System management mode is an execution mode in x86 processors that can only be entered via an SMI (Intel SDM, Vol.1 3.1). The handlers that execute in this mode are implemented in the firmware (BIOS, UEFI) and so usually their source code cannot be easily consulted or modified. These handlers execute with high permission levels and can perform a variety of tasks such as power management, temperature management, and emulation of legacy hardware. All of these tasks are performed in a way that is completely transparent for the operating system.