Write to file with handle (INT 21h, 40h)

        Entry:
           AH = 40h
           BX = file handle
           CX = number of bytes to write
           DS:DX = pointer to buffer
        Return:
           CF = 1 if error
                  AX = Error Code
           CF = 0 successful
                  AX = number of bytes written

        Note: If CX is zero, no data is written, and the file is
              truncated or extended to the current position