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A certain microprocessor requires $4.5$ microseconds to respond to an interrupt. Assuming that the three interrupts $\text{I}_1, \text{I}_2$ and $\text{I}_3$ require the following execution time after the interrupt is recognized:

  1. $\text{I}_1$ requires $25$ microseconds
  2. $\text{I}_2$ requires $35$ microseconds
  3. $\text{I}_3$ requires $20$ microseconds

$\text{I}_1$ has the highest priority and $\text{I}_3$ has the lowest. What is the possible range of time for $\text{I}_3$ to be executed assuming that it may or may not occur simultaneously with other interrupts?

  1. $24.5$ microseconds to $39.5$ microseconds
  2. $24.5$ microseconds to $93.5$ microseconds
  3. $4.5$ microseconds to $24.5$ microseconds
  4. $29.5$ microseconds $93.5$ microseconds
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3 Answers

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Best answer

Min time required [without simultaneous occurrence ]

Only one instruction at a time i.e. I3 .

Response time + Execution time=4.5+20=24.5

Max time required

All instructions are present.

i.e.=I1+I2+I3 ={(4.5+25) +(4.5+35) +(4.5+20) }

                  =93.5

So possible range of I3 24.5 microsecond to 93.5 microsecond .

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The maximum time for $I_3$ would be when $I_1$ and $I_2$ run before it. So, $4.5 + 25 + 4.5 +35+4.5+20=93.5\mu s$

Minimum time for $I_3$ would be when it executes first, and $I_1$, $I_2$ arrive later. So, $4.5+20=24.5\mu s$

 

Option B

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Consider case-1: I3 is executed without other interrupts:

Time interval = Interrupt processing time(I3) + Execution time(I3) = 4.5 + 20 microseconds = 24.5 microseconds

Consider case-2: I3 is executed simultaneously with other interrupts:

Time interval =( Interrupt processing time + Execution time) for I1, I2, I3 = 4.5 + 25 + 4.5 + 35 + 4.5 + 20 = 93.5 microseconds
Answer:

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