The 21H function and the 22H function of the 21H interruption are the ones in charge of realizing the random readings and writings in that order. The random register number and the recent block are used to calculate the virtual position of the register to read or write. The AL register gives identical information for the in order reading of writing. The information to be read will be returned on the transfer region of the disk, similarly the information to be written resides on the DTA.
To close a file we utilize the 10H function of the 21H interruption. If after calling up this function, the AL register holds the FFH value, it means that the file has altered position, the disk was altered or there is error of disk access.