Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
Figure 5-23 Root System Description Pointer and Table
All system description tables start with identical headers. The primary purpose of the system
description tables is to define for OSPM various industry-standard implementation details. Such
definitions enable various portions of these implementations to be flexible in hardware requirements
and design, yet still provide OSPM with the knowledge it needs to control hardware directly.
The Extended System Description Table (XSDT) points to other tables in memory. Always the first
table, it points to the Fixed ACPI Description table (FADT). The data within this table includes
various fixed-length entries that describe the fixed ACPI features of the hardware. The FADT table
always refers to the Differentiated System Description Table (DSDT), which contains information
and descriptions for various system features. The relationship between these tables is shown in
Figure 5-24.
Fixed ACPI
Description Table
FACP
Differentiated System
Description Table
DSDT
Firmware ACPI
Control Structure
FACS
Header
Static info
FIRM
DSDT
BLKs
Header
Differentiated
Definition
Block
ACPI
Driver
Wake Vector
Shared Lock
Software
Hardware
...
GPx_BLK
PM2x_BLK
OEM-Specific
PM1x_BLK
Located in
port space
Device I/O
Device Memory
PCI configuration
Embedded Controller space
Figure 5-24 Description Table Structures
• OSPM finds the RSDP structure as described in Figure 5.2.5.1 (“Finding the RSDP on IA-PC
Systems”) or Figure 5.2.5.2 (“Finding the RSDP on UEFI Enabled Systems”).
When OSPM locates the structure, it looks at the physical address for the Root System Description
Table or the Extended System Description Table. The Root System Description Table starts with the
signature “RSDT”, while the Extended System Description Table starts with the signature “XSDT”.
102 April, 2015 Version 6.0