ACPI Component Architecture (ACPI-CA)

An ACPI-aware operating system must include code that accesses the ACPI BIOS, registers, and tables. It must also include an AML byte-code interpreter. This upper layer of core ACPI software is shown in Figure 1. Intel has implemented an OS-independent implementation of this layer of software called ACPI Component Architecture or ACPI-CA [2]. ACPI-CA is used by many open source operating systems including FreeBSD and Linux.

ACPI-CA provides a high-level ACPI API to the operating system. The OS uses this API to implement power management, device configuration, and thermal management. All fixed features and some access to AML names are wrapped by the exported function. But some ACPI-specific devices have to access ACPI namespace. The ACPI-CA API is shown in Table 1.

Table 1: ACPI-CA API

Function Class	Functions
ACPI subsystem management	AcpiInitializeSubsystem, AcpiEnableSubsystem, AcpiTerminate, AcpiSubsystemStatus, AcpiDisable, AcpiGetSystemInfo, AcpiFormatException, AcpiPurgeCachedObjects
memory management	AcpiAllocate, AcpiFree
ACPI table management	AcpiFindRootPointer, AcpiLoadTables, AcpiLoadTable, AcpiUnloadTable, AcpiGetTableHeader, AcpiGetTable AcpiGetFirmwareTable,
namespace interface	AcpiWalkNamespace, AcpiGetDevices, AcpiGetName, AcpiGetHandle, AcpiAttachData, AcpiDetachData, AcpiGetData
object manipulation	AcpiEvaluateObject, AcpiGetObjectInfo, AcpiGetNextObject, AcpiGetType, AcpiGetParent
event handler interface	AcpiInstallFixedEventHandler, AcpiRemoveFixedEventHandler, AcpiInstallNotifyHandler, AcpiRemoveNotifyHandler, AcpiInstallAddressSpaceHandler, AcpiRemoveAddressSpaceHandler, AcpiInstallGpeHandler, AcpiAcquireGlobalLock, AcpiReleaseGlobalLock, AcpiRemoveGpeHandler, AcpiEnableEvent, AcpiDisableEvent, AcpiClearEvent, AcpiGetEventStatus,
resource interfaces	AcpiGetCurrentResources, AcpiGetPossibleResources, AcpiSetCurrentResources, AcpiGetIrqRoutingTable,
hardware interface	AcpiSetFirmwareWakingVector, AcpiGetFirmwareWakingVector, AcpiEnterSleepStatePrep, AcpiEnterSleepState, AcpiLeaveSleepState

Operating systems that use ACPI-CA must provide it with some basic low-level functions. Intel has implemented the low-level part for Linux. A list of these functions is shown in Table 2.

Table 2: OS functions provided to ACPI-CA

Function Class	Functions
library initialization	AcpiOsInitialize, AcpiOsTerminate
semaphore control	AcpiOsCreateSemaphore, AcpiOsDeleteSemaphore, AcpiOsWaitSemaphore, AcpiOsSignalSemaphore
memory allocation	AcpiOsAllocate, AcpiOsCallocate, AcpiOsFree, AcpiOsMapMemory, AcpiOsUnmapMemory, AcpiOsGetPhysicalAddress
interrupt control	AcpiOsInstallInterruptHandler, AcpiOsRemoveInterruptHandler
process control	AcpiOsGetThreadId, AcpiOsQueueForExecution, AcpiOsSleep, AcpiOsStall
device access	AcpiOsReadPort, AcpiOsWritePort, AcpiOsReadMemory, AcpiOsWriteMemory, AcpiOsReadPciConfiguration, AcpiOsWritePciConfiguration, AcpiOsReadable, AcpiOsWritable
signal/timer control	AcpiOsGetTimer, AcpiOsSignal
diagnostic functions	AcpiOsPrintf, AcpiOsVprintf, AcpiOsGetLine, AcpiOsDbgAssert

One of the main user-visible differences between FreeBSD ACPI and Linux ACPI is the user interface. FreeBSD uses sysctl to export ACPI-related kernel variables, while Linux uses the procfs /proc filesystem to export them.