Operation method of portable terminal

A method of operating a portable terminal, the method comprising:

• entering a window screen movement mode in which a window screen of a foreground task is displayed on top and window screens of background tasks are displayed under the window screen of the foreground task;
• displaying the window screens of the background tasks to be stacked in a plurality of layers in accordance with an order in which the background tasks are used;
• selecting one of a plurality of window screens including the window screen of the foreground task and the window screens of the background tasks according to a movement command of a user;
• changing a display state of the selected window screen;
• changing transparency of at least some regions of at least one of window screens stacked on the selected window screen among the plurality of window screens;
• displaying the selected window screen as a foreground screen and finishing the window screen movement mode, when input of deciding the selected window screen is received;
• and
• deleting an arbitrary window screen according to settings among the window screens of the background tasks,
• wherein the displaying the window screens includes displaying a window screen of a first task of a first task group and a window screen of a first task of a second task group on a layer which is positioned above a layer including a window screen of a specific task when the specific task and the second task group are sequentially used after the specific task and the first task group are used in turn, wherein the specific task, the first task group, and the second task group are included in the background tasks,
• wherein the changing transparency includes increasing transparency of at least some regions of the at least one window screen stacked on the selected window screen,
• wherein the movement command of the user is input of dragging or tilting the portable terminal in a predetermined direction,
• wherein the selecting one of the plurality of window screens includes changing the selected window screen according to a direction of the dragging or tilting input,
• wherein the selecting one of the plurality of window screens includes changing the selected window screen to a window screen of a different layer among the plurality of layers when the dragging or tilting input is performed in a first direction and changing the selected window screen to a window screen of the same layer when the dragging or tilting input is performed in a second layer, and
• wherein the selecting one of the plurality of window screens includes selecting a more recently used window screen when a plurality of window screens corresponds to the movement command of the user.