RISC (reduced instruction set computer)

RISC ( reduced instruction set computer ) is an implementation of fewer types of computer instructions to the microprocessor, originated in the 80's MIPS host ( namely RISC ), RISC machines adopting microprocessor referred to as RISC processor. In this way, it can be faster to perform the operation ( millions of instruction per second more, namely MIPS ). Because the computer executes each instruction type requires additional transistor and circuit elements, the set of computer instructions more will make the microprocessor is more complex, perform the operation will be more slow.

New York York town of IBM research center of the John Cocke proved, computer in about 20% of the command assumed the job in 1974 80%, he put forward the concept of RISC. First thanks to the discovery of the computer is IBM in 1980 PC / XT. Later, IRISC

BM RISC System / 6000 also use this ideology. The word RISC itself belongs to the University of California at Berkeley in a teacher David Patterson. This concept was also used in the RISC Sun SPARC microprocessor, and contributed to what is now called the MIPS technology to build, it is part of the Silicon Graphics. Many current microchip are now using the RISC concept.

The concept of RISC has led the microprocessor design with a deeper thinking. Design must be taken into account: instruction should how better mapped to the clock speed of the microprocessor on the ( ideally, one instruction in one clock cycle after executing ); system structure requires a" simple"; as well as in without resorting to the software help, microchip itself can do much work and so on.

Concept analysis

The concept of RISC has led the microprocessor design with a deeper thinking. Design must be taken into account: instruction should how better mapped to the clock speed of the microprocessor on the ( ideally, one instruction in one clock cycle after executing ); system structure requires a" simple"; as well as in without resorting to the software help, microchip itself can do much work and so on.

In addition to the performance improvement, RISC some of the advantages and the related design improvement and:

If a new microprocessor, one of its goals is not so complicated, so its development and testing will be faster.

@ use microprocessor instruction operating system and applications programmers will find, the use of smaller instruction set makes code easier.

@ RISC simple the choice of how to use the space on the microprocessor has more freedom.

Compared to the past, high-level language compiler can produce more efficient code, because the compiler using RISC machine on a smaller instruction set.

In addition to RISC, any full instruction set computer use is complex instruction set computing ( CISC ).

RISC typical examples such as: MIPS R3000, HP - PA8000 series, Motorola M88000 belong to the RISC microprocessor.

Characteristic

Improved features

And CISC than in addition to performance improvements, RISC some of the advantages and the related design improvement and:

@ use microprocessor instruction operating system and applications programmers will find, the use of smaller instruction set makes code easier.

@ RISC simple the choice of how to use the space on the microprocessor has more freedom.

@ compared to the past, high-level language compiler can produce more efficient code, because the compiler using RISC machine on a smaller instruction set.

In addition to RISC, any full instruction set computer use is complex instruction set computing ( CISC ).

RISC typical examples such as: MIPS R3000, HP - PA8000 series, Motorola M88000 belong to the RISC microprocessor.

Main characteristics

RISC microprocessor not only reduced instruction system, using a superscalar and superpipeline structure; they number only a few instructions, but greatly enhances the parallel processing capability. Such as: in 1987 Sun Microsystem launched the SPARC chip is a superscalar processor of RISC. SGI launched the MIPS processor using pipelined structure, these RISC processor in building parallel reduced instruction set multiprocessor plays a core role.

RISC processor is the UNIX domain 64 mainstream chip multiprocessor

Performance characteristics

Performance characteristics of a simplified: since the instruction set, assembly line and commonly used instructions are available hardware implementation;

Performance characteristics of two: use a large number of registers, so that most instructions are in the register for, to improve the processing speed;

Performance characteristics of three: the cache main memory disk - - three level storage structure, the fetch and store instructions performed separately, so that the processor can complete the work as much as possible, and not from memory access information and slow processing speed.

Application characteristics; because the RISC processor instructions, using simple hard wired control logic, strong processing capacity, fast speed, the vast majority of the world's UNIX workstation and server manufacturers are using RISC chip as CPU. If the original DEC Alpha21364 IBM Power PC, G4, HP PA - 8900, SGI R12000A and SUN Microsystem 's Ultra SPARC .

Operation characteristics

The working frequency of RISC chip in 400MHZ magnitude. The clock frequency is low, less power consumption, little Wen Sheng, machine failure is not easy and aging, improve the reliability of the system. A single instruction cycle to accommodate multiple parallel operation. RISC microprocessors in the development process. Had a very long instruction word ( VLIW ) microprocessor, it uses the very long instruction combination, put together many instructions can be executed in parallel, to. VLIW processor of the basic model is the scalar code execution model, so that each machine cycle with multiple operation. Some RISC processor also uses a VLIW instruction to improve the processing speed. Pentium 4 microprocessor architecture, fully uses the RISC architecture.

Difference

RISC and CISC is the design and manufacture of the microprocessor two kinds of typical technology, although they were tried in the system structure, operation, hardware and software, compile time and run time many factors make some kind of balance, in order to achieve the purpose of efficient, but the methods are different, therefore, in many respects, differences, they are mainly there are:

( 1 ) the instruction system: RISC designers to focus on those frequently used instruction, make them has simple and efficient characteristics. The commonly used functions, Chang Tong assembly instructions. Therefore, in the RISC machines to achieve specific function, efficiency may be low. But can use pipelining and superscalar technology to improve and make up. While the CISC computer command system is relatively rich, have special instructions to accomplish a specific function. Therefore, a specific task with high efficiency.

( 2 ) a memory operation: RISC the memory operation is limited, so that the control is simple; and the CISC machine memory operation instruction, direct operation.

( 3 ) program: RISC assembly language program generally requires a larger memory space, realize the special function of complex procedure, is not easy to design; while the CISC assembly language programming is relatively simple, scientific computing and complex operation of the program design is relatively easy, high efficiency.

( 6 ) the design cycle: RISC microprocessor has the advantages of simple structure, compact layout, short design cycle, and easy to use the latest technology; CISC microprocessor is a complex, long design period.

( 7 ) the user: RISC microprocessor has the advantages of simple structure, easy to hold regular instruction, performance, easy to use; CISC microprocessor complex structure, powerful function, easy to realize the special function.

( 8 ) scope of application: the RISC command system is identified with a particular application area about, the RISC machine is more suitable for the special machine; and the CISC machine is more suitable for general purpose machine.