As shown in figure 1, here the entire heap, 1024 M, is set aside to be used (the entire bar), but only the initial 256 M is allocated to be used, as shown by the shaded area.

如图1所示，整个heap的1024M被放置起来备用(整个条)，只有最初的256 M被分配使用，如图中阴影区域所示。