ACPI Enable and Disable Behavior

ACPI Enable
OSPM performs this transition by writing the ACPI_ENABLE value to the SMI_CMD, which generates an event to the hardware to transition the platform into ACPI mode. When hardware has finished the transition, it sets the SCI_EN bit and returns control back to OSPM.

ACPI Disable
While in the G0 “working state,” OSPM can request a transition to Legacy mode by writing the ACPI_DISABLE value to the SMI_CMD register, which results in the hardware going into legacy mode and resetting the SCI_EN bit LOW.

=======================================================================

ACPI Enable
由OS去寫ACPI_ENABLE的值到SMI_CMD port，會由BIOS所註冊的ACPI Enable的SMI handler去處理將SCI_EN bit設起來，在這handler裡，OEM可以自行加入程式碼以達到一些特殊的需求，再將控制權交還給BIOS。

ACPI Disable
在ACPI_ENABLE下，也可由OS去寫ACPI_DISABLE的值到SMI_CMD port，會由BIOS所註冊的ACPI Disable的SMI handler去處理將SCI_EN bit清為零。