CISC and RISC adopt different philosophies in the management of instructions that are able to perform. The microprocessors CISC (Complex Instruction Set Computing) favor the expressiveness and the power of the instructions and the amplitude of choice offered, even if this involves a greater circuit complexity, with a consequent greater difficulties to rise in frequency and then, in the final analysis , the risk of a limit to the possibility of an increase in performance. Currently are paradigmatic examples of CISC CPU x86 and compatible, i.e. family members Celeron, Centrino, Pentium, Xeon, Core and Core2 Intel, and their competitors Sempron, Turion, Athlon, and AMD Opteron.
The opposite philosophy of RISC (Reduced Instruction Set Computing) in which the instruction set has been streamlined and reduced to the minimum required to perform any task, however, and we focused instead on a much optimized workflow, facilitated from simpler circuitry. The space saved in the CPU allows to compensate for the relative poverty expressive of instructions integrating a large amount of cache, or decidedly superscalar architecture building, or even increasing the number of registers, while the simplicity of instructions can execute in a short time and then to increase the frequency of the CPU. Classic RISC processors are mainly used in smart phones and tablets. The ARM architecture is a typical representative. Intel and AMD processors are seen as typical CISC processors that operate internally as well as with RISC-like structures.
RISC and CISC stand for the terms reduced instruction set computer or complex instruction set computer. Thus two basic philosophies are meant for computers.
CISC or Complex Instruction Set Computing
CISC stands for Complex Instruction Set Computing. The CISC processor is characterized by a wide range of commands and complex addressing. Although an extensive command set is available, in practice only a few commands and addressing modes can be used. Because only a few parts of the processor to be used which is extremely inefficient.
The terms CISC and RISC are about how the CPU of a computer works. In CISC CPU model, usually there are few registers (storage locations in the CPU), some of which also usually have special tasks for this purpose, there are many commands, including very powerful, for example, the process in a loop several registers. The commands usually have unequal lengths command. The most commonly used one byte less common two or three bytes. This code can be kept compact. Typically these commands are implemented by microcode. This is a partition of the instructions into smaller units, a kind of interpreter on the CPU. In modern CPUs this microcode is even changeable, so the manufacturer can import bug fixes.
When a CPU is being developed, it is often found that it moves more and more towards the CISC. The reason is quite simple. One has with progressing time, the technology to integrate more and more transistors on a chip. So the first 8086 CPU had 29,000 transistors and a Pentium 4 has 42 ...