In sequential allocation, a block with value 0 is untaken.
In linked allocation, a block containing {a, b}belongs to
the file with file identifier a and carries a link to
block b. a is 0 if the block is untaken, and
b is -1 if the block is not linked to any other block.
In indexed allocation, a block with value 0 is untaken and, an
index block carries an 'I' along with the file identifier.