Two methods are widely used:
1- using a linked list of disk blocks, with each block holding as many free disk block numbers as will fit.
each block on the free list holds the numbers of 255 free blocks.
2- bitmap. A disk with n blocks requires a bitmap with n bits
Free blocks are represented by 1s in the map, allocated blocks by 0s (or vice versa).
requires less space, since it uses 1 bit per block,