Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
ReleaseGlobalLock:
mov ecx, GlobalLock ; ecx = Address of Global Lock in FACS
rel10: mov eax, [ecx] ; Get current value of Global Lock
mov edx, eax
and edx, not 03h ; Clear owner and pending field
lock cmpxchg dword ptr[ecx], edx ; Attempt to set it
jnz short rel10
; If not set, try again
and eax, 1
; Was pending set?
; If one is returned (we were pending) the caller must signal that the
; lock has been released using either GBL_RLS or BIOS_RLS as appropriate
ret
Although using the Global Lock allows various hardware resources to be shared, it is important to
notice that its usage when there is ownership contention could entail a significant amount of system
overhead as well as waits of an indeterminate amount of time to acquire ownership of the Global
Lock. For this reason, implementations should try to design the hardware to keep the required usage
of the Global Lock to a minimum.
The Global Lock is required whenever a logical register in the hardware is shared. For example, if
bit 0 is used by ACPI (OSPM) and bit 1 of the same register is used by SMI, then access to that
register needs to be protected under the Global Lock, ensuring that the register’s contents do not
change from underneath one environment while the other is making changes to it. Similarly if the
entire register is shared, as the case might be for the embedded controller interface, access to the
register needs to be protected under the Global Lock.
5.2.11 Definition Blocks
A Definition Block consists of data in AML format (see Section 5.4 “Definition Block Encoding”)
and contains information about hardware implementation details in the form of AML objects that
contain data, AML code, or other AML objects. The top-level organization of this information after
a definition block is loaded is name-tagged in a hierarchical namespace.
OSPM “loads” or “unloads” an entire definition block as a logical unit. OSPM will load a definition
block either as a result of executing the AML Load() or LoadTable() operator or encountering a
table definition during initialization. During initialization, OSPM loads the Differentiated System
Description Table (DSDT), which contains the Differentiated Definition Block, using the DSDT
pointer retrieved from the FADT. OSPM will load other definition blocks during initialization as a
result of encountering Secondary System Description Table (SSDT) definitions in the RSDT/XSDT.
The DSDT and SSDT are described in the following sections.
As mentioned, the AML Load() and LoadTable() operators make it possible for a Definition Block
to load other Definition Blocks, either statically or dynamically, where they in turn can either define
new system attributes or, in some cases, build on prior definitions. Although this gives the hardware
the ability to vary widely in implementation, it also confines it to reasonable boundaries. In some
cases, the Definition Block format can describe only specific and well-understood variances. In
other cases, it permits implementations to be expressible only by means of a specified set of “built
in” operators. For example, the Definition Block has built in operators for I/O space.
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