Question

Consider a one word instruction ADD R2,@R1 executed on Hypothetical CPU R1 and R2 are processing registers. Fetching operation takes 2 clock cycles, memory operand read takes 2 clock cycles and any ALU operation takes one clock cycles and extra clock is not required for storing the result in destination regiseter. No. of memory cycles needed to complete the insturction cycle of the above ADD R2,@R1 is .........

Answer 1

ADD r2,@r1 => r2 is in reg direct AM, r1 is in reg-indirect AM

IF + ID => Fetch operation → 2 cycles

OF → s1 takes 1 register reference (ignore), s2 take 1 register ref (ignore) and 1 memory ref (2 cycle) → total 2 cycles

PD → processing takes 1 ALU operation → 1 cycle

WB → 1 reg ref (ignore)

Total 5 cycles.

Comments

But It was asked Memory Cycle na ? Actually I am bit confused between Memory cycle and Cpu cycle?

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yup… it says memory cycles… I did not read carefully. But anyway, in such questions asking for memory cycles doesn’t make much sense. Because you see… they haven’t given any information for memory cycles. Even if we want to calculate, we may assume 1 mem ref takes 1 mem cycle, answer could be 1 mem cycle. But then again, the fetching operation will also access memory, there too must be memory cycles utilized. So, question make more sense when talking about cpu cycles only.