system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does the computer do differently in its processing when a game is in full screen as opposed to borderless window?

I understand the difference between the two modes... but what is the computer doing differently? Does one mode allow the game to run faster or have different performance compared to the other?
gpt
In fullscreen mode, the fullscreen application has "exclusive use" of a monitor, otherwise the OS has to manage all the displaying for that monitor.

Basically, when you run an application in a window, the OS has to "intercept" the drawing commands for each application that is running on that monitor, so that it can composite all of the drawings into their own little frames, and possibly do other fancy tricks. For example, on Windows 7/8 you can hover over the icon of an open application in the taskbar, and a small preview window pops up that displays what the application is drawing. The application doesn't handle this special case, it's all the OS's doing, and it can do this because it's intercepting the drawing of each application.

In fullscreen mode, all of this is bypassed. The application has full control over what is being displayed on the monitor. It controls when to refresh the frame, it controls the resolution that is being sent to the monitor, etc. This can have some performance benefits, since the OS will no longer be drawing anything onto that monitor, but those are minor compared to simply having complete control. In windows 8 for example, windowed applications have VSync force-enabled, since windows 8 will not draw a partially rendered frame into the application window. Although many people prefer VSync anyway since it eliminates tearing, it also introduces some input lag, which for some gamers is a big downside. G-Sync or other variable refresh rate solutions also cannot work in windowed mode because you can't force all other applications being drawn to match the refresh rate of a single application.