Number of surfaces =16
Tracks per surface=128
Sectors per track=256
Data which will store per sector=512 bytes
Capacity of the disk = 16 surfaces X 128 tracks X 256 sectors X 512 bytes = 256 Mbytes.
The number of bits required to access a sector =Total number of sectors.
= 16 surfaces X 128 tracks X 256 sectors
=2^4x2^7x2^8=2^19