Implement file control block (FCB) functions:
  INT21_OpenFileUsingFCB [0x0f],
  INT21_CloseFileUsingFCB [0x10],
  INT21_SequenialReadFromFCB [0x14],
  INT21_SequenialWriteToFCB [0x15],
  INT21_ReadRandomRecordFromFCB [0x21],
  INT21_WriteRandomRecordToFCB [0x22],
  INT21_RandomBlockReadFromFCB [0x27],
  INT21_RandomBlockWriteToFCB [0x28].