In early 1980s, some CPU designers realised that many instructions supported by a CISC-based CPU are rarely used. Hence, they came up with the idea of greatly reducing design complexity of a CPU by implementing in hardware circuitry only bare minimum basic set of instructions and some of the frequently used instructions. The instruction set of the CPU need not support other complex instructions because a computer can implement them in software by using the basic set of instructions. While working on simpler CPU design, the designers also came up with the idea of making all instructions of uniform length so that decoding and execution of all instructions becomes simple and fast.
Furthermore, to speed up computation and to reduce the complexity of handling a number of addressing modes they decided to design all instructions in such a way that they retrieve operands stored in registers in CPU rather from memory. These design ideas resulted in processing faster and less expensive processors. CPUs with a small instruction set, fixed-length instructions, and reduced references to memory to retrieve operands are called CPUs based on RISC (Reduced Instruction Set Computer) architecture. Since RISC processors have a small instruction set, they place extra demand on programmers who must consider how to implement complex computations by combining simple instructions. However, RISC processors are faster for most applications, less complex, and less complex expensive to produce than CISC processors because of simpler design.
Popular RISC processors used in workstations are POWER (used in IBM workstations), SPARC (used in SUN workstations), and PA-RISC (used in HP workstations).
Supporters of RISC technology claim that increased processing speed and lower cost of RISC processors easily offset limitations of a reduced instruction set. However, critics of RISC technology are of the opinion that RISC processors has to process more of these simple programmed instructions to complete a task, placing additional burden on system software. There seems to be no clear answer as to which technology is better, The answer may be that each technology lends itself best to certain applications, and so both technology will coexist.
A Reduced Instruction Set Computer is a type of microprocessor architecture that utilizes a small, highly-optimized set of instructions rather than the highly-specialized set of instructions typically found in other architectures. RISC is an alternative to the Complex Instruction Set Computing (CISC) architecture and is often considered the most efficient CPU architecture technology available today. With RISC, a central processing unit (CPU) implements the processor design principle of simplified instructions that can do less but can execute more rapidly. The result is improved performance. A key RISC feature is that it allows developers to increase the register set and increase internal parallelism by increasing the number of parallel threads executed by the CPU and increasing the speed of the CPU's executing instructions. ARM, or “Advanced RISC Machine” is a specific family of instruction set architecture that’s based on reduced instruction set architecture developed by Arm Ltd. Processors based on this architecture are common in smartphones, tablets, laptops, gaming consoles and desktops, as well as a growing number of other intelligent devices.