC machines). The Burroughs large systems architecture uses this approach. The B5000 was designed in 1961, long before the term "RISC" was invented. The architecture puts six 8-bit instructions in a 48-bit word, and was a precursor to VLIW design (see below: 1990 to Today).

The Burroughs architecture was one of the inspirations for Charles H. Moore's Forth programming language, which in turn inspired his later MISC chip designs. For example, his f20 cores had 31 5-bit instructions, which were fit four to a 20-bit word.

RISC chips now dominate the market for 32-bit embedded systems. Smaller RISC chips are even becoming common in the cost-sensitive 8-bit embedded-system market. The main market for RISC CPUs has been systems that require low power or small size.

Even some CISC processors (based on architectures that were created before RISC became dominant), such as newer x86 processors, translate instructions internally into a RISC-like instruction set.

These numbers may surprise many, because the "market" is perceived to be desktop computers. x86 designs dominate desktop and notebook computer sales, but desktop and notebook computers are only a tiny fraction of the computers now sold. Most people in industrialised countries own more computers in embedded systems in their car and house than on their desks.