You can find the int() function in the [CATALOG]. Of course you can always type a function name, too. 

Here are some results of the [x] (int()) function. 

By default, the TI89 will connect the segments of a graph. Since we know that the graph of y = [x] is discontinuous, press [F6Style] and select "2: Dot". 

Here is the resulting graph (in the "2: ZoomStd" window). 

You won't find "int(" in the [2nd][MATH] menu, but the "6 floor(" function in the "1: Number" submenu is the same thing. 

Here are some results of the floor() function  compare to the int() function results above. 