Advanced Configuration and Power Interface Specification

ACPI Overview
necessary. Figure 3-8 is an example mobile PC diagram that depicts a single thermal zone with a
central processor as the thermal-coupled device. In this example, the whole notebook is covered as
one large thermal zone. This notebook uses one fan for active cooling and the CPU for passive
cooling.
Thermal
Zone
P
L
L
(Passive Cooling)
CPU
Fan
(Active Cooling)
L
2
D
R
A
M
CPU/
Memory/
PCI Bridge
Graphics
D
R
A
M
LCD
CRT
NVRAM
PCI/PCI
Bridge
L
A
N
M
P
E
G
Momentary
USB
Port 1
Keyboard
Docking
HDD
0
HDD
1
F0: PIC, PITs, F2:
DMA, RTC, EIO, ... USB
F1: BM
IDE
Embedded
Controller
PS/2
Ports
Mouse
DPR0
EPROM
SIO:
COMs,
LPT,
FDC,
ACPI
DPR1
COM
LPT
FDD
Figure 3-8 Thermal Zone
The following sections are an overview of the thermal control and cooling characteristics of a
computer. For some thermal implementation examples on an ACPI platform, see Section 11.6,
“Thermal Zone Interface Requirements.”
3.10.1 Active and Passive Cooling Modes
ACPI defines two cooling modes, Active and Passive:
Passive cooling
OS reduces the power consumption of devices at the cost of system performance to
reduce the temperature of the machine.
Active cooling
OS increases the power consumption of the system (for example, by turning on a fan)
to reduce the temperature of the machine.
These two cooling modes are inversely related to each other. Active cooling requires increased
power to reduce the heat within the system while Passive cooling requires reduced power to decrease
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