css – With one lined up to base of web page, exactly how to make an additional use up remainder of the upright room in web page?

On my web page I have 2 DIVs, “primary” and also “footer”:

<< div id=" primary"> > things ... <.

<< div id=" footer"> > things ... <.

I lined up << div id=" footer">> to the base of the web page in this manner:

 #footer {
setting: dealt with;
size:100%;
lower:0;
}

What's the most effective method to make << div id=" primary">> use up all the room from top of web page to top of << div id=" footer">>?

For the grandfather clause where the home window is so tiny that << div id=" footer">> uses up the whole web page room, << div id=" primary">> can be relocated off the web page completely, or be lined up on top of the web page, I uncommitted as long as it does not damage anything.