RISC (reduced instruction set computer) and CISC (Complex Instruction Set Computer) architecture are the two current CPU. The difference between them lies in the different design concepts and methods of CPU.
Early CPU are all CISC architecture, it is designed to use the least machine language instructions to complete the required computing tasks. For example, for multiplication, on CISC architecture CPU, you may need one instruction: MUL ADDRA, ADDRB can be ADDRB number ADDRA and multiplying the result stored in ADDRA in. The ADDRA, ADDRB data is read into the register, and multiplying the result back to memory write operation is dependent on all the logical CPU designed to achieve. This architecture increases the complexity of the process and requirements of the CPU CPU architecture, but for compiler development is very beneficial. Such as the above example, C programs a * = b can be directly translated to a multiply instruction. Today, only Intel and compatible CPU is still in use CISC architecture.
RISC architecture requires software to specify the various steps. To achieve the above example on the RISC architecture, the ADDRA, ADDRB data is read into the register, multiplication and write the result back to memory operations must be implemented by software, such as: MOV A, ADDRA; MOV B, ADDRB ; MUL A, B; STR ADDRA, A. This architecture may reduce the complexity of a CPU, and permits the production of a more powerful CPU processes at the same level, but the compiler design have higher requirements.
Complex instruction set computer (CISC)
Long-term to improve computer performance is often achieved by increasing the complexity of the hardware. As integrated circuit technology. In particular, the rapid development of VLSI (Very Large Scale Integration) technology, in order to facilitate software programming and improve the operating speed of the program, hardware engineers approach adopted is the increasing complex functions implementable instructions and addressing a variety of flexible ways. Even some instructions to support complex operations after the high-level language statements classified. To make the hardware more complex, the cost is increased accordingly. To achieve complex operations, the microprocessor and the various registers in addition to providing similar functions to the external programmer machine instructions. Is also stored in read only memory by (ROM) micro program to achieve its highly functional, proud of the primary series of processing execution command calculation after analyzing each instruction to complete the required function, this type of design It is referred to as a complex instruction set computer (complex instruction set computer-CISC) architecture. Usually the number of instructions contained in the CISC computer at least 300
or more, and some even more than 500.
Reduced Instruction Set Computer (RISC)
The use of complex instruction set computer has a strong ability to deal with high-level language. This increase computer performance is beneficial. When the computer design development along this road. Some people do not follow the crowd. They go back and look at the road traveled in the past, began to suspect that this traditional approach: IBM company did in New York Yorktown's JhomasI.Wason research center in the rationality of organizational strength studies instruction in 1975. Because I was already feeling the increasingly complex instruction set is not only easy to achieve. But also may reduce system performance. Also he began to carry out this research at the University of Berkeley plus books in 1979 by Professor Patterson led a group of scientists. The results showed many shortcomings CISC. First of all. In this computer. Differences between the various usage instructions: a typical procedure of the calculation process used by 80% of the instruction. Only one processor instruction is 20%. In fact the most frequently used instructions is taken, stored and processing these simple instructions. In this way - to the long-term commitment to design complex instruction, in fact, it is the lingua franca in the processor instruction set in practice a rare design. Simultaneously. Complex instruction set will inevitably bring about structural complexity. This not only increases the time and cost of the design also likely to cause design errors. In addition. Although the VLSI technology has now reached a very high level, but it is difficult to put all the hardware to do on a CISC chip, which hinder the development of single-chip computer. In CISC, many complex instructions require a very complex operation, most of these instructions is a direct replica of some high-level language, so common is poor. As a result of two microcode implementation, it also reduces the operating speed of the system that simple instructions frequently called. thus. For these ills of CISC. Patterson, who proposed the idea of a RISC instruction set that is should contain only a small amount of high frequency of use of those instructions. And to provide the necessary instructions to support the operating system and high-level language. Computer evolved in accordance with this principle is known as reduced instruction set computer(Reduced Instruction Set Computer-RISC) architecture. Referred to as RISC.
CISC with the difference of RISC
We often talk about the "PC" with the topic "Macintosh", but how many people know to Intel's X86 core of the PC Series is based on CISC architecture, while Apple's Macintosh is based on RISC architecture, CISC and What is the difference between RISC in the end?
From a hardware perspective CISC dealing with unequal length instruction set, it must be unequal division of command, so when performing a single instruction requires more processing. And RISC is performed as long as a reduced instruction set, the CPU executes instructions faster and the time stability. Therefore, in the parallel processing RISC significantly better than CISC, RISC can execute multiple instructions simultaneously, which may be partitioned into a plurality of instruction threads or processes, referred to the plurality of processors concurrently. Since RISC reduced instruction set is executed, so it is simple and inexpensive manufacturing process.
From a software perspective, CISC running is that we are familiar DOS, Windows operating system. And it has a large number of applications. Because the world has more than 65% of management software vendors are based on CISC architecture PC and compatibles services, like the well-known Microsoft it is one of them. The RISC in this regard was a bit weak. Although also be run on RISCDOS, Windows, but requires a translation process, so speed is much slower.
Currently CISC and RISC is gradually moving toward integration, Pentium Pro, Nx586, K5 is one of the most obvious example, their cores are based on RISC architecture. They accepted after CISC instruction decomposed classified into RISC instructions in order to be able to execute multiple instructions in case of a time. Thus, the next generation of CPU fusion CISC and RISC two technologies, from the perspective of both software and hardware will learn from each other.