The CPU Performance Equation

# Structured Computer Organization (4th Edition)

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Unformatted text preview: The: EPIJ Parlor-manna Equltlnn Most computers are constructed using a clock running at a constant rate. These discrete time events are called ticks, stock ticks. stock periods, stocks. weir-s, or cfoei: eycies. Computer designers refer to the time of a clock period by its dura— tion (cg. 2 us) or by its rate (cg, SDI] MHz}. CPU time for aprogram can then he espresscd two ways: CPU time = CPU clock cycles for a pmgram at Elle-cit cycle time [if . _ CPUclockcyclesforaEy'mtt mum" Clockrate In addition to the number of cloclt cycles needed to execute a pmg'am, we can afso count the number of instructions executed—the instruction path length or in- struction cotter (HZ). If we know the number of clock cycles and the instruction count we can calculate the average number [If stock cycles per instruction {CPI}: CPI = CPU clock cycles for a my D: This CPU ﬁgure of merit provides insight into different styles of instruction sets and implementations, and we 1still use it extensively in the next four chapters. By transposing instruction count in the ahme formula, clock cycles can he de- ﬁned as 1:: K CPI. "this allows us to use CPI in the execution time fonnula: CPU time = [CHEPTsﬂn-elteyelemue _ tescpi ”Um-m Expanding the ﬁrst formula into the units of measure shows how the pieces ﬁt together: Program Innh'uction K Clock cycle _ Program tnstrueuons H Clock cycles Seconds Somalis = CPU ﬁrrle As this formula demonstrates. CPU performance is dependent upon three charac— teristics: clock cycle {or rate), clock cycles per instmctiom and instruction count. Furﬂlenuore, CPU time is eqmtty dependent on these three characteristics: A 10% improvement in any one of Iltem leads to a 10% improvement in CPU time. MtPS cat“ Willlﬁ't’l 'WC’J-tum FEW Fetter-“Kl S H L I Clo-cl: nerE‘i PM “ geese cm ma ...
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