3. (15) Consider a paging system with the page table in memory. A. If a memory r
ID: 3730665 • Letter: 3
Question
3. (15) Consider a paging system with the page table in memory. A. If a memory reference takes 100 nanoseconds, how l take? long does a paged memory reference B. If we add TLBs, and 75 percent of all page-table references are found in the TLBs, what is the effective memory reference time? (Assume that finding a page-table entry for the TLBs takes 20 nanoseconds.) C. It takes 750 milliseconds to service a page fault. The page fault rate is .001. What is the EAT? Include the TLB hit and miss portions of the equation. D. What is the purpose of a TLB? E. What is the purpose of paging?Explanation / Answer
A.
access time = time required to access the paging table + time required to access the page
=100+100=200ns
B.
access time = time required to access the page using tlb + time required to access the page using page table
=0.75 * (20 + 100) + 0.25 *( 20+ 100 +100)
=90+55
=145
C.
ma=memory-access time
p=probability of a page fault
EAT = effective access time= (1 - p) x ma + p x page fault time.
=(1-0.001)*145 +0.001*( 750000000)
=144.85+750000=750144.85microseconds=750.14485ms
D.
The purpose of a TLB is to reduce the effective memory access time of a page. It is based on locality of reference phenomena i.e. most of the page requests exists in TLB. The TLB is associative, high-speed memory. Each entry in the TLB consists of two parts: a key (or tag) and a value. When the associative memory is presented with an item, the item is compared with all keys simultaneously. If the item is found, the corresponding value field is returned.The TLB contains only a few of the page-table entries. When a logical address is generated by the CPU, its page number is presented to the TLB. If the page number is found, its frame number is immediately available and is used to access memory.
E.
Paging is a memory-management scheme that permits the physical address space of a process to be noncontiguous. Paging avoids external fragmentation and the need for compaction. It also solves the considerable problem of fitting memory chunks of varying sizes onto the backing store.
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