Memory is divided up into allocation units using a bitmap.
Corresponding to each allocation unit is a bit in the bitmap, which is zero (0) only if the unit is free and one (1) only if the unit is occupied and vice-versa.
An important design issue is the size of the allocation unit.
The smaller the allocation unit, the larger the bitmap.
A bitmap gives an easy way to keep track of memory words in a fixed amount of memory only because the bitmap size depends on the memory size and the allocation unit size.