TLB and page fault

Question

in a virtual memory system which of the following is not possible

a)TLB miss with no page fault

b)TLB miss with page fault

c)TLB hit and page fault

d) none

the answer given is option d

but.........TLB is a memory that stores a part of the pagetable....and if its a hit , then it directly goes to the process and need not refer the pagetable right?

option a) TLB is checked, its a miss and the pagetable is checked and its present there , therefore no page fault ------->this is posssible

option b)TLB is checked, its a miss and the pagetable is checked and its not present there , therefore  page fault ------->this is posssible

option c)TLB is checked, its a hit...when its a hit, the pagetable is not checked itself....how can u consider a pagefault there??

may be there is something wrong in the concept that i have understood

please help

Comments

Refer https://gateoverflow.in/139373/self-doubt

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http://pages.cs.wisc.edu/~cao/cs537/lecture17.txt

Answer 1

All of these are valid options..Let us see one by one :

Option 1 is possible as if there is TLB miss , then page table is referred , then the address translation is done and the actual page then is possible that it is present in the main memory..

Option 2 is possible :

a) If it consults to a page table entry where the valid bit is set to '0' indicating that it is an illegal memory access..

b) If the page table entry is absent ; indicating the page is not in physical memory address space.

Option 3 is also possible : 

Say there is TLB hit but on access we get a trap , possibly bcoz the area where we are trying to write something is write protected(read only)..

Ref : https://stackoverflow.com/questions/6398811/can-a-tlb-hit-lead-to-page-fault-in-memory

So all of the above scenarios are possible ..Hence D) should be the correct answer..

Answer 2

there are two things that take place while a process is executed , first is virtual address is converted into physical address and then the page at that physical address is to be accessed. TLB just helps in doing the first thing i.e conversion of Virtual address to Physical address and it may be possible that while accessing the page at the physical address page fault occurs . so option 3 is also right.