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35 votes
35 votes

A paging scheme uses a Translation Look-aside Buffer (TLB). A TLB-access takes $10$ ns and the main memory access takes $50$ ns. What is the effective access time(in ns) if the TLB hit ratio is $\text{90%}$ and there is no page-fault?

  1. $54$
  2. $60$
  3. $65$
  4. $75$
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5 Answers

Best answer
45 votes
45 votes
Effective access time $=$ hit ratio $\times$ time during hit $+$ miss ratio $\times$ time during miss

In both cases TLB is accessed and assuming page table is accessed from memory only when TLB misses.

$= 0.9 \times (10+50) + 0.1 \times (10 + 50 + 50)$

$= 54 + 11=65$

Correct Answer: $C$
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9 votes
9 votes
Answer is (C) Part.

Just one more approach -->

$Effective Access Time = Address Translation Time + Main Memory Access Time$

= $10 + 0.1(50) + Main Memory Access Time = 15 + 50 = 65ns$
1 votes
1 votes
answer :- (c) 65

reason-

Given : TLB access time =  10 ns

main memory access time = 50 ns

TLB hit ratio= 90%  i.e (0.9)

now we first need to find miss ratio that is [1- TLB hit ratio]

miss ratio= 1- 0.9= 0.1

now calculating the effective access time by formula,

hit ratio X (TLB access time + memory access time) + miss ratio X (TLB access ratio + 2 X memory access ratio)

= 0.9X(10 ns + 50 ns)+ 0.1 X (10 ns + (2 X 50 ns))

=0.9 X 60 ns + 0.1 X 110ns

=54 ns + 11 ns

=65 ns
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