Calculate Number Of Disk Block using bitmap method

Question

For a Hard disk of Capacity 20 MB . Disk block address of 16 bits and disk block size of 1 KB . The number of disk blocks required to store all the Free Blocks using Bitmap method is

•  1
• 2
• 3
• None

Answer 1

1 block=1kB=2^10*8 bits 

no of blocks=20*2^20/2^10=20*2^10 blocks

no of blocks to require to store bit map=ceil(20*2^10/2^10*8)

                                                         =ceil(2.5)=3

so 3 blocks required

Comments

Why we can not calculated no. of disk blocks = 2^(16)   where 16 is DBA bits?
Then for what purpose DBA bits used?

Answer 2

I am getting  3

total number of bolck in File is  =  20MB/1KB

                                                        =  20 K blocks

and each Block Need  1 bit in Bit map then total 20 K bits are needed to represent Bit Map in memory

memory for Bit map in Byte  = 20 K bit / 8 bit

                                                   =  2.5 KB

as we know the size of 1 block is 1KB,  then to hold 2.5 KB bit map file we need atleast ceil(2.5)=3 block