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A non-pipelined CPU has $12$ general purpose registers $(R0,R1,R2, \dots ,R12)$. Following operations are supported

  • $\begin{array}{ll} \text{ADD Ra, Rb, Rr} & \text{Add Ra to Rb and store the result in Rr} \end{array}$
  • $\begin{array}{ll} \text{MUL Ra, Rb, Rr} & \text{Multiply Ra to Rb and store the result in Rr} \end{array}$

$\text{MUL}$ operation takes two clock cycles, $\text{ADD}$ takes one clock cycle.

Calculate minimum number of clock cycles required to compute the value of the expression $XY+XYZ+YZ$. The variable $X,Y,Z$ are initially available in registers $R0,R1$ and $R2$ and contents of these registers must not be modified.

  1. $5$
  2. $6$
  3. $7$
  4. $8$
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option B ) 6 cycles xy+xyz+yz = y(x+xz+z)
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Best answer
Let's first rewrite the expression as: y*(x + z + x*z)

the instructions are:
ADD R0, R1, R3

MUL R0, R1, R4

ADD R3, R4, R3

MUL R2, R3, R3

Since it is a non-pipelined processor it will take 2*2 + 2*1 = 6 cycles
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Would the answer be different if the CPU was pipelined?
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let us rewrite the equation as Y(X+Z+X*Z)

Let us consider the following instruction

ADD R0, R2, R3

MUL R0, R2, R4

ADD R3, R4, R3

MUL R1, R3, R3

So tye nos of clock cycle will be 1+2+1+2=6 clock cycles.
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Is it allowed to optimze the instruction to determine minimum number of clock cycle needed?
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Answer:

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