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  Function 21H (decimal 33) reads one record from a random location in a
  file. To use this function, open a file with an FCB. Then store the record
  number of the record you want to read in the random-record field of the
  FCB. When you call function 21H with DS:DX pointing to the FCB, DOS reads
  the specified record into the DTA.

  AL is set with the same codes as it is for a sequential read: AL = 00H
  indicates a successful read; AL = 01H indicates end-of-file, with no more
  data available; AL = 02H means that insufficient space exists in the DTA
  segment; and AL = 03H indicates an end-of-file, with a partial data record
  available.

  Contrast this function with function 27H, which can read more than one
  random record at a time, or with function 14H, which reads sequential
  records. See function 24H for more on setting the random-record field.