You are investigating the possible benefits of a way-predicting level 1 cache. Assume that the 32 KB two-way set-associative single-banked level 1 data cache is currently the cycle time limiter. As an alternate cache organization you are considering a way-predicted cache modeled as a 16 KB direct-mapped cache with 85% prediction accuracy. Unless stated otherwise, assume a mispredicted way access that hits in the cache takes one more cycle.
a. What is the average memory access time of the current cache versus the way-predicted cache?
b. If all other components could operate with the faster way-predicted cache cycle time (including the main memory), what would be the impact on performance from using the way-predicted cache?
c. Way-predicted caches have usually only been used for instruction caches that feed an instruction queue or buffer. Imagine you want to try out way prediction on a data cache. Assume you have 85% prediction accuracy, and subsequent operations (e.g., data cache access of other instructions, dependent operations, etc.) are issued assuming a correct way prediction. Thus a way misprediction necessitates a pipe flush and replay trap, which requires 15 cycles. Is the change in average memory access time per load instruction with data cache way prediction positive or negative, and how much is it?
d. As an alternative to way prediction, many large associative level 2 caches serialize tag and data access, so that only the required data set array needs to be activated. This saves power but increases the access time. Use
CACTI's detailed Web interface for a 0.090 μm process 1 MB four-way setassociative cache with 64-byte blocks, 144 bits read out, 1 bank, only 1 read/ write port, and 30-bit tags. What are the ratio of the total dynamic read energies per access and ratio of the access times for serializing tag and data access in comparison to parallel access?