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  Function 1AH (decimal 26) establishes the disk transfer area that DOS will
  use for file I/O. The location of the DTA is specified by the register
  pair DS:DX. Normally, you should specify a DTA address before you use any
  of the interrupt 21H functions that access a DTA. If you do not, DOS uses
  the default 128-byte DTA at offset 80H in the program segment prefix.