Imiating Frames Through CSS

Right now, I am trying to create a look that I only know how to create by using frames, but I actually have no pratical use for frames other than for the design aspect.

I am trying to create the 'top-bottom' frameset by using CSS. By using frames the bottom frame would be 100 pixels in height, and the top frame's height would be whatever the bottom frame isnt taking up.

What I am trying to do is create a page where the content is on top and the central nav/logo is always on the bottom and never hidin. So far I've got the bottom CSS layer at the bottom, and 100 pixels in height, but when I go to attempt to create the top portion there seems to be no way to set the top layer's height to be whatever the bottom frame isnt taking up.

I'll attach an image of waht i hope to accomplish, and maybe you can suggest some methods of achieving it.

Calculate the window height, and work from that, using something like:

screenHeight = (document.all) ? parseInt(document.body.scrollHeight) : parseInt(window.innerHeight) ;

topHeight = screenHeight - 100

is that java or something? i understand the concept but dont i need to use some sort of script to get it to work?