The main function of a computer is to run programs that reside in memory, and typically are previously read from the hard disk. In general, you can imagine a program as a sequence of instructions. A statement is a bit string that specifies the processor to "best" and "where to do it." The processors are able to perform directly only the so-called machine language instructions. However, the development of programs in machine language would be too complicated and, in fact, this approach is used only in special cases where you need to create optimized code for demanding applications (drivers, industrial, etc.).. Programmers usually write programs using high level languages \u200b\u200blike C, C + +, Java, etc.. and make use of so-called compilers to translate it into machine language.
The set of instructions executable by a particular processor is called the Instruction Set . This represents the portion of the machine visible to the programmer and compilers. Generally, indicating that two processors have different instruction set, it is said that these differ in the ' Instruction Set Architecture (ISA). It must be said that two CPUs that differ in the ISA, surely also differ in the internal architecture. Commercial processors can be classified into two broad categories according to the ISA, reduced instruction set computer (RISC ) and Complex Instruction Set Computer ( CISC). RISC architectures can generally run a relatively small number of simple instructions quickly. In contrast, the CISC implement in hardware a greater number of more complex instructions: CISC think that education is equivalent to about four or five RISC instructions. The above change means that the machine language of RISC processors is more low-level machine language of CISC processors. This means that in the second case the compiler to translate the high-level instructions, is responsible for more work. Only after the introduction of RISC architecture, which occurred around 1985, was finally recognized that this allows for performance.
Often, when designing new processors, it had to take into account commercial needs that have not allowed the introduction of specific innovations in the ISA. In fact, the designers had to take into account the need to maintain backward compatibility, ie to ensure that even after the introduction of a new processor it can use the old programs. For this reason, the type of x86 CISC ISA, first used by the cpu, is still implemented in the latest generation of processors. However, it must be said that the Pentium Pro onwards, the processors its plant products, although they kind of x86 ISA interface, internally they are in fact based on RISC architecture and include special circuits that allow the translation into hardware x86 to RISC.
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