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A hardwired CPU uses $10$ control signals $S_1$ to $S_{10}$, in various time steps $T_1$ to $T_5$, to implement $4$ instructions $I_1$ to $I_4$ as shown below:


Which of the following pairs of expressions represent the circuit for generating control signals $S_5$ and $S_{10}$ respectively?

($(I_j + I_k)T_n$ indicates that the control signal should be generated in time step $T_n$ if the instruction being executed is $I_j$ or $l_k$)

  1. $S_5 = T_1 + I_2 \cdot T_3$ and

    $S_{10} = (I_1 + I_3) \cdot T_4 + (I_2 + I_4) \cdot T_5$

  2. $S_5 = T_1 + (I_2 + I_4) \cdot T_3$ and

    $S_{10} = (I_1 + I_3) \cdot T_4 + (I_2 + I_4) \cdot T_5$

  3. $S_5 = T_1 + (I_2 + I_4) \cdot T_3$ and

    $S_{10} = (I_2 + I_3 + I_4) \cdot T_2 + (I_1 + I_3) \cdot T_4 + (I_2 + I_4) \cdot T_5$

  4. $S_5 = T_1 + (I_2 + I_4) \cdot T_3$ and

    $S_{10} = (I_2 + I_3) \cdot T_2 + I_4 \cdot T_3 + (I_1 + I_3) \cdot T_4 + (I_2 + I_4) \cdot T_5$

asked in CO & Architecture by (20.8k points)  
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option d) is the answer right?

option (d) is the answer right?

Yes Boss

1 Answer

+14 votes
Best answer
4. is the option for this question.
If we look at the table, we need to find those time-stamps and instructions which are using these control signals.

For example, $S_5 = T_1$ has used control signal $S_5$ for all the instructions, or we can say irrespective of the instructions. Also, $S_5$ is used by instructions $I_2$ and $I_4$ for the time stamp $T_3$ so that comes to:
$$S_5 = T_1 + I_2 \cdot T_3 + I_4 \cdot T_3 = T_1 + (I_2+I_3) \cdot T_3$$

In the same way, we'll calculate for $S_{10}$.
It's an example of Hardwired CU Programming used in RISC processors. It gives accurate result, but isn't good for debugging since minor change will cause to restructure the control unit.
answered by (6k points)  
edited by


No option is matching with your S5 expression.

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