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How much time would a program of ten thousand cycles be completed if it were run on a A/RISC B/ CISC based computer architecture given the following parameters of information. 

  1. Assume that 80% of executed instructions being simple and 20% complex 
  2. on a CISC machine simple instructions take 4 cycles, complex instructions take 8 cycles; cycle time is 100 ns (10-7 s); 
  3. on a RISC machine simple instructions are executed in one cycle; complex operations are implemented as a sequence of instructions; we consider on average 14 instructions (14 cycles) for a complex operation; cycle time is 75 ns (0.75 * 10-7 s). 
  4. Explain the significance of the difference in answers or results.

1 Answer

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80% instructions – simple

20% instructions – complex

Instruction count – 10000

CISC;                                                    RISC;

Cycle time = 100ns                               Cycle time = 75ns

Cycles per simple instruction = 4          Cycle time per simple inst. = 14

Cycles per complex instruction = 8       Cycle time per complex inst. = 14

Time for CISC

= (8000 x 4 x 100 x 10-9) + (2000 x 8 x 100 x 10-9)

=

Time for RISC

= (8000 x 1 x 75 x 10-9) + (2000 x 14 x 75 x 10-9)

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