You purchased an Acme computer with the following features:

• 95% of all the memory access are found in the cache.
• Each cache block is two words, and the whole block is read on any miss.
• The processor sends references to its cache at the rate of 109 words per second.
• 25% of those references are writes.

Assume that the memory system can support 109 words per second, reads or writes. The bus reads or writes a single word at a time (the memory cannot read or write two words at once). Assume at any one time, 30% of the blocks in the cache have been modified. The cache uses write-allocate on a write miss.

You are considering adding a peripheral to the system and you want to know how much of the memory system bandwidth is already used. Calculate the percentage of memory-system bandwidth used on the average in the two cases below. Be sure to state your assumptions.

(a) The cache is write-through.

(b) The cache is write-back.