Int 21 Fn 40  - DOS 2+ - "write" - Write To File Or Device                 [D]

   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 5.0-6.0); 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

See Also: AH=28h,AH=3Fh,AH=93h,INT 2F/AX=1109h