Caches

Question 1

Effective access time (EAT) formula

Answer 1

EAT = hit ratio * TC + (1 - hit ratio) * TM

Where:

• TC = cache access time
• TM = memory access time
• hit ratio = total # of hits divided by total # of references

Question 2

Unified vs. split caches

Answer 2

Unified caches keep data and instructions together in the same cache, while split cache systems keep an instruction cache and a data cache separately.

Split caches allow better access in parallel but this is costly

Question 3

Fully associative cache behavior

Answer 3

• Any memory block can go into any empty cache line
• All lines are checked for hits or misses
• Requires hardware to compare lines in parallel
• A valid bit is set for each line if it’s occupied
• Block loaded into vacant cache line if miss occurs
• If no vacant lines, replacement is required