Previous to we can perform writing to the disk it is essential to define the
data transfer region using for this end the 1AH function of the 21H
interruption.
The 1AH function does not go back any state of the disk nor or the
operation, other than the 15H function, which is the one we will utilize to write to
the disk, does it on the AL register, if this one is equal to zero there
was no mistake and the fields of the current register and block are dated.