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CISC(Complex Instruction Set Computer) 


A computer processor contains instructions or microinstructions that implement various functions. The richer the instruction set, the easier it is to write programs for the microprocessor, but the rich microinstruction set will affect its performance. The design strategy of the complex instruction set computer (CISC) architecture is to use a large number of instructions, including complex instructions. Compared with other designs, programming in CISC is easier than in other designs, because each simple or complex task has a corresponding instruction. The programmer does not need to write a lot of instructions to complete a complex task. But the complexity of the instruction set makes the circuit of the CPU and the control unit very complicated.

CISC includes a rich set of microinstructions that simplifies the creation of programs that run on the processor. The instructions are composed of assembly language, and some common functions that were originally realized by software are replaced by hardware instruction systems. The programmer's work is therefore much reduced, and some low-level operations or operations are processed simultaneously in each instruction period to improve The execution speed of a computer is called a complex instruction system.

Among the various instructions of the CISC instruction set, the frequency of use differs greatly. About 20% of the instructions will be used repeatedly, accounting for 80% of the entire program code. The remaining 80% of the instructions are not used often, and only account for 20% of the programming.

Compared with RISC

The RISC (Reduced Instruction Set Computer) design scheme, as its name implies, has a simplified instruction set that increases the efficiency of the processor but requires more complex external programs. The RISC structure preferentially selects the simple instructions with the highest frequency of use to avoid complex instructions; fixes the instruction length, reduces the instruction format and the type of ground-finding methods; mainly uses control logic, and does not need or use microcode control to improve the operation speed.

The RISC design is based on the work done by John Cocke in IBM. John Cocke found that about 20% of computer instructions completed about 80% of the work. Therefore, RISC-based systems are usually faster than CISC systems. Its 80/20 rule promotes the development of RISC architecture.

Of course, compared with the CISC architecture, although the RISC architecture has the above advantages, it cannot be considered that the RISC architecture can replace the CISC architecture. In fact, RISC and CISC have their own advantages, and the boundaries are not so obvious. Modern CPUs often use the periphery of CISC, which incorporates the features of RISC. For example, the ultra-long instruction set CPU combines the advantages of RISC and CISC and becomes one of the future CPU development directions.


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