System Management Mode: Difference between revisions
| [unchecked revision] | [unchecked revision] |
Content deleted Content added
Rewrite with more detail - needs an expert's input |
|||
Line 14:
== SMM and ACPI ==
SMM predates [[ACPI]]. Without ACPI, it makes sense that the firmware would want to stay resident in memory even once an OS is running, because the OS may not have drivers for all of the thermal sensors, fans, and CPU performance settings that are available on that particular motherboard/chipset/CPU combination. With ACPI, however, the firmware stores [[AML]] code for the OS to interpret and execute; that AML tells the OS how to handle all those things. Theoretically, this means that SMM is not nearly as critical on a modern system as on a pre-ACPI system. However, at boot time the firmware has no way of knowing whether or not the OS includes a complete ACPI implementation or whether the OS is willing to take responsibility to managing power, thermal properties, and CPU performance. So by default, the firmware will still use SMM to handle all those things itself.
If, however, at some point the OS indicates to firmware that it ''does'' have a complete ACPI implementation, the firmware will stop performing those low-level management operations and will expect the OS to perform them instead. Even once the OS takes over, the chipset will still occasionally signal SMIs and the CPU will enter SMM, but the firmware will not have very much to do.
Line 21:
A simple example of SMM and ACPI is the power button. Pressing the power button causes the chipset to signal an SMI, sending the CPU into SMM and jumping to the SMM entry point. The firmware determines that the power button was pressed. By default, before the OS takes over low-level system management responsibilities, the firmware will handle the event by telling the chipset to tell the power switching component of the motherboard to turn off, and the system powers off immediately. If, however, the OS has indicated that ''it'' will handle low-level system management responsibilities, the firmware ignores the button press and updates the ACPI structures in memory to note that the event occurred. The OS can detect the event by examining the ACPI structures and perform any clean-up operations (like saving files) before ''it'' tells the chipset to tell the power switching component of the motherboard to turn off.
== Transparency ==
| |||