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RISC vs. CISC. By: Frank Norris. Topics of Discussion:. Technological limitations which lead up to the design philosophies of RISC and CISC RISC and CISC Processor Methodologies Processing Comparisons Today’s Processors – Combining RISC and CISC for better performance.

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RISC vs. CISC

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Risc vs cisc l.jpg

RISC vs. CISC

By: Frank Norris


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Topics of Discussion:

  • Technological limitations which lead up to the design philosophies of RISC and CISC

  • RISC and CISC Processor Methodologies

  • Processing Comparisons

  • Today’s Processors – Combining RISC and CISC for better performance


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Technological Limitations leading to CISC design philosophy:

  • In the 1970’s & 80’s memory was both slow and expensive.

  • High cost and poor performance facilitated the need for compact and efficient code. (Assembly Language for direct execution)

  • Code written in a high level language took much longer to translate into assembler, which lead to bloated code and slower program execution.

  • The CISC philosophy was to shift some of the programming burden to the hardware level.


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CISC and Performance:

  • time/program = [ (instructions/program)x (cycles/instruction) x (time/cycle) ]

    • CISC seeks to reduce the number of instructions executed to perform a task thereby increasing overall performance


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CISC Methodology: Use additional hardware to perform code translation and optimization.

  • Complex instructions written in a high level language translate directly into exactly one instruction in assembler.

  • Reduced difficulty in writing compilers, improve code compaction, and ease debugging.

  • Improve the efficiency of programs written in high level languages.

  • Reduce software development costs as well as the size and complexity of programs/systems.


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Complex Instructions:

To multiply two numbers, first load each operand from a location in main memory (locations 1:1 through 6:4) into one of the six registers (A, B, C, D, E, or F). Once loaded, they can be multiplied by the execution unit (or ALU).

1.LOAD [A, 2:3]

2.LOAD [B, 5:2]

3.MULT [A, B]

4.STORE [2:3, A]


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Complex Instructions:

  • CISC ‘rolls up’ this instruction set into one compact instruction to be handled by the decoder.

  • MULT [2:3, 5:2]

  • Microcode engine within the CPU decodes the complex instructions and executes microcode programs to carry out the task


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Pro:

Complex instructions operate directly on main memory.

Programmer is no longer required to do a direct call to LOAD and STORE operations as they are now handled by hardware.

Compiler has less work to translate statements in a high level language to assembly language.

Con:

Microcode became more difficult to test and debug as systems became more complex requiring numerous patches to fix bugs.

Programmers weren’t using the more complex instructions sets in favor of smaller instructions that accomplished the same result.

The use of memory operands caused structural hazards preventing concurrent execution of instructions. (pipelining)

Pros and Cons of CISC:


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Observations leading to RISC methodology:

  • Only 20% of the available instructions are being used. Transistors currently allocated to these complex instruction sets could be better utilized elsewhere to gain performance.


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RISC Methodology: Simple instructions and the return of direct execution

  • Reestablish the direct execution model since most complex instructions were not utilized.

  • Do away with the microcode engine (eliminate the overhead involved in decoding)

  • Reduce the instruction set by eliminating all but the most necessary instructions

  • Replace the complex instructions with groups of smaller ones


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RISC and Performance:

  • time/program = [ (instructions/program) x(cycles/instruction)x (time/cycle) ]

    • RISC seeks to reduce the number of CPU cycles per instruction executed to increase overall performance


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RISC and Performance:

  • RISC instructions, wherever possible, should only take one instruction cycle to complete.

  • Pipelining is only really feasible where instructions of varying degrees of complexity are not dealt with, which yields a lower number of average cycles per instruction, thereby increasing performance.


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Advantages of RISC:

  • RISC uses only register to register operations

  • Only LOAD and STORE operations have access to memory

  • Separation of LOAD and STORE instructions allows the compiler to shift these operations around for maximum efficiency during execution.

  • Simple instructions require fewer transistors which make the chips easier to design and cheaper to produce


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Today’s Processors:

  • Most CPU’s today are based on the CISC methodology, but with the increase in transistor resources the RISC methodology is also in use within the same processor.(Intel, AMD, etc.) All modern processors utilize the following advancements:

    • on-chip cache clocked as fast as the processor

    • additional functional units for superscalar execution

    • support for floating point operations

    • branch prediction

    • out-of-order execution

    • While keeping backward compatibility with older x86 standards and utilizing a RISC processing core increases overhead, market factors have dictated that this be the case for manufacturers.


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Today’s Processors:

  • Intel’s newest processors blur the barrier between RISC and CISC even more through the following advancements:

    • Rapid Execution Engine

      • Two Arithmetic Logic Units allowing basic integer instructions to execute in 1/2 a clock cycle.

    • Execution Trace Cache

      • Stores 12K decoded microcode operations in the order of program execution. This removes the decoder from the main execution loop and improves cache efficiency.

    • Data Prefetch Logic

      • Anticipates data needed by an application and pre-loads it into the Advanced Transfer Cache.

    • Streaming SIMD Extensions 2 (SSE2) Instructions

      • Special instruction set to accelerate video, speech, image processing, encryption, and science/engineering applications.

    • Hyper-Threading Technology

      • two logical processors that can execute different tasks simultaneously using shared hardware resources.


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Summary:

  • CISC processors are more expensive to build, and maintaining the microcode for these processors becomes increasingly more complex.

  • RISC processors gain a performance advantage through the elimination of microcode, pipelining and caching, and the use of a direct execution control unit.

  • Technological advancements have blurred the boundary between true RISC and CISC architectures in recent years, and this trend continues today. Designers are actively looking for things to integrate into the chips to make use of the increased transistor resources and make performance improvements.


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Sources:

http://cpusite.examedia.n1/docs/cisc_vs_risc.html

http://amiga.emugaming.com/riscisc.html

http://www.heyrick.co.uk/assembler/riscvcisc.html

http://www.arstechnica.com/cpu/4q99/risc-cisc/rvc-1.html

http://developer.intel.com/design/Pentium4/prodbref/#netburst


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