I'm given the following problem:
TLB lookup time is given as 8ns and memory access time for a computer system is given as 40ns. What should be the TLB miss rate to achieve effective memory access time 60ns ?
I use the following formula where x is the TLB Hit Rate, and (1-x) is the TLB miss rate:
TLB Hit Rate * TLB Lookup Time + TLB Miss Rate * (TLB Lookup Time + Memoery Access Time) = effective memory access time
8x + (1 – x)(8 + 40) = 60
And solving for x I get:
8x + (1 – x)(8 + 40) = 60 8x + (1 – x)(48) = 60 8x + 48 – 48x = 60 -40x = 12 X = -0.3 TLB Hit Rate = -30% TLB Miss Rate = (1 – TLB Hit rate) TLB Miss rate = (1 – (-.3)) TLB Miss Rate = 1.3 TLB Miss Rate = 130%
Is it possible in this case for the hit rate to be negative, and miss rate to be over 100%? Or am I messing up on either the formula or the calculations?