Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
Field
Byte
Length
Byte
Offset
Description
Creator ID 4 28 Vendor ID for the ASL Compiler.
Creator Revision 4 32 Revision number of the ASL Compiler.
Definition Block n 36 n bytes of AML code (see Section 5.4, “Definition Block
Encoding”)
5.2.11.2 Secondary System Description Table (SSDT)
Secondary System Description Tables (SSDT) are a continuation of the DSDT. The SSDT is
comprised of a system description table header followed by data in Definition Block format. There
can be multiple SSDTs present. After OSPM loads the DSDT to create the ACPI Namespace, each
secondary system description table listed in the RSDT/XSDT with a unique OEM Table ID is
loaded.
Note: Additional tables can only add data; they cannot overwrite data from previous tables.
This allows the OEM to provide the base support in one table and add smaller system options in
other tables. For example, the OEM might put dynamic object definitions into a secondary table such
that the firmware can construct the dynamic information at boot without needing to edit the static
DSDT. A SSDT can only rely on the DSDT being loaded prior to it.
Table 5-43 Secondary System Description Table Fields (SSDT)
Field
Byte
Length
Byte
Offset
Description
Header
Signature 4 0 ‘SSDT’ Signature for the Secondary System Description Table.
Length 4 4 Length, in bytes, of the entire SSDT (including the header).
Revision 1 8 2
Checksum 1 9 Entire table must sum to zero.
OEMID 6 10 OEM ID
OEM Table ID 8 16 The manufacture model ID.
OEM Revision 4 24 OEM revision of DSDT for supplied OEM Table ID.
Creator ID 4 28 Vendor ID for the ASL Compiler.
Creator Revision 4 32 Revision number of the ASL Compiler.
Definition Block n 36 n bytes of AML code (see Section 5.4 , “Definition Block
Encoding”)
5.2.11.3 Persistent System Description Table (PSDT)
The table signature, “PSDT” refers to the Persistent System Description Table (PSDT) defined in the
ACPI 1.0 specification. The PSDT was judged to provide no specific benefit and as such has been
deleted from follow-on versions of the ACPI specification. OSPM will evaluate a table with the
“PSDT” signature in like manner to the evaluation of an SSDT as described in Section 5.2.11.2,
“Secondary System Description Table.”
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