ACPI Advanced Configuration and Power Interface
The standard that lets the operating system control power management and hardware configuration instead of the BIOS. Created in 1996 by Intel, Microsoft and Toshiba; replaced APM.
Firmware exposes tables — DSDT, SSDT, MADT, FADT — describing the machine's devices, interrupt routing and power states. The strange part is that those tables contain code in a bytecode language called AML, which the OS interprets in its own virtual machine to read temperatures, handle lid buttons and power down components. Hence the classic Linux problems: an AML implementation only ever tested against Windows. It defines S-states (S0 running, S3 suspend to RAM, S4 hibernate), C-states for CPU idle and P-states for frequency scaling. The acpi_osi flags exist precisely to lie about which OS is running.