INT 21 - DOS 2+ - "WRITE" - WRITE TO FILE OR DEVICE
        AH = 40h
        BX = file handle
        CX = number of bytes to write
        DS:DX -> data to write
Return: CF clear if successful
            AX = number of bytes actually written
        CF set on error
            AX = error code (05h,06h) (see AH=59h)
Notes:  if CX is zero, no data is written, and the file is truncated or
          extended to the current position
        data is written beginning at the current file position, and the file
          position is updated after a successful write
        the usual cause for AX < CX on return is a full disk
BUG:    a write of zero bytes will appear to succeed when it actually failed
          if the write is extending the file and there is not enough disk
          space for the expanded file (DOS 5.0); one should therefore check
          whether the file was in fact extended by seeking to 0 bytes from
          the end of the file (INT 21/AX=4202h/CX=0/DX=0)
        under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
SeeAlso: AH=3Fh,AH=93h,INT 2F/AX=1109h