Memory Address Lines?

Question

How many address lines are required to address 20MB of Memory? What are the steps of working this out?

Answer 1

25 address lines. 20MB (exactly 20MB) - 1 = 1001100010010110011111111 in binary, or 1312CFF in HEX.

to convert just take the size (20MB), subtract 1 (address 0 is a valid address) and figure out what it is in binary, and the number of bits = the number of address lines. I cheat and use the scientific calculator in windows.

another example would be 32MB (which is actually 33554432 bytes, but for ease is called 32MB).
32mb - 1 = 1111111111111111111111111 in binary or 1FFFFFF in Hex. So the number of address lines would be 25.