#author("2018-07-13T21:15:21+09:00","","")
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*Processor Performance Equation [#efa936d1]
#ref(perfeq.png,center,40%)

- calculates the time it takes for the program to execute by multiplying three independent factors
- the three factors are instruction count, clocks per instruction, and clock time

**Instruction Count (IC) [#ze771296]
- can reduce instruction count by adding powerful instructions to the instruction set--> increases either CPI or clock time
- CISC processor instructions can accomplish two or three RISC processor instructions.

**Clocks per Instruction (CPI) [#h63af5cc]
- reduce CPI by instruction-level parallelism
-- complex instruction increases CPI

**Clock Time (φ) [#c16ca972]
- depends on transistor speed and the complexity of the work
-- can be reduced when transistor size decreases
---increases power consumption and heat generated

**Example Problem((Problem Source: https://www.acm.org/hennessy-patterson-turing-lecture)) [#h444de24]
   Problem:
   Suppose a program (or a program task) takes 1 billion instructions to execute on a processor running at 2 GHz. 
   Suppose also that 50% of the instructions execute in 3 clock cycles, 30% execute in 4 clock cycles, and 20% execute in 5 clock cycles.
   What is the execution time for the program or task?
   ----
   Solution:
   We have the instruction count: 109 instructions. The clock time can be computed quickly from the clock rate to be 0.5×10-9 seconds. 
   So we only need to to compute clocks per instruction as an effective value:
   Value   Frequency   Product
   3	     0.5        1.5
   4	     0.3        1.2
   5	     0.2        1.0
   CPI =	3.7
   Then we have Execution time = 1.0×109 × 3.7 × 0.5×10-9 sec = 1.85 sec.


iron law of processor performace: risc is 4x faster than cisc because cisc executes fewer instructions per program (3/4), but need more clock cycles per instruction (6*CPI)
----
Sources:
+[[Performance Equation:https://www.d.umn.edu/~gshute/arch/performance-equation.xhtml]]
+[[Basic Performance Equation:http://www0.cs.ucl.ac.uk/teaching/B261/Slides/lecture2/tsld015.htm]]
+[[Components of CPU Performance and Performance Equation:http://www.ece.lsu.edu/ee4720/2012/lsli02.pdf]]
+[[CPU Performance Equation:https://www.cl.cam.ac.uk/teaching/0607/CompArch/store-boosting-isca90.pdf]]


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