In electronics and computer science, a reduced instruction set computer (RISC, pronounced "risk") is a computer architecture designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a complex instruction set computer (CISC), a RISC computer might require more machine code in order to accomplish a task because the individual instructions perform simpler operations. The goal is to offset the need to process more instructions by increasing the speed of each instruction, in particular by implementing an instruction pipeline, which may be simpler to achieve given simpler instructions. The key operational concept of the RISC computer is that each instruction performs only one function (e.g., copy a value from memory to a register). The RISC computer usually has many (16 or 32) high-speed, general-purpose registers with a load–store architecture in which the instructions that perform arithmetic and tests operate only on the registers, and the instructions that access data in the main memory of the computer only load data from memory into registers or store data from registers into memory. The design of the CPU allows RISC computers few and simple addressing modes and predictable instruction times that simplify design of the system as a whole. The conceptual developments of the RISC computer architecture began with the IBM 801 project in the late 1970s, but these were not immediately put into use. Designers in California picked up the 801 concepts in two seminal projects, Stanford MIPS and Berkeley RISC. These were commercialized in the 1980s as the MIPS and SPARC systems. IBM eventually produced RISC designs based on further work on the 801 concept, the IBM POWER architecture, PowerPC, and Power ISA. As the projects matured, many similar designs, produced in the mid-to-late 1980s and early 1990s, such as ARM, PA-RISC, and Alpha, created central processing units that increased the commercial utility of the Unix workstation and of embedded processors in laser printers, the routers, and similar products. RISC processors are used in supercomputers, such as the Fugaku. The varieties of RISC processor design include the ARC processor, the DEC Alpha, the AMD Am29000, the ARM architecture, the Atmel AVR, Blackfin, Intel i860, Intel i960, LoongArch, Motorola 88000, the MIPS architecture, PA-RISC, Power ISA, RISC-V, SuperH, and SPARC.
History and development A number of systems, going back to the 1960s, have been credited as the first RISC architecture, partly based on their use of the load–store approach. The term RISC was coined by David Patterson of the Berkeley RISC project, although somewhat similar concepts had appeared before. The CDC 6600 designed by Seymour Cray in 1964 used a load–store architecture with only two addressing modes (register+register, and register+immediate constant) and 74 operation codes, with the basic clock cycle being 10 times faster than the memory access time. Partly due to the optimized load–store architecture of the CDC 6600, Jack Dongarra says that it can be considered a forerunner of modern RISC systems, although a number of other technical barriers needed to be overcome for the development of a modern RISC system.
IBM 801 Michael J. Flynn views the first RISC system as the IBM 801 design, begun in 1975 by John Cocke and completed in 1980. The 801 developed out of an effort to build a 24-bit high-speed processor to use as the basis for a digital telephone switch. To reach their goal of switching 1 million calls per hour (300 per second) they calculated that the CPU required performance on the order of 12 million instructions per second (MIPS), compared to their fastest mainframe machine of the time, the 370/168, which performed at 3.5 MIPS. The design was based on a study of IBM's extensive collection of statistics gathered from its customers. This demonstrated that code in high-performance settings made extensive use of processor registers, and that they often ran out of them. This suggested that additional registers would improve performance. Additionally, they noticed that compilers generally ignored the vast majority of the available instructions, especially orthogonal addressing modes. Instead, they selected the fastest version of any given instruction and then constructed small routines using it. This suggested that the majority of instructions could be removed without affecting the resulting code. These two conclusions worked in concert; removing instructions would allow the instruction opcodes to be shorter, freeing up bits in the instruction word which could then be used to select among a larger set of registers. The telephone switch program was canceled in 1975, but by then the team had demonstrated that the same design would offer significant performance gains running just about any code. In simulations, they showed that a compiler tuned to use registers instead of operating directly on memory would run code about three times as fast as traditional designs. Somewhat surprisingly, the same code would run about 50% faster even on existing machines due to the improved register use. In practice, their experimental PL/8 compiler, a slightly cut-down version of PL/I, consistently produced code that ran much faster on their existing mainframes. A 32-bit version of the 801 was eventually produced in a single-chip form as the IBM ROMP in 1981, which stood for 'Research OPD [Office Products Division] Micro Processor'. This CPU was designed for "mini" tasks, and found use in peripheral interfaces and channel controllers on later IBM computers. It was also used as the CPU in the IBM RT PC in 1986, which turned out to be a commercial failure. Although the 801 did not see widespread use in its original form, it inspired many research projects, including ones at IBM that would eventually lead to the IBM POWER architecture.
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