[數學] cumulative function

The joint probability density function of the continuous random variables X and Y is given by
f(x,y)= 256xy if 0<x<(1/2) and 0<y<(1/4)
0 otherwise
Determine F(x,y), the corresponding joint cumulative density function of x and y,
for -inf.<x<inf. and -inf.<y<inf..

I would like to ask the following questions:
1. What is the required cumulative function?
2. How can we present the cumulative function of x and y when 0<x<(1/2) and y<0?