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  Service 03H (decimal 3) writes one or more cassette data blocks of 256
  bytes each. (See service 02H.) As with service 02H, the CX register gives
  the count of bytes requested, and ES:BX points to the data area in memory.
  If the amount of data being written is not a multiple of 256 bytes, the
  last data block is padded out to full size.

  After the service is completed, CX should contain 00H, and ES:BX should
  point just past the last memory byte that was written.

  Curiously, no error signals are provided for this service, essentially
  because a cassette tape recorder can't inform the computer of any
  difficulties. This forces the ROM BIOS to write data in blind faith that
  all is well. Needless to say, it would be a good idea to read back any
  data written, just to check it.