Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
while legacy systems use some type of transparent interrupt handler to respond to these events (that
is, an SMI interrupt handler). ACPI-compatible hardware can choose to support both legacy and
ACPI modes or just an ACPI mode. Legacy hardware is needed to support these features for non-
ACPI-compatible operating systems. When the ACPI OS loads, it scans the BIOS tables to
determine that the hardware supports ACPI, and then if the it finds the SCI_EN bit reset (indicating
that ACPI is not enabled), issues an ACPI activate command to the SMI handler through the SMI
command port. The BIOS acknowledges the switching to the ACPI model of power management by
setting the SCI_EN bit (this bit can also be used to switch over the event mechanism as illustrated
below):
SCI_EN
PM1x_CNT.0
Power
Management
Event Logic
0
Dec
1
SMI_EVNT
SCI_EVNT
Shareable
Interrupt
Figure 4-20 Power Management Events to SMI/SCI Control Logic
The interrupt events (those that generate SMIs in legacy mode and SCIs in ACPI mode) are sent
through a decoder controlled by the SCI_EN bit. For legacy mode this bit is reset, which routes the
interrupt events to the SMI interrupt logic. For ACPI mode this bit is set, which routes interrupt
events to the SCI interrupt logic. This bit always returns set for ACPI-compatible hardware that does
not support a legacy power management mode (in other words, the bit is wired to read as “1” and
ignore writes).
The SCI interrupt is defined to be a shareable interrupt and is connected to an OS visible interrupt
that uses a shareable protocol. The FADT has an entry that indicates what interrupt the SCI interrupt
is mapped to (see Section 5.2.6, “System Description Table Header”).
If the ACPI platform supports both legacy and ACPI modes, it has a register that generates a
hardware event (for example, SMI for IA-PC processors). OSPM uses this register to make the
hardware switch in and out of ACPI mode. Within the FADT are three values that signify the
address (SMI_CMD) of this port and the data value written to enable the ACPI state
(ACPI_ENABLE), and to disable the ACPI state (ACPI_DISABLE).
To transition an ACPI/Legacy platform from the Legacy mode to the ACPI mode the following
would occur:
• ACPI driver checks that the SCI_EN bit is zero, and that it is in the Legacy mode.
• OSPM does an OUT to the SMI_CMD port with the data in the ACPI_ENABLE field of the
FADT.
• OSPM polls the SCI_EN bit until it is sampled as SET.
To transition an ACPI/Legacy platform from the ACPI mode to the Legacy mode the following
would occur:
• ACPI driver checks that the SCI_EN bit is one, and that it is in the ACPI mode.
84 April, 2015 Version 6.0