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Assuming pages of size 128 words each and array is stored in row major, how many page faults will be generated in the following C program?

int A[128][128];

for(int j=0;j<128;j++)

for(int i=0;i<128;i++)

A[i][j]=0;

 

a)128

b)16384

c)0

d)None
in Operating System by Active (3.9k points) | 193 views
0
Is it B? Assuming size of an element=size of one word
+1
@ mini panda yes it would be B as for every element we need new page so page fault would be 128*128=16384....if i would have been first loop than size thing must have created problem but here in this qsn its okk
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Yes right. And since size is not mentioned so for the convenience i assumed it=4B
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@mini panda even if you take 2 bytes answer wont change..:)
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But how? Then each page will contain 256 array elements right? That is 2 rows(128*2). Then the answer would get halved right?
+1
look when you have each element of size 2 bytes than as array is store row wise in one page you will have 64 elements i.e a[0][0].....a[0][64]...so we are accessing a[0][0] first ..................than a[1][0] so that will be stored in 3rd page i.e a[1][0]...a[1][64] so you need to bring new page in that will cause page fault............ now siilalrly for every i value you will need page fault ...
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MiNiPanda and @ Shubham Shukla 6

consider word size is = size of int, otherwise you can get many answer

 

how can you say how many page faults without giving how many frames are allocated?

if no.of frames allocated =128 ====> it produce 128 page faults only

if no.of frames allocated < 128 ====> it produce 1282 page faults only

 

+1
@shaik masthan 1 word by default=1 byte in size..
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Shubham Shukla 6, who says it?

+1
@shaikmasthan here page size= 128 byte

now whether int element take 2 bytes or 4 bytes in abve qsn  our page fault would be 128*128 only

But what if i would have been outer loop and j would have been inner loop than page fault would have been different and that would have dpend on integr size taken

if it would have been 2 than 1 page could have 64 elements so than page fault would have been 2*128 as for every i value you will get max 2 page fault

if size would have been 4 bytes than 1 page would have contained 32 elements than page fault would have been 4*128=512...as for every i value page fault would have been 4.
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@ shaikmasthan in CO you must have studied
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Whatever be the size of int but it has to be equal to word size for this Q to get 214 as answer.

And by default word size is taken as 4B(or 2B sometimes) and not 1B as far as i know after studying CO..

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Shubham Shukla 6 , 

for generalizing those two implementations, i considered as word size is = size of int

not only for implementation given in this question...

i know what you said....  and more over how people arguing take int size=4 Bytes by default, like that people arguing that word = 4 Bytes...

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@shaikmasthan when nothing said in CO qsns and they talk about 1 word what you take while solving??
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@minipanda any refernce where 1 word=4byte or 2bytes.???
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not that here is a controversy with size of int, therefore you can't assume by default...

 

in your case, there is no controversy.... therefore you can assume by default as your wish..
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@minipanda any qsn of gate where 1 word=2bytes or 4bytes..(until and unless mentioned)..???
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This is from

Computer Organization
Book by Safwat G. Zaky, V. Carl Hamacher, and Zvonko Vranesic
 
+1
In gate you can't guess anything by yourself untill it is mentioned ...and in gate they will surely mention the mapping...

so i thing any answer according to the guessing will  correct in this senario
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@sushant yadav so what should we consider  here they have mentioned nothing about word size what according to you must be default size of word..?
+1
I always take it as 1 byte whenever not given ..but it differs for different people..

1 Answer

+1 vote
Best answer
The answer seems to be $none\space of\space these$ to me, but trying to reach one of the other options by taking some assumptions.

Given that $128$ x $128$ words are stored in $row\space major$ order, ie; elements are stored row by row. so there are total $128$ pages, each page containing elements $A[i][0]$ to $A[i][127]$. [Word addressible memory is taken because the size of a word and integer are not given in bytes.]

Now in the loop of the program, the words are accessed in $column\space major$ order $\rightarrow A[0][0]$, $A[1][0]$, $A[2][0]$.. (inner loop is $i$).

Therefore a new page has to be accessed each time $A[i][j]$ is accessed.[Another assumption here, number of pages in memory at a time $<= 128$]
 
Hence the total number of page faults $= 128 * 128 = 16384$
So, the answer is $(B)$
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