risc architecture

RISC ARCHITECTURERISC ARCHITECTURE

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Guan Hang SuGuan Hang Su

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-> RISC design philosophy-> RISC design philosophy

-> Features of RISC-> Features of RISC

-> Case Study-> Case Study

-> The Success of RISC processors-> The Success of RISC processors

-> CRISC -> CRISC

What is RISC ?What is RISC ?

RISC = RISC = Ridiculously Simple ComputerRidiculously Simple Computer

OrOr

RISC = RISC = Reduced Instruction Set ComputersReduced Instruction Set Computers

RISC design philosophyRISC design philosophy

RISC is a designRISC is a design Philosophy Philosophy where you reduce the where you reduce the COMPLEXITYCOMPLEXITY of the instruction of the instruction set, which will reduce the amount set, which will reduce the amount of of spacespace, , time, cost, powertime, cost, power, , heatheat and other things it takes to and other things it takes to implement the instruction set part implement the instruction set part of a processor. of a processor.

Features of RISCFeatures of RISC

1) small instruction set 1) small instruction set

2) load/store architecture 2) load/store architecture

3) fixed length coding and hardware 3) fixed length coding and hardware decoding decoding

4) large register set 4) large register set

(continue)(continue)

5) delayed branching 5) delayed branching

6) processor throughput of one 6) processor throughput of one instruction per cycle in averageinstruction per cycle in average

Benefits of RISC Benefits of RISC

RISC processor will cost less to design -- RISC processor will cost less to design -- since a significant cost of the chip can be the since a significant cost of the chip can be the actual R&D costs to create it, this can be actual R&D costs to create it, this can be substantial on its ownsubstantial on its own

easier to design (and fewer bugs) means that easier to design (and fewer bugs) means that

the processor will have a faster time to the processor will have a faster time to market market

faster time to market means the processor faster time to market means the processor can use newer processescan use newer processes


newer manufacturing processes mean smaller newer manufacturing processes mean smaller transistors -- smaller transistors means more transistors -- smaller transistors means more space on chip, less cost to manufacture, lower space on chip, less cost to manufacture, lower power (which creates less heat) and it means power (which creates less heat) and it means more MHz -- or some balance of all of those more MHz -- or some balance of all of those rewards rewards

simpler to design means that they have more simpler to design means that they have more time for adding other things (things that can time for adding other things (things that can make the processor do more in each cycle) make the processor do more in each cycle)

more design time also means that you can tune more design time also means that you can tune the processor more to get more MHz (more the processor more to get more MHz (more cycles) and many other effects cycles) and many other effects

Case StudyCase Study

Intel i386 for the most part Intel i386 for the most part processed only one instruction at a processed only one instruction at a time and took, on the average, five time and took, on the average, five to ten clock cycles to execute each to ten clock cycles to execute each one. The first RISC processors were one. The first RISC processors were fully pipelined, typically with five fully pipelined, typically with five stages, and averaged between 1.3 stages, and averaged between 1.3 and 2.0 clock cycles to execute an and 2.0 clock cycles to execute an instruction.instruction.

Case Study (x86 and RISC processor Case Study (x86 and RISC processor

of the early RISC era in 1987).of the early RISC era in 1987).


The MIPS R2000 processor is a smaller The MIPS R2000 processor is a smaller device built in an older semiconductor device built in an older semiconductor process with less than half the number process with less than half the number of transistors as the Intel i386DX, yet it of transistors as the Intel i386DX, yet it blows its CISC counterpart right out of blows its CISC counterpart right out of the water in performance: more than the water in performance: more than twice the Dhrystone MIPS rating and twice the Dhrystone MIPS rating and more than four times the SPECmark89 more than four times the SPECmark89 performance (even with a smaller performance (even with a smaller external cache). external cache).


In this case study, RISC processor In this case study, RISC processor shows huge advantage of the shows huge advantage of the design concept for the upcoming design concept for the upcoming era of VLSI-based era of VLSI-based microprocessors is clear. microprocessors is clear.

* VLSI (* VLSI (Very-large-scale integrationVery-large-scale integration))

The success of RISCThe success of RISC

The first system The first system that would today be that would today be known as RISC but known as RISC but wasn’t at that time, wasn’t at that time, it was the it was the CDC 6600CDC 6600 supercomputersupercomputer, , designed in designed in 19641964 by by Seymour CraySeymour Cray..

1925-19961925-1996

Cray-1 memory boardCray-1 memory board It requires its own It requires its own

electrical sub-station to electrical sub-station to provide with power. provide with power. The unit electrical bill The unit electrical bill was $30,000-$35,000 was $30,000-$35,000 per month. $20,000 for per month. $20,000 for the air-condition room the air-condition room and about $10,000-and about $10,000-$15,000 for the $15,000 for the machine. Cost per machine. Cost per board was about board was about $50,000 to replace with $50,000 to replace with the $1,000,000 per the $1,000,000 per year maintenance year maintenance contract. contract.

(Are the quotes were (Are the quotes were from the paper's in San from the paper's in San Francisco ExaminerFrancisco Examiner

The Success of RISCThe Success of RISC

There are a number of factors which There are a number of factors which have transformed RISC technology have transformed RISC technology into a success in the marketplaceinto a success in the marketplace

1)1) First factor simplicity of design (most First factor simplicity of design (most important).important).

**Original RISC processors contained less than Original RISC processors contained less than 300,000 logic gates and even today. RISC 300,000 logic gates and even today. RISC processors are typically much more compact processors are typically much more compact and leaner than CISC processors. and leaner than CISC processors.

22) The second factor which has made ) The second factor which has made possible the performance gains possible the performance gains associated with RISC processors is associated with RISC processors is better compilersbetter compilers..

Compiler technology has changed in the last few Compiler technology has changed in the last few years and many techniques which just yesterday years and many techniques which just yesterday were considered very sophisticated are now were considered very sophisticated are now fairly common. Optimizing compilers have driven fairly common. Optimizing compilers have driven

assembly language programmers to extinctionassembly language programmers to extinction. .


There are four major players in There are four major players in RISC market: SUN, Hewlett-RISC market: SUN, Hewlett-Packard, IBM and MIPS. Packard, IBM and MIPS.

These four companies account for These four companies account for most of the RISC chips sold for most of the RISC chips sold for Workstations. The four are Workstations. The four are mutually incompatible.mutually incompatible.

Apple iMAC: Using a RISC ProcessorApple iMAC: Using a RISC Processor

What is CRISC?What is CRISC?

CRISC = CISC + CRISC = CISC + RISCRISC

(Complex/Reduced Instruction Set (Complex/Reduced Instruction Set Computing) Computing)

CRISC is a 32-bit single-chip VLSI CRISC is a 32-bit single-chip VLSI processor architecture that processor architecture that achieves high performance by achieves high performance by means of RISC and multiple means of RISC and multiple functional unit approaches functional unit approaches

One computer that exhibits features One computer that exhibits features of both CISC and RISC is the Cyrix of both CISC and RISC is the Cyrix M1. M1.

It has 32 general-purpose registers It has 32 general-purpose registers

as a typical reduced instruction set as a typical reduced instruction set computers, but by using a technique computers, but by using a technique called dynamic register naming, called dynamic register naming, only 8 registers are visible at a time. only 8 registers are visible at a time.

The Pentium is another CISC/RISC The Pentium is another CISC/RISC hybrid. It uses variable-length hybrid. It uses variable-length instructions and few general-instructions and few general-purpose registers as a complex purpose registers as a complex instruction set computer would, instruction set computer would, but it adopts RISC-like features, but it adopts RISC-like features, pipelining and a floating-point pipelining and a floating-point unit.unit.

ReferencesReferences

http://granite.sru.edu/~dawna/paper.hhttp://granite.sru.edu/~dawna/paper.htmltml

http://en.wikipedia.org/wiki/RISChttp://en.wikipedia.org/wiki/RISC

http://www.mackido.com/Hardware/x8http://www.mackido.com/Hardware/x86RISC.html6RISC.html

http://www.osdata.com/system/http://www.osdata.com/system/physical/processors.htmphysical/processors.htm

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