Reading up on wiki on bitmap as a method hard drives use to find out sectors that are available to store data to or not.
Examples are given showing the size of a bit map. with hard drive size and number of sectors.
I'm failing on where these answers are coming from.
One example from the wiki is
For explanatory purposes, we will use a 4 GiB hard drive with 4096 byte sectors, and assume the bitmap itself is stored elsewhere. The example disk would require 1,048,576 bits, one for each sector, or 128 KiB.
Can someone explain how this solution is achieved, and if possible add a small explanation regarding why,