computer organization

Question

Given that 4 GB and 2 MB are the respective sizes of main memory and cache. Find the most appropriate tag bit length. Assume the block size is same in both cache and main memory and direct mapping is used with byte addressable memory.
 

• (A)10
• (B)11
• (C)12
• (D)13

Ans given op(b)........but how it possible bez there is no block size given.....plz someone explain this

Answer 1

Suppose block size=2^x B
no of main memory blocks=2³²/2^x =2^32-x 
no of cache memory blocks or lines =2²¹/2^x =2^21-x 
suppos tag bits =y
hence 2^y is no of main memory block matches to one cache memory line
2^y=2^32-x /2^21-x =2¹¹
hence tag bits= 11