ISI2017-PCB-CS-5(b)

Question

Consider a paging system with the page table stored in memory. If a memory reference takes $200$ nanoseconds, how long does a paged memory reference take? If we add a Translation Lookaside Buffer (TLB) and $75$ percent of all page-table references are TLB hits, what will then be the effective memory reference time? Assume that finding a page-table entry in the TLB takes $20$ nanoseconds, if the entry is present.

Comments

EMAT = 0.75*(20 ns + 200 ns) + 0.25*(20 ns + 200 ns + 200 ns)

           = 165 ns + 105 ns = 270 ns.

If it is a hit, the time taken for reference will be(20 + 200)( TLB + Mem.Access)

If it is a miss, we need another 200 ns for page table table access which is also in memory.

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If a memory reference takes 200 nanoseconds, how long does a paged memory reference take

 Paged memory reference = Time to access page table + Time to access main memory frames

                                            = $\text{200 ns + 200 ns}$

                                            = $\text{400 ns}$

Answer 1

Effective memory access time would be --

EMAT = 0.75*(20 + 200) + 0.25*(20 + 200 + 200) = 270 ns