Subhan Waizi
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Erstellt / Aktualisiert 12.05.2016 / 16.06.2017
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0 Exakte Antworten 38 Text Antworten 0 Multiple Choice Antworten
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State the main features of CISC and RISC processors.

CISC (Complex Instruction Set Computer):

  • Extensive instruction set
  • complex and efficient machine instructions of variable length and variable execution time
  • Micro-coding of the machine instructions
  • Extensive addressing capabilities for memory operations
  • Relatively few registers

RISC (Reduced Instruction Set Computer)

  • Reduced Instruction Set
  • Less complex, simple instructions of fixed length which typically take 1 clock cycle per instruction
  • Simple control unit and no microcode
  • Few addressing schemes for memory operands with only two basic instructions, namely LOAD and STORE
  • Many registers organized in a register file
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How can multicore systems be used to save power?

Calculate power consumption of single-core: \(P = C * V_{dd}^2*f\) with \(Performance = const*f\)

If more cores are considered this will change to \(P = 2*C*V_{dd}^2*f\) with new \(Performance = 2*const*f\)

If you decrease the frequency to get the same performance you can also reduce \(V_{dd} \) as it is roughly proportional to f.

Thus you will get \(P = 2*C*V_{dd}^2/4*f/2\)

Rough calculation, as it does not take leakage power consumption and communication overhead into account, however it can serve as rough estimate.

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What are the main advantages of the RISC architecture considering embedded applications?

  • Simpler hardware
    • small and simple instruction set leads to less chip space
    • extra functions, such as memory mangament units or floating point arithmetic units can be easily extended and placed on the same chip
    • smaller chip allows placing more chips on a single silicon waver, which can significantly lower the per-chip cost which is very important to embedded applications with high manufacturing volumes
  • short design cycle
    • RISC architecture is simpler than equivalent CISC processors, thus they can be designed and teste more quickly
    • Can adapt to advantages of technological developments sooner than CISC designs due to its simplicity, thus result in greater leaps in performance between generation
  • Low energy consumption
    • simpler hardware, smaller control logic and the option to scale down voltage via pipelining can lead to low energy consumption
    • Especially in embedded designs aimed at mobile consumer markets, power consumption plays a signifcant role
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What are disadvantages of the RISC processor?

  • Code Quality
    • RISC performance greatly depends on the quality of software code that it is executing
    • harder to program efficiently. if instruction scheduling in a program is poor the processor can spend a lot of time stalling, which means waiting for the result of one instruction before being able to proceed with subsequent instruction
    • instruction scheduling in general complex, which makes it hard for a compiler to generate efficient and good assembly
  • System Design
    • ​requires more instructions than a CISC for the sampe application which implies larger program memory
    • requires fast memory systems to provide the instructions. Thus these systems typically contain large memory caches, mostly on the chip itself, often referred to as L1-chaes


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Looking at the advantages of RISC, why would CISC still be around and why is it still being developed?


The prominent reason here is backward compatibility. As the iBM compatible PC is the most common computer in the world, Intel wanted a CPU taht would run all the applications that are in the hands of more than 100 million users.

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In modern processors today, is there huge differences between CISC and RISC architectures?

The trend has shown that the RISC and CISC processors are more and more converging. Each of them are adapting the main advantages of the other processor architecture which results in some sort of RISC-CISC mix. Some examples are

  • pipelining, branch prediction and hardware stacks are adapted into CISC
  • on the other way current high degree of pipelining, caches and parallelizm does not inherit RISC's basic idea of simplicity
  • clock speeds of modern CISC and RISC are more and more becoming the same (not much difference)
  • real performance issue is not the architecture, but optimization of instruction set ands and details of machine organization
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What can be understood as a CISC-RISC architecture?

This kind of architecture has been emerging from common RISC and CISC architectures. It tries to adapt the advantages of both design methodologies into one design. For example in the CISC architecture, there will be a decoder that converts the CISC instructions into a number of less complex RISC instructions that are executed by RISC core. Popular examples are the modern Intel cores and cores from the AMD family which are compatible with program written for their CISC predecessors, however can still perform competitively against processors based on RISC.

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What are the advantages of single-bit processing?

  • Extreme simplicity of hardware leading to minimum area overhead
  • Low area often results in very low power consumption
  • easy re-configuration of 1-bit systems
    • vector processing (SIMD = Single Instruction Multiple Data)
    • pipelining is straightforward
    • less wire overhead for communication again resulting in less power
    • no need for switching for reconfiguration again resulting in less power and area