Plot of square region of points

Hi, how can I plot a square lattice of points in a central region of my axes ?

if true
  x1=0;
x2=1;
y1=0;
y2=1;
x = [x1, x2, x2, x1, x1];
y = [y1, y1, y2, y2, y1];
plot(x, y, 'b-', 'LineWidth', 3);
hold on;
x1=150;
x2=350;
y1=150;
y2=350;
x = [x1, x2, x2, x1, x1];
y = [y1, y1, y2, y2, y1];
% plot(x, y, 'b-');
xlim([1, 512]);
ylim([-1, 512]);
end

I mean, to plot points in the rectangular region that is possible to have with this code