seeking the upper left buffer position of the window

[Dieter Wilhelm]

Hi

Please would you mind to give me a pointer for the access function of
the upper left position of the current window?  I'm sure there is one
but I couldn't find it and I'd like to place a temporary line (with
momentary-string-display) exactly at the uppermost visible line in the
current buffer window.

-- 
    Best wishes

    H. Dieter Wilhelm
    Darmstadt, Germany