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This document discusses the differences between RISC and CISC processor architectures. RISC processors are simpler with fewer instruction types that can each be executed quickly. CISC processors support more complex instructions but are harder to design and can have performance issues. The document also addresses some misconceptions, noting that hybrid architectures may provide the best balance and that the optimal ISA may have some complex instructions in addition to the simplicity of RISC.

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RISC vs. CISC Rui Wang Tibor Horvath
Towards CISC Wired logic microcode control Temptingly easy extensibility Performance tuning HW implementation of some high-level functions Marketing Add successful instructions of competitors New feature hype Compatibility: only extensions are possible
CISC Problems Performance tuning unsuccessful Rarely used high-level instructions Sometimes slower than equivalent sequence High complexity Pipelining bottlenecks lower clock rates Interrupt handling can complicate even more Marketing Prolonged design time and frequent microcode errors hurt competitiveness
RISC Features Low complexity Generally results in overall speedup Less error-prone implementation by hardwired logic or simple microcodes VLSI implementation advantages Less transistors Extra space: more registers, cache Marketing Reduced design time, less errors, and more options increase competitiveness
RISC Compiler Issues The compilers themselves Computationally more complex More portable The compiler writer Less instructions probably easier job Simpler instructions probably less bugs Can reuse optimization techniques
RISC vs. CISC misconceptions Arguments favoring RISC: simple design, short design time, speed, price Study of RISC should include hardware/software tradeoffs, factors influencing computer performance and industry-side evaluation.
RISC vs. CISC misconceptions Incorrect implication from the two acronyms: RISC and CISC. They are not bifurcations between which designers have to choose Carelessly leaving out the participation of Operating System
RISC vs. CISC misconceptions Reduced design time? academic <-> industrial Performance claims of RISC proponent do not decouple design features like MRSs. MRSs can have a remarkable effect on program execution
Conclusion RISC vs. CISC? CISC Effectively realizes one particular High Level Language Computer System in HW - recurring HW development costs when change needed RISC Allows effective realization of any High Level Language Computer System in SW - recurring SW development costs when change needed
Conclusion Optimum? Hybrid solutions RISC core & CISC interface Still has specific performance tuning Optimal ISA Between RISC & CISC Few, carefully chosen, useful complex instructions Still has complexity handling problems

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Tibor

  • 1. RISC vs. CISC Rui Wang Tibor Horvath
  • 2. Towards CISC Wired logic microcode control Temptingly easy extensibility Performance tuning HW implementation of some high-level functions Marketing Add successful instructions of competitors New feature hype Compatibility: only extensions are possible
  • 3. CISC Problems Performance tuning unsuccessful Rarely used high-level instructions Sometimes slower than equivalent sequence High complexity Pipelining bottlenecks lower clock rates Interrupt handling can complicate even more Marketing Prolonged design time and frequent microcode errors hurt competitiveness
  • 4. RISC Features Low complexity Generally results in overall speedup Less error-prone implementation by hardwired logic or simple microcodes VLSI implementation advantages Less transistors Extra space: more registers, cache Marketing Reduced design time, less errors, and more options increase competitiveness
  • 5. RISC Compiler Issues The compilers themselves Computationally more complex More portable The compiler writer Less instructions probably easier job Simpler instructions probably less bugs Can reuse optimization techniques
  • 6. RISC vs. CISC misconceptions Arguments favoring RISC: simple design, short design time, speed, price Study of RISC should include hardware/software tradeoffs, factors influencing computer performance and industry-side evaluation.
  • 7. RISC vs. CISC misconceptions Incorrect implication from the two acronyms: RISC and CISC. They are not bifurcations between which designers have to choose Carelessly leaving out the participation of Operating System
  • 8. RISC vs. CISC misconceptions Reduced design time? academic <-> industrial Performance claims of RISC proponent do not decouple design features like MRSs. MRSs can have a remarkable effect on program execution
  • 9. Conclusion RISC vs. CISC? CISC Effectively realizes one particular High Level Language Computer System in HW - recurring HW development costs when change needed RISC Allows effective realization of any High Level Language Computer System in SW - recurring SW development costs when change needed
  • 10. Conclusion Optimum? Hybrid solutions RISC core & CISC interface Still has specific performance tuning Optimal ISA Between RISC & CISC Few, carefully chosen, useful complex instructions Still has complexity handling problems