System Management Mode (SMM, sometimes called ring -2 in reference to protection rings) is an operating mode of x86 central processor units (CPUs) in which all normal execution, including the operating system, is suspended. An alternate software system which usually resides in the computer's firmware, or a hardware-assisted debugger, is then executed with high privileges.
It was first released with the Intel 386SL. While initially special SL versions were required for SMM, Intel incorporated SMM in its mainline 486 and Pentium processors in 1993. AMD implemented Intel's SMM with the Am386 processors in 1991. It is available in all later microprocessors in the x86 architecture.
Some ARM processors also include the Management Mode, for the system firmware (such as UEFI).
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