Chun ChiuChun Chiu

Overview• What is RISC?• Characteristics of RISC• What is CISC?• Why using RISC?• RISC Vs. CISC• RISC Pipelines• Advantage of RISC / disadvantage of RISC• Advantage of CISC / disadvantage of CISC• Example of CISC/RISC• Combine RISC and CISC• History

What is RISC?

• RISC is the abbreviation of “Reduced Instruction Set Architecture”.

• Reduced Instruction Set Architecture is a type of microprocessor that recognizes a relatively limited number of instructions.

Characteristics of RISC• One instruction per cycle

– Instruction length, addressing mode, the formats are uniform: this can make full use of pipelining.

• Register-to-register operations– Only load and store instructions access memory.

• Simple addressing modes– Almost all instructions use simple register addressing. Other, more

complex modes can be synthesized in software from simple ones.• Simple instruction formats

– It uses same length instructions so that the instructions are aligned on word boundaries and may be fetched in a single operation.

RISC PipelinesA RISC processor pipeline operates in much the same way, although the stages in the pipeline are different. While different processors have different numbers of steps, they are basically variations of these five, used in the MIPS R3000 processor: - fetch instructions from memory - read registers and decode the instruction - execute the instruction or calculate an address - access an operand in data memory - write the result into a register

Basic five-stage pipeline in a RISC machine:IF = Instruction FetchID = Instruction DecodeEX = ExecuteMEM = Memory accessWB = Register write back

In the fourth clock cycle (the green column), the earliest instruction is in MEM stage, and the latest instruction has not yet entered the pipeline.

And what is CISC?

• CISC is the abbreviation of “Complex Instruction Set Computer (or computing)”

• It is a type of microprocessor design. The CISC architecture contains a large set of computer instructions that range from very simple to very complex and specialized. Which was Invented earlier than RISC.

Why using RISC?Traditional CISC structure has inherent shortcomings,

the development of computer technology is constantly introducing new and complex set of instructions.

To support these new instructions, the computer will be more complex architecture, however, in many instructions of CISC’s instruction set, almost 20% instructions will be use repeatedly, and it accounted for 80% of the entire code. The remaining 80% of the instruction is not frequently used in the programming, but it still take 20% of the rest code.

Obviously, this structure is not reasonable.

Based on the above irrationality, 1979, University of California at Berkeley in the United States put forward the concept of RISC, RISC is not simply to reduce the instruction, but to focus on how to make computers simple and reasonable way to improve operations Speed .

RISC structure of the highest priority to select the simple instructions frequently used to avoid complex instructions; to fixed instruction length, to reduce instruction format and the ways to search.

Based on control logic, do not use or use less amount of microcode to achieve above purpose.

RISC Vs. CISC

• The RISC machine executes instructions faster because it does not have to go through a microcode conversion layer.

• The RISC compiler generates more instructions than the CISC compiler for the same processing.

CISC RISC Emphasis on hardware Emphasis on software Includes multi-clock

complex instructions Single-clock,

reduced instruction only Memory-to-memory:

"LOAD" and "STORE"incorporated in instructions

Register to register:"LOAD" and "STORE"are independent instructions

Small code sizes,high cycles per second

Low cycles per second,large code sizes

Transistors used for storingcomplex instructions

Spends more transistorson memory registers

• The way RISC call subroutine is different with CISC:– CISC need to keep the context in stack and also

operate in memory, When it call or return a function.

– RISC do it in different way, RISC keep them in register, and it also pass parameters by register.

– RISC interrupt subroutine can be regarded as a special link.

– When CISC interrupt subroutine, will be push all the data in the register to stack.

Advantage of RISC1. RISC is that they can execute their instructions very

fast because the instructions are so simple. The instructions usually finish in 4 or 5 processor cycle.

2. Because instruction operand has to store in the register, thus the operation time is unified.

3. RISC are used pipelining, cache memory, less or do not use microcode.

4. More important advantage is that RISC chips require fewer transistors, which makes them cheaper to design and produce.

Disadvantage of RISC

• RISC architectures put a greater burden on the software.– Is this worth the trouble because conventional

microprocessors are becoming increasingly fast and cheap anyway?

Example of CISC/RISC

• The PowerPC microprocessor(old Mac) , used in IBM's RISC System/6000 workstation and Macintosh computers, is a RISC microprocessor.

• Intel's Pentium microprocessors are CISC microprocessors.

Combine RISC and CISC

• RISC takes each of the longer, more complex instructions from a CISC design and reduces it to multiple instructions that are shorter and faster to process.

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History• The original RISC prototype computer, called the 801 Minicomputer, was

built at IBM's Research Division in 1980 -- the result of a project that began in the 1970's.

• 1950s IBM instituted a research program• 1964 Release of System/360• Mid-1970s improved measurement tools demonstrated on CISC• 1975 801 project initiated at IBM’s Watson Research Center• 1979 32-bit RISC microprocessor (801) developed led by Joel Birnbaum• 1984 MIPS developed at Stanford, as well as projects done at Berkeley• 1988 RISC processors had taken over high-end of the workstation market• Early 1990s IBM’s POWER (Performance Optimization With Enhanced

RISC) architecture introduced with the RISC System/6k– AIM (Apple, IBM, Motorola) alliance formed, resulting in PowerPC

References

Internet sources:- http://

www.computerhistory.org/collections/accession/102657026- http://www.karbosguide.com/books/pcarchitecture/chapter12.htm- http://en.wikipedia.org/wiki/Instruction_pipeline- http://www.webopedia.com/TERM/R/RISC.html

Book sources:- Computer Organization And Architecture, 8th Edition.