Processor Architecture Whats the minimum prediction accuracy

(Processor Architecture) What’s the minimum prediction accuracy one should achieve in order to get benefits using way-prediction (in terms of average memory access time).

N3. You are asked to evaluate the way-prediction in a particular cache design problem. Without the way-prediction, a cache with a set-associative organization has an access time of 1.5 ns. With the way-prediction, on a way- prediction hit, the access time is 1.2 ns. However, if the way prediction is wrong, the processor has to spend extra 1.0 ns to obtain the data from other ways. Assuming the cache miss rate is 5% with miss penalty of 80 ns for the both cases, i.e. with and without way prediction.

Solution

averageAccessTime = h*Tc +(1-h)*M

where h = hit rate
(1-h) = miss rate
Tc = time to access information from cache
M = miss penalty (time to access main memory)
     
     
   ===>5*1.5 + 5*80 ==> 7.5 + 400 ==> 407.5
   ==> 407/1000 ==> 40.7%
     
Minimum prediction accuracy = 40.7%