4. A computer system has 256K words of main memory and 16K words of direct-mapped cache memory. The block size is 256 words. The access time for the main memory is 15 times that of the cache memory. a) What is the size of the tag field? b) If a fully associative mapping scheme were used instead, what would be size of the tag field? c) Determine the efficiency of the system assuming a cache hit ratio of 0.9 d) If the cache access time is 200 nanoseconds, what hit ratio would be required to achieve an average access time equal to 500 nanoseconds? (Note: I K-1024 or 210 . That would make 256K-2sX 210-218)