The size of the data bus indicate how many bit we can transfer at a time. Size of data bus is mainly specified by the data storage capacity of each location of memory module.
We have to resolve the issues how to specify a particular memory location where we want to store our data or from where we want to retrive the data.
This can be done by the memory address. Each location can be specified with the help of a binary address.
If we use 4 signal lines, we have 16 different combinations in these four lines, provided we use two signal values only (say 0 and 1).
To distingush 16 location, we need four signal lines. These signal lines use to identify a memory location is termed as ADDRESS BUS. Size of address bus depends on the memory size. For a memory module of capacity of 2n location, we need n address lines, that is, an address bus of size n.