Abstract:
An advanced configuration and power management system is described which supports an unload command by identifying those objects that are to be unloaded and, rather than simply attempting to delete the objects, detaches them from the namespace while keeping them in memory in a location accessible by the system. In this way, the system avoids the problem of trying to synchronize accesses to the objects, yet the objects not longer exist in the tree, so namespace collisions are avoided.

Description:
REFERENCE TO RELATED APPLICATION 
   The present patent application claims priority from U.S. Provisional Patent Application No. 60/276,101, entitled System and Method for Unloading Namespace Devices, filed on Mar. 15, 2001. 

   BACKGROUND OF THE INVENTION 
   Power management and control is an important and evolving part of computing technology. Greater demand for computing devices with more functionality and smaller size increases the need for an efficient power management and configuration system. For instance, many computing systems, particularly in smaller, laptop computers, have moved toward hot-pluggable components so that they may be dynamically removed to conserve power and weight when not needed. ACPI (Advanced Configuration and Power Interface) is an emerging industry specification that defines a flexible and extensible interface for power and configuration management. The interface enables and supports power management through improved hardware and operating system coordination. ACPI allows the operating system to control the power states of many hardware components, and to pass information to and from some hardware components, such as the temperature of a thermal sensor or the power remaining in a battery. It should be noted that a computer system may include both hardware components that are designed to interact with an ACPI system and some that are not. 
   Stated generally, to interact with the ACPI system, hardware components provide a platform-independent description of themselves (termed “ACPI tables”) to an ACPI portion of the computer&#39;s operating system. At boot time, the ACPI subsystem builds a namespace that describes each of the several ACPI-compliant hardware devices by loading the several ACPI tables. Each ACPI table may include methods that allow the ACPI subsystem or other program modules to interact with the ACPI-compliant hardware devices. 
   More specifically, the ACPI specification defines two types of ACPI tables that are pertinent to the present discussion: a Differentiated System Description Table (DSDT) and a Secondary System Description Table (SSDT). The DSDT is part of a fixed system description loaded at system boot and it cannot be unloaded. SSDTs are description tables that may be loaded after system boot. There may be multiple SSDTs but there is only one DSDT. The theory is that an OEM can provide base system support in one table (the DSDT) and add smaller system options on an as needed basis in other tables (the SSDTs). 
   The ACPI specification provides for an Unload function to unload SSDTs that may be loaded, however, existing ACPI implementations have not implemented the Unload function for several reasons. For instance, there has not been a real need to use SSDTs in addition to the DSDT. Creating and loading a DSDT to describe the typical computing system has been sufficient because most computing systems simply don&#39;t have components which are routinely removed or that require an ACPI description to be removed from the namespace. In addition, difficult synchronization issues have acted to dissuade ACPI system developers from implementing an unload capability. 
   Moreover, removing entries from the ACPI namespace creates collision issues with entries that would subsequently be loaded. For mostly that reason, in the few instances where computing components are commonly removed from a computing system (such as undocking a laptop computer), the ACPI objects are simply left in the namespace. If another ACPI table is subsequently loaded in the namespace and has the same name or is configured to support the same device, a collision would occur. Until now, namespace collisions have been avoided by simply loading multiple ACPI descriptions for each alternative type of hardware device that may be present and then not unloading them. The number of alternative hardware devices has, until recently, been small enough that the DSDT can be loaded with each alternative hardware device without a need for SSDTs. Alternatively, the number of SSDTs that may be loaded is small enough that there has not been a need to unload them. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the problems identified above by providing a system and method for unloading objects from the namespace of an advanced configuration and power management system. Briefly stated, a software component, such as an ACPI driver, in accordance with the present invention manages the ACPI namespace. The component supports an Unload command by identifying those objects that are to be unloaded and, rather than simply attempting to delete the objects, detaches them from the namespace while keeping them in memory in a location accessible by the system. Events that may surface through the ACPI subsystem are temporarily suspended during the unload event to avoid synchronization conflicts. In this way, the described system overcomes the problems of trying to synchronize accesses to the objects, yet the objects no longer exist in the tree, so namespace collisions are avoided. 
   More specifically, the invention provides a method for unloading an object from an advanced configuration and power management system by, in response to an Unload command, disabling general purpose events and flushing various work queues. These tasks put the ACPI subsystem in a steady state. At that point, the ACPI subsystem identifies those ACPI namespace objects to be unloaded, such as from a parameter passed with the Unload command. The ACPI subsystem may, in addition, temporarily terminate access to the ACPI namespace, such as by acquiring a lock on the ACPI namespace. With access to the ACPI namespace suspended, the ACPI subsystem detaches each of the objects from the ACPI namespace while maintaining pointers to the actual locations of the objects in memory. This operation preserves the direct children of the objects to be unloaded, which allows the ACPI subsystem to find any control methods that it thinks belong to the object. 
   The ACPI subsystem may then mark the detached objects and any associated namespace objects as invalid, which informs various components of the ACPI subsystem that they should not try to execute or evaluate those objects. The ACPI system can then release the lock on the ACPI namespace, resume handling general purpose events, and issue a notification to the kernel that the objects that have been removed from the ACPI namespace. In response to the notification, the kernel may unload any device drivers that were supporting the devices associated with the unloaded objects. When a final device driver has been unloaded, the ACPI subsystem may free any data structures associated with the unloaded objects, including the namespace objects that were detached from the namespace. 
   There are several advantages to the present invention. For instance, the computer operating system does not have to make the entire unload process synchronous. In other words, in the absence of the present invention, to simply delete a device description from the namespace the ACPI subsystem would have to block until the operating system has stopped using the device. With the invention, the ACPI subsystem still has control of the device descriptions that have only been detached from the namespace but not deleted, which avoids the blocking problem. In addition, the ACPI subsystem may handle an Unload command followed by a Load command without the namespace collision problem identified above because the device description has been removed from the namespace. Moreover, an existing facility, such as an operating system kernel, maintains reference counts on the various devices in the system, then reference counting by the ACPI subsystem is not required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary computing environment which may be adapted to implement one embodiment of the present invention. 
       FIG. 2  is a functional block diagram of one exemplary ACPI system as implemented in the computer system shown in FIG.  1 . 
       FIG. 3  is a graphical representation of one possible ACPI namespace which essentially represents a working version of the ACPI tables as shown in FIG.  2 . 
       FIG. 4  is a functional block diagram illustrating in greater detail the ACPI system illustrated in  FIG. 2 , with greater emphasis on certain components as they may implement one embodiment of the present invention. 
       FIG. 5  is a functional block diagram illustrating the ACPI system shown in  FIG. 4  after particular objects have been detached from the namespace. 
       FIG. 6  is a logical flow diagram from the perspective of an AML interpreter generally illustrating a process for unloading a device from an ACPI namespace, in accordance with one embodiment of the present invention. 
       FIG. 7  is a logical flow diagram from the perspective of an ACPI driver generally illustrating a process for unloading a device from an ACPI namespace, in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Illustrative Operating Environment 
   With reference to  FIG. 1 , an exemplary system for implementing the invention includes a computing device, such as computing device  100 . In a very basic configuration, computing device  100  typically includes at least one processing unit  102  and system memory  104 . Depending on the exact configuration and type of computing device, system memory  104  may be volatile (such as RAM), non-volatile (such as ROM, flash memory, etc.) or some combination of the two. System memory  104  typically includes an operating system  105 , one or more application programs  106 , and may include program data  107 . This basic configuration is illustrated in  FIG. 1  by those components within dashed line  108 . 
   Computing device  100  may also have additional features or functionality. For example, computing device  100  may also include additional data storage devices (removable and/or non-removable) such as, for example, magnetic disks, optical disks, or tape. Such additional storage is illustrated in  FIG. 1  by removable storage  109  and non-removable storage  110 . Computer storage media may include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, program modules or other data. System memory  104 , removable storage  109  and non-removable storage  110  are all examples of computer storage media. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by computing device  100 . Any such computer storage media may be part of device  100 . Computing device  100  may also have input device(s)  112  such as keyboard, mouse, pen, voice input device, touch input device, etc. Output device(s)  114  such as a display, speakers, printer, etc. may also be included. All these devices are well know in the art and need not be discussed at length here. 
   Computing device  100  may also contain communications connection(s)  116  that allow the device to communicate with other computing devices  118 , such as over a network. Communications connection(s)  116  is an example of communication media. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. The term computer readable media as used herein includes both storage media and communication media. 
   Illustrative Configuration Management System 
     FIG. 2  is a functional block diagram of an ACPI system  200  as implemented in the computer system  100  of FIG.  1 . The ACPI system  200  shown in  FIG. 2  generally illustrates a configuration management system that may benefit from the present invention. Although illustrated in FIG.  2  and described here generally, certain components of the ACPI system  200  are shown in greater detail in  FIG. 4  together with a detailed discussion of those components and how they implement one embodiment of the present invention. Although the present invention may be described here with reference to the ACPI configuration management system, there is no intention to limit the present invention to ACPI. Rather, the present invention is intended to operate with and provide benefits with any operating system, architecture, and/or configuration management system. 
   As shown, the application programs  106  may interface with a kernel  201 , which is a part of the operating system  105 , generally through application programming interface (API) calls or the like. The kernel  201  can be generally considered as one or more software modules that are responsible for performing many operating system functions. One such function is passing information between the application programs  106  and the lower level components of the ACPI system  200 , such as the ACPI driver  202  (described below) and various device drivers (e.g., device driver  211 ). 
   The kernel  201  interfaces with Operating System Power Management (OSPM) system code  203 . The OSPM system code  203  comprises one or more software modules that may be a part of the operating system  105  and that may be used to modify the behavior of certain components of the computer system  100 , typically to conserve power in accordance with pre-configured power conservation settings. As is generally known, the various device drivers  211  interface with and generally control the hardware installed in the computer system  100 . 
   The ACPI driver  202  is, generally speaking, a module that controls the functioning of much of the ACPI system  200 . The ACPI driver  202  may be supplied as part of the operating system  105  or as a separate component. In the described system, the ACPI driver  202  is loaded during system start-up at the base of a tree of devices where it acts as an interface between the operating system  105  and the ACPI BIOS  220 . The ACPI driver  202  is responsible for many tasks, such as initiating and maintaining the ACPI system  200  by populating an ACPI namespace  209  (illustrated in FIG.  3  and described below) at system startup, loading and unloading description blocks from the ACPI namespace at run time, handling certain general purpose events triggered by ACPI hardware, handing off other general purpose events to modules registered to handle those events, and the like. 
   A driver communicates with other drivers and the operating system components (e.g., an I/O manager or the kernel  201 ), for example in the Windows® 2000 operating system, by passing messages called I/O request packets (IRPs) up and down a “driver stack.” As will be understood by those skilled in the art, drivers for a particular hardware device may be “stacked” such that messages directed either down to the hardware device or back up to the operating system (or other program module) are passed through a chain of drivers in a driver stack before reaching their destination. Applications and systems, such as the ACPI system  200 , may insert a driver, such as the ACPI driver  202 , in a driver stack to add functionality to the hardware device. 
   To that end, for each device described in an ACPI namespace  209  (described below), the ACPI driver  202  creates either a filter Device Object (filter DO) or a Physical Device Object (PDO) in the driver stack for that device. If the device is capable of being enumerated by an element of another subsystem, such as a Plug-n-Play subsystem, that element of the other subsystem may create the PDO for the device and the ACPI driver  202  may put a filter DO on top of the PDO. The ACPI system  200  provides power management features to the device stack by means of these device objects. For more information on filter DOs, PDOs and Functional DOs (FDOs), refer to the Microsoft Windows® 2000 Driver Development Kit, publicly available from the Microsoft Corporation of Redmond, Wash. 
   The ACPI driver  202  makes use of several components when performing the functions of the ACPI system  200 . One component is the ACPI BIOS  220 , which refers to the portion of system firmware that is compatible with the ACPI specification. Generally stated, the ACPI BIOS  220  is part of the code that boots the machine (similar to the BIOS present in most conventional computer systems) and implements interfaces for power and configuration operations, such as sleep, wake, and some restart operations. The ACPI BIOS  220  contains definition blocks used to construct ACPI Tables  222 , as is described in greater detail below. Note that although the BIOS  26  and the ACPI BIOS  220  are illustrated as separate components in  FIG. 2 , they may be implemented as one component in the computer system  100 . 
   The ACPI Tables  222  include at least one definition block (e.g., a DSDT described later) that contains data, control methods, or both. Each set of data or control methods defines and provides access to a respective hardware device. The definition blocks are written in an interpreted language called ACPI Machine Language (AML), the interpretation of which is performed by an AML interpreter  205 . One such definition block, a Differentiated Definition Block (DDB), describes the base computer system. Other definition blocks may be provided to describe additional hardware devices. One ACPI Table  222 , known as a Differentiated System Description Table (DSDT) describes the base computer system, that is, the DSDT contains a Differentiated Definition Block (DDB), which describes the root system. The DSDT is like other data blocks, except that it cannot be unloaded. 
   There may be one or more Secondary System Descriptor Tables (SSDTs), each of which describes hardware components in addition to the base computer system. The ACPI Specification provides that SSDTs may be unloaded, although until now that capability has been unrealized. The tables also include header data structures that contain information about what the block contains, for example, whether it is a Differentiated System Description Table (DSDT) or a Secondary System Descriptor Table (SSDT). Other definition blocks may be provided to describe additional ACPI devices. 
     FIG. 3  is a graphical representation of one possible ACPI namespace  209  which essentially represents a working version of the ACPI tables  222 . The ACPI Namespace  209  is a hierarchical tree structure in protected memory that contains named objects which describe the ACPI-aware devices installed in the computer system  100 . The objects may be data objects, control method objects, bus/device package objects, or the like. As mentioned above, at boot time, the operating system  105  (via the ACPI driver  202 ) reads the ACPI tables  222  from the ACPI BIOS  220  and loads certain definition blocks (e.g., the DDB) from the ACPI tables  222  to construct the ACPI namespace  209 . In accordance with the teachings of the invention, the ACPI driver  202  may dynamically change the contents of the ACPI namespace  209  at run time by loading and/or unloading additional definition blocks from the ACPI Tables  222 . 
   As mentioned, a device in the ACPI namespace  209  may contain control methods. A “control method” is a software module that defines how the ACPI system  200  performs a hardware-related task. For example, the ACPI system  200  may invoke a control method to read the temperature of a thermal zone. Control methods are written in AML, are stored in the definition blocks within the ACPI BIOS  220 , and are loaded into the ACPI namespace  209 , typically at system boot up. Once in the ACPI namespace  209 , the control methods may be invoked by other components in the ACPI system  200 , such as device drivers or the like, and are then interpreted and executed by a virtual machine in the AML Interpreter  205 . The use and structure of the ACPI namespace, and the objects within, are described in greater detail below with respect to FIG.  3 . 
   The illustrative ACPI namespace  209  shown in  FIG. 3  includes a namespace root  301 , several illustrative branches under the root  301 , and several other objects of various types. For instance, power resource objects, such as power resource object \_PID 0   304 , may reside under the namespace root  301 . The object \PID 0   304  may define a power resource for a particular device, such as an on-off switch for an IDE device. The \_SB namespace  311  includes namespace objects that define ACPI-compliant components attached to the system bus. One example of a such namespace object is the PCI bus namespace object  312 . Each namespace object may contain other objects, such as data objects  313 , control methods, or other namespace objects (e.g., IDE namespace objects IDE 0   314  and IDE 1   321 ). Several control methods may be loaded in the ACPI namespace  209  in connection with various objects. For example, control methods related to the status and maintenance of a particular power resource object may be loaded within the scope of the power resource object, such as control methods  305  under the power resource object \PID 0   304 . 
     FIG. 4  is a functional block diagram illustrating in greater detail the ACPI system illustrated in  FIG. 2 , with greater emphasis on certain components as they may implement one embodiment of the present invention. Several components illustrated in  FIG. 4  are similar to those illustrated in FIG.  2  and are shown with similar element numbers for simplicity. For example, the ACPI driver  202 , the AML interpreter  205 , the ACPI registers  224 , and the kernel  201  each exhibit similar functionality to the similarly-identified components illustrated in  FIG. 2. A  namespace  405  is also illustrated and is similar to the ACPI namespace  209  illustrated in FIG.  2  and FIG.  3 . The namespace  405  illustrated in  FIG. 4 , however, includes simplified objects for illustrative purposes only. The ACPI driver  202  may also maintain other data structures  406  that contain information about the objects in the namespace  405 . The other data structures  406  may contain device-specific information for more rapid access by the ACPI driver  202 . 
   Application  401  is shown that may interact with the kernel  201  or other components of the ACPI system  400 . Other components of the ACPI system  400  include a GPE event mask  407 , which presents to the ACPI system  400  a current state of the ACPI registers  224  so that the ACPI system  400  may identify the existence and the originator of any GPE events raised by ACPI-compliant hardware, such as hardware device  410 . The hardware device  410  is accessed by the computing system through the use of a driver stack  412  that includes the ACPI driver  202 , a device object  420 , and may include filter drivers, such as filter driver  414  and filter driver  416 . In this discussion, a device object (sometimes referred to as a Physical Device Object or PDO) is a kernel-mode object, defined by an I/O manager portion of the operating system  105 , that represents a physical hardware device. The device object may be created by an underlying bus driver when the physical device is enumerated. Each object in the namespace  405  is associated with a device object, such as device object  420 , for the particular hardware device which the namespace object represents. The filter drivers ( 414 ,  416 ) are inserted into the driver stack  412  by other processes and provide additional functionality to the hardware device  410 . 
   A device extension  422  is associated with the ACPI driver  202  and contains information used by the ACPI driver  202 , such as at least a pointer to a corresponding device-type object (e.g., object A) and additionally may have pointers to other namespace objects, such as method objects (e.g., object B), associated with that device-type namespace object. In this discussion, a device extension is a data structure associated with a device object that includes context information, such as device state information, spin locks, driver data, and other device-object information. In this embodiment, the device extension  422  includes a reference count field that includes a reference count for each resource that refers to the device object  420 , such as the device-type namespace object, any child device extensions, and the like. Preferably, that reference count field is managed by the ACPI driver  202  but is based on a reference count performed by the kernel  201  for each device object enumerated in the system. By making use of the reference counting already performed by the kernel  201 , the ACPI system  200  need not reproduce the reference counting that is already being performed. It should be noted that the ACPI driver  202  may also maintain reference counts for its own convenience in addition to any external reference count. The device extension  422  also includes pointers to those resources that refer to the device object  420 , including objects in the namespace  405 . 
   The operation of the components is generally described with reference to an example of an unload operation. In this example, an unload event is generated in response to the hardware device  410  either being removed or indicating that it is about to be removed (e.g., a hardware switch or button being pressed). The unload event may be generated by raising a GPE signal in the ACPI registers  224 , which is transmitted, via the GPE event mask  407 , to the AML interpreter  205  as an instruction to unload the hardware device  410 . Alternatively, the unload event may be generated in software, such as by the operating system in response to a user request to remove a piece of hardware or the like. In response to the unload event, the AML interpreter  205  first (prior to actually beginning the unload operation) informs the ACPI driver  202  that an unload operation is about to occur. This pre-notification allows the ACPI driver  202  to do some pre-processing, like disabling system power management and other miscellaneous events. 
   In addition, the ACPI driver  202  may disable general purpose events from being generated and sent to the AML interpreter  205 . Disabling GPE signals at the beginning of the unload operation avoids potential problems that could otherwise occur during the unload operation. For instance, the ACPI driver  202  does not know which hardware device, such as a laptop docking station, is associated with which GPE pins. Thus, if any part of the ACPI namespace  405  is being unloaded, disabling all the GPE pins is the safest way to avoid improperly attempting to service a GPE event for a piece of hardware whose definition is being unloaded. For that reason, the GPE enable pins are cleared at the start of the unload operation so that the GPE mask  407  may be rebuilt at the conclusion of the unload operation. 
   Next, the AML interpreter  205  informs the ACPI driver  202  which entries in the namespace  405  are disappearing. For example, the unload message may have indicated that the hardware device  410  is being removed or otherwise made unavailable, and the namespace  405  may include one or more objects associated with the hardware device  410 . For instance, the objects within box  431  may be associated with the hardware device  410 , and as such are identified to the ACPI driver  202 . The object labeled “A” may be an ACPI device-type namespace object, and the object labeled “B” may be a control method or other namespace object associated with namespace object “A.” 
   In response to the notice from the AML interpreter  205 , the ACPI driver  202  severs any links in its data structures  406  between the disappearing namespace entries (objects  431 ) and the parents of those disappearing entries. The ACPI driver  202  maintains reference counts in the parent for each child, so the ACPI driver  202  decrements those reference counts as appropriate. The ACPI driver  202  also marks the parent as needing to be updated and indicates to the kernel  201  that the hardware device  410  no longer exists so that the kernel  201  may begin tearing down the driver stack  412  associated with the hardware device  410 . It should be noted that the kernel  201  may wait until no running applications or other processes have open handles to the unloaded hardware device before tearing down the driver stack. 
   Concurrently, the AML interpreter  205  unlinks the disappearing entries (objects  431 ) from the namespace  405  and marks any control methods (e.g., namespace object “B”) associated with the disappearing entries as not executable. If an attempt is made to execute a method marked as not executable, the AML interpreter  205  would either return a failure code or success without actually executing the method. The AML interpreter  205  detaches the disappearing entries from the namespace tree  405  without severing the links between the ACPI devices (e.g., namespace object “A”) and their children (e.g., namespace object “B”). Preserving these relationships makes implementation simpler because it allows for certain assumption to remain true, such as caching pointers to certain namespace objects. Moreover, by detaching the entries from the namespace, new entries may be added to the namespace  405  in place of the unloaded entries without fear of a namespace collision. 
   Once the AML interpreter  205  has completed cutting the disappearing entries from the namespace, it notifies the ACPI driver  202  that the AML interpreter&#39;s portion of the unload operation is complete. In response to that notification, the ACPI driver  202  may re-enable any of the support that was disabled at the start of the unload operation. For example, the ACPI driver  202  may rebuild the GPE mask  407  by looking at the GPE Control Methods in the ACPI namespace  405  to calculate a bit mask. The ACPI driver  202  may additionally look at the devices contained in the namespace to see which pins they use for Wake-Support and add those pins to the bit mask. 
   Referring to  FIG. 5 , there are now a number of objects  431  (including object “A” and object “B”) floating in memory that are not attached to the ACPI namespace  405 . However, as mentioned above, the device extension  422  associated with the ACPI driver  202  includes a pointer  451  to the ACPI device-type namespace object (i.e., object “A”) that has been cut from the namespace  405 . For that reason, the operating system  105  and the ACPI driver  202  may still access those objects. In such a case, because any control methods or data structures associated with the objects have been marked as invalid, the accessing entity (e.g., the kernel  201 ) is notified that the device no longer exists and may respond in a graceful manner, such as with a surprise remove event or the like. 
   As mentioned above, the ACPI driver  202  keeps track of the resources that point to the device extension  422 . The kernel  201  begins deleting those resources and tearing down the driver stack  412  ( FIG. 4 ) when notified by the ACPI driver  202  of the unload operation. As each resource pointing to the device extension  422  is destroyed, the reference count  453  in the device extension  422  is decremented. Thus, the reference count  453  will eventually reflect only a single referring entity, the corresponding namespace object. At that point, the ACPI driver  202  may safely notify the AML interpreter  205  that the namespace objects  431  that were cut are no longer being referenced by any process outside of the ACPI system  200 , and it is safe to free the memory associated with the unloaded namespace objects  431 . In response, the AML interpreter  205  may destroy the unloaded namespace objects  431  and clean up any memory allocated to them. 
   If the unloaded objects  431  do not have a corresponding device object, then one of three situations applies: the objects have zero, one, or more than one reference. First, if the data structure has only one reference, then there is not an error and the operations outlined above apply to free the memory. However, zero references or more than one reference indicates a problem with the reference counting. For instance, zero references would not normally be a valid case and would indicate a bug in the driver. More than one reference may indicate to the system that there is an error in the ACPI tables themselves and it will react accordingly. It should be noted that the case where there are two references can occur even if the unloaded objects did not have corresponding device object. The particular case is where an unloaded object has a child device that is not part of the table being unloaded. The operating system treats this case as an error in the ACPI bios. 
   While the invention has been described thus far with reference to functional block diagrams of software components implementing embodiments of the invention, it may also be described with reference to logical flow diagrams illustrating processes for implementing embodiments of the invention. The logical operations making up the described embodiments of the present invention may be referred to variously as operations, structural devices, acts, or modules. It will be recognized by those skilled in the art that these operations, structural devices, acts, or modules may be implemented in software, in firmware, in special purpose digital logic, or any such combination without deviating from the spirit and scope of the present invention. 
     FIG. 6  is a logical flow diagram from the perspective of the AML interpreter  205  generally illustrating a process  600  for unloading a device from the ACPI namespace  405 , in accordance with one embodiment of the present invention. The process  600  begins at block  601 , where an unload event has been generated indicating that a hardware device either has disappeared or will shortly disappear. Processing continues at block  602 . 
   At block  602 , the AML interpreter  205  informs the ACPI driver  202  that an unload event has been generated. In this embodiment, the ACPI driver  202  is provided notice at the beginning of the unload operation so that the ACPI driver  202  may begin any preprocessing activities that may be helpful or necessary for the unload operation to proceed properly. For example, the ACPI driver  202  may clear the GPE mask  407  and disable the GPE enable pins (within the ACPI registers  224 ) until the AML interpreter  205  has had time to conduct its portion of the unload operation. In this way, new GPE events will not be created until the unload operation is complete. Other preprocessing activities may also be performed. 
   At block  604 , the AML interpreter  205  identifies the namespace objects which are affected by the unload operation and passes that information to the ACPI driver  202 . For instance, the AML interpreter  205  may first acquire a lock on the ACPI namespace  405  and then walk the namespace to build a list of objects within the ACPI namespace  405  that are intended to be unloaded. It should be noted that the ACPI driver  202  is only interested in device-type namespace objects at this point because the linkages are maintained between those objects and their children (e.g., control methods, static values, packages, and the like). The information describing the affected namespace objects is then passed to the ACPI driver  202  to be handled as appropriate (and as described more fully below). The AML interpreter  205  may release the lock it acquired on the ACPI namespace  405  at this point (e.g., when it has finished walking the namespace to build the list). 
   At block  606 , the AML interpreter  205  detaches the affected ACPI namespace objects from the namespace  405 . The AML interpreter  205  may walk the namespace  405  and sever any links between the affected objects and their parents. The AML interpreter  205  may set a flag in the affected objects indicating that they are being removed. It may be helpful to note again that the affected device-type namespace objects are detached from their parents, while any children of those objects remain linked. In other words, those namespace objects that may be found in memory through the use of a device extension are severed from their parent, but any children that do not have a corresponding device extension remain linked to their parent so that the ACPI driver  202  continues to have a link to those objects. 
   At block  608 , once the AML interpreter  205  has detached the affected objects from the namespace  405 , the AML interpreter  205  informs the ACPI driver  202  of the success and that the unload event is complete. In other words, the AML interpreter  205  indicates to the ACPI driver  202  that the AML interpreter  205  has completed its portion of the unload operation. At that point, the affected ACPI namespace objects remain accessible to the ACPI driver  202  but have been removed from the namespace  405 . For that reason, the AML interpreter  205  may reload a new table in the namespace  405  to support a similar hardware device without fear of a collision with the unloaded objects. 
   At decision block  610 , the AML interpreter  610  continues normal processing until a notification that the ACPI driver  202  has completed its portion of the unload operation is received. The AML interpreter  205  may continue to perform routine ACPI functions and operations while awaiting that notification. When the notification is received from the ACPI driver  202 , at block  612 , the AML interpreter  205  may destroy the affected objects and any associated data structures or the like. It will be appreciated that this notification may happen several times. For instance, the operating system may not unload device drivers in such a way that a single call to free deleted namespace objects is possible. The notification from the ACPI driver  202  indicates that the device corresponding to the affected objects is no longer in use (or referred to by other processes) and the memory may be freed and the process ends at ending block  614 . 
     FIG. 7  is a logical flow diagram from the perspective of the ACPI driver  202  generally illustrating a process  700  for unloading a device from the ACPI namespace  405 , in accordance with one embodiment of the present invention. The process  700  begins at block  701  where an unload operation of ACPI components associated with a hardware device has been begun. 
   At block  702 , the ACPI driver  202  receives notification from the AML interpreter  205  that the unload operation has begun. In response to the notification, at block  704 , the ACPI driver may perform any preprocessing activity that may be necessary or helpful to the unload operation. In one example, as described above, the ACPI driver  202  may clear the GPE mask  407  and the GPE enable pins until the completion of the unload operation. 
   At block  706 , the ACPI driver  202  receives from the AML interpreter  205  a notification of the affected ACPI namespace objects and the affected hardware devices. In addition, the ACPI driver  202  may set flags in the parent of any affected object indicating that the parent now has invalid relations and should be reevaluated at the conclusion of the unload operation. At block  708 , the ACPI driver  202  passes that information to the kernel  201  so that the kernel  201  may begin tearing down any driver stack associated with the hardware device being unloaded. 
   At decision block  710 , the ACPI driver  202  continues processing normally until a notification is received from whatever facility (e.g., the kernel  201 ) tracks reference counts that the affected namespace objects are no longer in use by any outstanding tasks or processes. In this particular embodiment, a device extension associated with a device object in the driver stack for the hardware device is used to track the reference count. When a reference count within that device extension indicates that the only reference remaining is to the affected namespace objects themselves, then the ACPI driver  202  determines that it is safe to destroy the affected objects and free the memory. If there are more references remaining, the ACPI driver  202  doesn&#39;t do anything. 
   At block  712 , the ACPI driver  202  notifies the AML interpreter that the affected objects are no longer in use, indicating that it is safe to destroy the objects and free the memory. The AML interpreter  205  may respond to the notification by doing exactly that, destroying the now-unused objects and freeing the memory. At block  714 , once the AML interpreter has informed it of success, the ACPI driver  202  performs any post-processing activities that may be appropriate, such as setting the appropriate GPE enable pins to reactivate the GPE events, and the like. 
   As can be seen from the above description, implementations of the invention make possible an unload operation of devices from an ACPI namespace. The above specification, together with the attached drawings, provide a complete description of the manufacture and use of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the appended claims.