Patent Publication Number: US-7584473-B2

Title: Highly componentized system architecture with loadable virtual memory manager

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. patent application Ser. No. 10/754,112 filed on Jan. 8, 2004 and issued as U.S. Pat. No. 7,409,694 on Aug. 5, 2008, which is a continuation of U.S. patent application Ser. No. 09/282,227 filed on Mar. 31, 1999 and now abandon, which claims priority from a provisional U.S. Patent Application No. 60/099,562 filed on Sep. 9, 1998. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention is related to computer operating systems and in particular to a computer operating system which is highly componentized and has dynamically loadable operating features which may be loaded and unloaded during system run time. 
   2. Background Art 
   The progressive computerization of society involves a number of diverse computing platforms beside the general-purpose computer:
         Embedded control systems, including consumer devices, intelligent sensors and smart home controls.   Communication-oriented devices such as digital cell phones and networking infrastructure.   Programmable peripherals and microcontrollers.       

   In all these cases, the general-purpose platform approach is either not applicable, or it is prohibitively expensive. The microprocessor might be a DSP, a VLIW, or a micro-controller; the memory budget is severely restricted; there might be no MMU; the network connection might be sporadic; and Real-Time support is essential. 
   Current operating systems are either inflexible, big, lack Real-Time support, have complex hardware requirements, or are so special purpose that good development tools are unavailable and code reusability is low. 
   Microkernels [Black92, Engler95] attempt to modularize the operating system. But they confuse modularity with security by mandating that system services be in separate address spaces. Many of the services moved into separate server processes are still necessary for these systems to function and often the services have to trust each other. 
   C++ and Java provide objects at a very fine granularity level, and they are extremely successful with application programmers. Unfortunately, both languages confine their objects to a single address space. Object Linking and Embedding (OLE) [Brockschmidt95] and other similar systems extend objects across address spaces and across machine boundaries. OLE seamlessly integrates independently developed components. When editing an Excel spreadsheet inside a Word document it is in fact the Excel process that operates on objects inside of Word&#39;s address space. Unfortunately, it only works for user mode applications. 
   Modularity has always been an important paradigm in software design. By breaking a complex system into pieces, the complexity becomes more manageable. Address spaces provide security by installing virtual-memory based firewalls between applications. These two issues are orthogonal, but the distinction has been lost in systems research that has been concentrating on so-called microkernels. These issues have been discussed in the following publications:
     [Bershad95] Brian Bershad, S. Savage, P. Pardyak, E. G. Sirer, M. Fiuczynski, D. Becker, S. Eggers, C. Chambers.  Extensibility, safety and performance in the Spin operating system . In 15 th  ACM Symposium on Operating System Principles, pages 267-284, Copper Mountain Resort, Colo., December 1995.   [Black92] David Black, David Golub, Daniel Julin, Richard Rashid, Richard Draves, Randall Dean, Alessandro Forin, Joseph Barrera, Hideyuki Tokuda, Gerald Malan, David Bohman.  Microkernel Operating System Architecture and Mach . In 1 st  USENIX Workshop on Micro-kernels and Other Kernel Architectures, pages 11-30, Seattle, April 1992.   [Brockschmidt95] K. Brockshmidt.  Inside OLE, Second ed . Microsoft Press, Redmond Wash., 1995.   [Cheriton94] David Cheriton, Kenneth Duda.  A Caching Model of Operating System Kernel Functionality . In 1 st  Symposium on Operating Systems Design and Implementation, Seattle, 1994.   [Cheriton88] David Cheriton.  The V distributed system . In Communications of the ACM, pages 314-333, March 1988.   [Draves97] Richard Draves, Scott Cutshall.  Unifying the User and Kernel Environments . Microsoft Research Technical Report MSR-TR-97-10, 16 pages, March 1997   [Engler95] D. R. Engler, M. F. Kaashoek, J. O&#39;Toole Jr.  Exokernel: an operating system architecture for application - specific resource management . In 15 th  ACM Symposium on Operating System Principles, pages 251-266, Copper Mountain Resort, Colo., December 1995.   [Ford97] Bryan Ford, Godmar Back, Greg Benson, Jay Lepreau, Albert Lin, Olin Shivers.  The Flux OSKit: A Substrate for Kernel and Language Research . In Proceedings of the 16th ACM Symposium on Operating Systems Principles, pages 38-51. ACM SIGOPS, Saint-Malo, France, October 1997.   [Golub90] David Golub, Randall Dean, Alessandro Forin, Richard Rashid.  UNIX as an application program . In USENIX 1990 Summer Conference, pages 87-95, June 1990.   [Helander94] Johannes Helander.  Unix under Mach: The Lites Server . Master&#39;s thesis, 71 pages, Helsinki University of Technology, 1994.   [Hildebrand92] D. Hildebrand.  An architectural overview of QNX . In 1 st  USENIX Workshop on Micro-kernels and Other Kernel Architectures, pages 113-126, Seattle, April 1992.   [ISI95] Integrated Systems Inc.  pSOSystem System Concepts . Part No. COL0011, May 1995, ISI, Sunnyvale Calif.   [Jones96] Michael B. Jones, Joseph S. Barrera, III, Richard P. Draves, Alessandro Forin, Paul J. Leach, Gilad Odinak.  An Overview of the Rialto Real Time Architecture . In Proceedings of the 7 th  ACM SIGOPS European Workshop, pagg. 249-256, September 1996.   [Jones97] Michael B. Jones et al.  CPU Reservations and Time Constraints: Efficient, Predictable Scheduling of Independent Activities . In Proceedings of the 16th ACM Symposium on Operating Systems Principles, pages 198-211. ACM SIGOPS, Saint-Malo, France, October 1997.   [Jones 97b] Michael B. Jones.  The Microsoft Interactive TV System: An Experience Report . Microsoft Research Technical Report MSR-TR-97-18, July, 1997.   [Julin91] Daniel Julin, Jonathan Chew, Mark Stevenson, Paulo Guedes, Paul Neves, Paul Roy.  Generalized Emulation Services for Mach  3.0 : Overview, Experiences and Current Status . In Proceedings of the Usenix Mach Symposium, 1991.   [Lee98] Dennis Lee, Patrick Crowley, Jean-Loup Baer, Tom Anderson, Brian Bershad.  Execution characteristics of desktop applications on Windows NT . In Proceedings of the 25 th  International Symposium on Computer Architecture, Barcelona, Spain, June 1998.   [Liedtke95] Jochen Liedtke.  On  μ- kernel construction . In 15 th  ACM Symposium on Operating System Principles, pages 237-250, Copper Mountain Resort, Colo., December 1995.   [Mogul87] Jeffrey Mogul, Richard Rashid, Michael Accetta.  The Packet Filter: an Efficient Mechanism for User - level Network Code . In 11 th  ACM Symposium on Operating System Principles, November 1987.   [Rashid87] Richard Rashid.  From RIG to Accent to Mach: The evolution of a network operating system . Carnegie Mellon University Technical Report, August 1987.   [Rozier88] M. Rozier, A. Abrassimov, F. Armand, I. Boule, M. Gien, M. Guillemont, F. Hermann, C. Kaiser, S. Langlois, P. Leonard, W. Neuhauser.  CHORUS distributed operating system . In Computing Systems, pages 305-370, Vol. 1-4, 1988.   [Young89] Michael Wayne Young.  Exporting a User Interface to Memory Management from a Communication - Oriented Operating System . Ph.D. Thesis CMU-CS-89-202, Carnegie Mellon University, November 1989.   

   Mach [Black92] defined an interface for external memory managers [Young89] and was able to split virtual memory into functionally distinct parts, allowing part of the functionality to reside outside the privilege-level component (the “kernel”). Mach also separated part of the Unix operating system services out of the kernel [Golub90, Helander94], achieving modularity but limited additional functionality. The multiserver project [Julin91] went further in the modularization by splitting the Unix services into multiple independent servers. The componentization added structure and generality to the services. However, keeping the services in multiple address spaces did not add any security or robustness since components had to be available and trusted in any case. The most interesting new functionality was in the ability to emulate multiple OS interfaces, at the same time. 
   Contemporary research systems take the minimization of the “kernel” concept even further by defining even lower level abstractions and demonstrating the ability to split states across address space boundaries. None of these systems defines a new application programming interface (API) different from the Unix they emulate. The API that their predecessors [Rashid87, Cheriton88, Rozier88] did define, based on RPC and message exchanges, were not very successful with programmers. 
   The Cache Kernel [Cheriton94] uses Mach&#39;s external memory manager metaphor uniformly for the management of all kernel objects. Threads, Address Spaces and User Kernels are all handled through this pagein-pageout logical interface. An actual application is statically linked with a number of libraries, which provide default implementations of the required User Kernel components (VM, scheduling, IPC). This offers some flexibility by letting untrusted applications have their custom application kernel. Overall complexity is not decreased; it seems an application kernel would have to be as complicated as any other operating system. The ability to write your own application kernel would seem useful for a limited number of users, in teaching operating systems for instance. 
   Exokernel [Engler95] goes along the same lines demonstrating further ability to run operating system code in user mode. While it is highly successful in this and offers some added flexibility, it is questionable whether the premises differ from that of microkernels. The main contribution is in the mechanisms for application-specific resource management. 
   [Liedtke95] argues that microkernels have failed exclusively on performance grounds, and that poor performance is their only cause for inflexibility. Our argument is the opposite: inflexibility is inherent in the design, and leads to unavoidable inefficiencies that can only be mitigated by good implementations, never eliminated. 
   Spin [Bershad95] addresses the issue of expensive address space crossings by letting user code compiled by a trusted compiler run inside the kernel. This can be viewed as smart proxies that can do a lot of the work locally that otherwise would require communication. It is similar to loading packet filters into network drivers [Mogul87], to running database application query language inside database engines [reference], or to sandboxing Java applets. Applying these techniques to operating systems is beneficial when a trust boundary must be crossed and the cost would otherwise be high. It does not address the issue of whether or not a trust boundary is necessary. Spin uses an object-based language (Modula3) to provide extensibility. The pointer-safety property of the language is what permits execution of untrusted code in privileged mode. Trust relationships, as in the user-versus-kernel separation, should not dominate system decomposition. It is important to return to a global system view. The present invention addresses the issue of how to minimize the set of base services, and how to dynamically extend them on demand. 
   [Ford97] shows how a base set of system components can be composed in different ways to build an operating system kernel. The granularity is fairly coarse, and the techniques are limited to static linking. Components that should be of interest to OS researchers (VM, IPC, scheduling, etc.) cannot be replaced or removed, neither statically nor dynamically. The decomposition is otherwise limited to the “OS” component; it is not meant as a whole-system approach. This does not go far enough in the componentization. It provides a few convenient components, such as bootstrap loader and filesystems, but is mostly concerned with reusing existing device drivers and Unix code. It fails to componentize the core kernel services or extend the paradigm to applications. 
   Componentization and location independence has also been studied in the context of filesystems and network protocols [Maeda93] and in a number of existing embedded systems, such as pSOS [ISI95]. In a typical embedded system there is no loader, and components can only be chosen at static link time when the load image is built. Services are extremely limited, sometimes exclusively to the scheduling component. The number and priority of threads might have to be specified statically as well. 
   Chorus [Rozier88] can be configured to use either a page-based or a segment-based VM system. 
   SUMMARY OF THE INVENTION 
   A preferred embodiment of the invention is directed to a flexible system architecture that is suitable for a wide range of applications. The system is built out of minimal but flexible components, which can be deployed as needed. Instead of mandating a fixed set of operating system services and hardware requirements, the system preferably provides a menu of well-defined components that can be chosen to compose a complete system depending on hardware capabilities, security needs, and application requirements. 
   Dynamic loading and unloading of components provides the flexibility that lets the system adapt to changing requirements. 
   The componentization makes it possible to change the implementation of a component without affecting the rest of the system. Minimalism makes it possible to use the system with severely restricted hardware budgets. It also forces the system to be understandable and flexible. Software components, when possible, are not tied to a particular layer of the system, but can be reused. For example, the same code that implements the system physical memory heap is used to provide application heaps over virtual memory. The key system building blocks are componentized. This includes the virtual memory system, IPC, and the scheduler in addition to filesystems, networking, drivers, and protection policies. Preferred embodiments of the present invention extend object-orientation both across address spaces and across protection levels. 
   In a preferred embodiment, components are located in separate address spaces only when there is a real reason for it, such as security or specific address space requirements. Thus, the price of multiple address spaces (and transitions thereof) is paid only where needed. 
   The present invention is directed to a loadable virtual memory manager, and generally to a computer operating system capable of supporting application programs running in a computer having a working memory, the computer operating system including a kernel resident in the working memory at run time, and a loadable virtual memory manager resident at link time outside of the working memory and dynamically loadable into the working memory at run time upon demand of one of the application programs. The kernel includes a loader for loading the virtual memory manager into the working memory in response to a demand from one of the application programs. The computer is able to access a storage memory separate from the working memory, the loadable virtual memory manager residing at link time in the storage memory. The loader loads the virtual memory manager from the storage memory to the working memory. The loadable virtual memory manager is removable from the working memory upon lack of demand therefor by the application programs. 
   The kernel of the operating system further includes a virtual memory fault handler for handling virtual memory faults occurring in the absence of the virtual memory manager in the working memory. The kernel of the operating system includes a Namespace for registering the virtual memory manager upon the virtual memory manager being loaded into the working memory, whereby the virtual memory manager becomes available to each application program through the Namespace. Preferably, the Namespace includes an object supporting plural interfaces exposable by the application programs, the plural interfaces including a query interface through which an application program invokes the virtual memory manager, an add reference interface by which the Namespace manages each request for the virtual memory manager from any of the application programs, and a release reference interface, by which Namespace manages termination of the virtual memory manager from the working memory. The loader is responsive to the Namespace in maintaining the virtual memory manager in the working memory whenever the add reference interface has a reference to the virtual memory manager. 
   The virtual memory manager includes an object with plural interfaces exposable to other objects, the plural interfaces of the virtual memory manager including a virtual memory space interface(VMSpace), a virtual memory map interface(VMMap), a virtual memory view interface (VMView); a query interface, and a virtual memory factory interface (VMFactory). The VMSpace interface provides plural methods which control access by the application programs to virtual memory locations of the virtual memory manager and preferably includes plural memory regions and a skip list linking the plural memory regions across areas of memory described by the regions. 
   The VMMap interface provides plural methods which control use by the application programs of virtual memory locations of the virtual memory manager. The VMView interface provides plural methods which control views by the application programs of virtual memory locations of the virtual memory manager. The VMView interface can provide different views of the same memory space to different ones of the application programs and can support context switching. 
   The VMFactory interface provides constructors of objects of the virtual memory manager. The VMFactory interface provides a Namespace interface exposable by applications which enumerates all the objects of the virtual memory manager independently of the Namespace. 
   In one embodiment, the operating includes plural virtual memories resident outside of the working memory at link time, the loader being capable of loading plural ones of the plural virtual memories into the working memory at run time. 
   In one embodiment, the loadable virtual memory manager can include a default virtual memory associated with a local address space and plural virtual memories stacked with the default virtual memory, each of the plural virtual memories being associated with respective external address spaces through which the default virtual memory provides access. The external address spaces can reside on external computers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a block diagram of an exemplary operating environment of the invention. 
       FIG. 1B  is a block diagram of an operating system embodying the present invention in the computer illustrated in  FIG. 1A . 
       FIG. 1C  illustrates one application of the invention to form stacked virtual memories with a local virtual memory. 
       FIG. 2  illustrates a page table registry structure of a virtual memory manager of the operating system of  FIG. 1B . 
       FIG. 3  illustrates the objects in the virtual memory manager. 
       FIG. 4  illustrates the virtual memory manager of  FIG. 3  with a set of interfaces. 
       FIG. 5  illustrates the structure of an object in the operating system of  FIG. 1B . 
       FIG. 6  illustrates the structure of the virtual memory view object of the operating system of  FIG. 1B . 
       FIG. 7  illustrates the objects in a preferred implementation of the virtual memory manager. 
       FIG. 8  illustrates the Load VMM process in the operating system of  FIG. 1B . 
       FIG. 9  illustrates the method of handling a virtual memory (VMM) fault in the operating system of  FIG. 1B . 
       FIG. 10  illustrates the operation of the VMM fault handler in the operating system of  FIG. 1B . 
       FIG. 11  illustrates the method for taking a VMM fault in the operating system of  FIG. 1B . 
       FIG. 12  illustrates the operation of the context switch process in the operating system of  FIG. 1B . 
       FIG. 13  illustrates the SwitchTo process in the operating system of  FIG. 1B . 
       FIG. 14  illustrates the operation for unloading the virtual memory manager in the system of  FIG. 1B . 
       FIG. 15  illustrates the process for handling a page fault in the system of  FIG. 1B . 
       FIG. 16  illustrates a process by which a constructor creates a thread in the operating system of  FIG. 1B . 
       FIG. 17  illustrates multiple views of memory provided by the VMView object of the operating system of  FIG. 1B . 
       FIG. 18  illustrates multiple views of memory that can be obtained in accordance with  FIG. 17 . 
       FIG. 19  illustrates the basic features of a loadable interprocess communication (IPC) manager in the operating system of  FIG. 1B . 
       FIG. 20  illustrates the process of loading of the IPC manager of  FIG. 19 . 
       FIG. 21  illustrates an interface between the IPC manager and other threads. 
       FIG. 22  illustrates intercommunication provided by the IPC manager between different address spaces. 
       FIG. 23  illustrates how an IPC trap is handled in the operating system of  FIG. 1B . 
       FIG. 24A  illustrates the operation of the IPC trap handler in the operating system of  FIG. 1B . 
       FIG. 24B  illustrates objects inside the loadable IPC system of a preferred embodiment of the present invention. 
       FIG. 24C  illustrates objects in different address spaces connected by the loadable IPC system of  FIG. 24B . 
       FIG. 25  illustrates the interface Imutate which provides object mutation in the operating system of  FIG. 1B . 
       FIG. 26  illustrates one application of object mutation in the operating system of  FIG. 1B . 
       FIG. 27  illustrates another application of object mutation applied to a Vtable. 
       FIG. 28  illustrates synchronization of object mutation by mutual exclusion. 
       FIG. 29  illustrates synchronization of object mutation by transactional synchronization. 
       FIG. 30A  illustrates the process of object mutation by swizzling in accordance with a preferred embodiment of the invention. 
       FIG. 30B  illustrates the structure of a thread relative to external objects prior to swizzling. 
       FIG. 30C  illustrates the structure of the thread relative to the external objects corresponding to  FIG. 30B  after swizzling. 
       FIG. 31  illustrates an application of object mutation to achieve object interposition. 
       FIG. 32  illustrates an application of object mutation to carry out dynamic software upgrading. 
       FIG. 33  illustrates an application of object mutation to carry out run-time code generation. 
       FIG. 34  illustrates how to achieve object mobility using object mutation. 
       FIG. 35  illustrates how proxies may be used with object mutation to communicate across address spaces. 
       FIG. 36  illustrates a mutatable structure of the virtual memory manager. 
       FIG. 37  illustrates a method embodying the programming model of the invention. 
       FIG. 38  illustrates operations carried out with the demand-loading NameSpace in accordance with the programming model of  FIG. 37 . 
       FIG. 39  illustrates the loading of an object in accordance with the programming model of  FIG. 37 . 
       FIG. 40  illustrates an application of the programming model of  FIG. 37  to plug-and-play technology. 
       FIGS. 41 and 42  illustrate an example of a conventional process for linking an executable image. 
       FIG. 43  illustrates an example of a conventional process for linking with shared libraries. 
       FIG. 44  illustrates a process in accordance with the present invention for linking an executable image using shared libraries. 
       FIG. 45  illustrates a process in accordance with the present invention for forming a dynamically linked library. 
       FIGS. 46A and 46B  illustrate an example of a jump shortcutting process of the present invention. 
       FIGS. 47A and 47B  illustrate an example of a jump shortcutting process as applied to data references in accordance with the present invention. 
       FIG. 48  illustrates an example of a post-link time compaction process of the present invention. 
       FIG. 49  illustrates a load time code synthesis process for virtual memory in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary Operating Environment: 
     FIG. 1A  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the invention may be implemented. Although not required, the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer. Generally, program modules include processes, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including inside various programmable peripheral interface cards such as  126 ,  128 ,  130 ,  144 ,  158 ,  148  in  FIG. 1A , inside programmable peripherals such as disks, game controllers and accessories, speakers, modems, printers and the like, in hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. Thus, for example, the present invention can be an operating system of an optimally minimized configuration, as described below, running inside a network interface card of the network interface  158  of  FIG. 1A  or in an embedded control system or in a communication-oriented device. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located both in local and in remote memory storage devices. 
   With reference to  FIG. 1A , an exemplary system for implementing the invention includes a general purpose computing device in the form of a conventional personal computer  120 , including a processing unit  121 , a system memory  122 , and a system bus  123  that couples various system components including the system memory to the processing unit  121 . The system bus  123  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  124  and random access memory (RAM)  125 . A basic input/output system  126  (BIOS), containing the basic process that helps to transfer information between elements within the personal computer  120 , such as during start-up, is stored in ROM  124 . The personal computer  120  further includes a hard disk drive  127  for reading from and writing to a hard disk, not shown, a magnetic disk drive  128  for reading from or writing to a removable magnetic disk  129 , and an optical disk drive  130  for reading from or writing to a removable optical disk  131  such as a CD ROM or other optical media. The hard disk drive  127 , magnetic disk drive  128 , and optical disk drive  130  are connected to the system bus  123  by a hard disk drive interface  132 , a magnetic disk drive interface  133 , and an optical drive interface  134 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  120 . Although the exemplary environment described herein employs a hard disk, a removable magnetic disk  129  and a removable optical disk  131 , it should be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROM), and the like, may also be used in the exemplary operating environment. 
   A number of program modules may be stored on the hard disk, magnetic disk  129 , optical disk  131 , ROM  124  or RAM  125 , including an operating system  135 , one or more application programs  136 , other program modules  137 , and program data  138 . A user may enter commands and information into the personal computer  120  through input devices such as a keyboard  140  and pointing device  142 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  121  through a serial port interface  146  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). A monitor  147  or other type of display device is also connected to the system bus  123  via an interface, such as a video adapter  148 . In addition to the monitor, personal computers typically include other peripheral output devices (not shown), such as speakers and printers. 
   The personal computer  120  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  149 . The remote computer  149  may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the personal computer  120 , although only a memory storage device  150  has been illustrated in  FIG. 1A . The logical connections depicted in  FIG. 1A  include a local area network (LAN)  151  and a wide area network (WAN)  152 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and Internet. 
   When used in a LAN networking environment, the personal computer  120  is connected to the local network  151  through a network interface or adapter  153 . When used in a WAN networking environment, the personal computer  120  typically includes a modem  154  or other means for establishing communications over the wide area network  152 , such as the Internet. The modem  154 , which may be internal or external, is connected to the system bus  123  via the serial port interface  146 . In a networked environment, program modules depicted relative to the personal computer  120 , or portions thereof, may be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
   Introduction to the Architecture 
   In a preferred embodiment of the invention, the operating system components contain code and other metadata for classes of objects. When a component is loaded into an address space it is instantiated. The instantiated component creates object instances that communicate with other objects, potentially in other components. The objects expose their methods through Component Object Model (COM) [Brockschmidt95] interfaces. Threads execute code and synchronize through Mutexes and Condition variables. System components are typically written in C or C++ but there is no fundamental bias towards any particular language. 
   COM enables late binding, version compatibility and checking, transparency through proxies, cross language support, and is reasonably lightweight and efficient. Each object needs a method table pointer and a reference count. Each call adds one indirection for fetching the actual method pointer. 
   Component implementations in the preferred embodiment of the invention are rarely aware of their intended system layer. The same code can be used in different address spaces or contexts and can be nested. A filesystem can be applied to a file provided by another filesystem as well as to one provided by a disk driver. A heap can be applied to any memory: physical memory, memory allocated from another heap, or memory provided by a virtual memory manager. The loader loads modules into any address space. 
   Selection of System Components 
   What components should be part of a deployed system depends upon the application itself and its interface requirements, application memory requirements, security requirements, and the target hardware capabilities. Flexible loading of modules was an important design goal for the operating system described herein. The loading of components can be deferred until they are actually used by an application. Device drivers and runtime services typically fall into this category. Others can be loaded just prior to running an application, such as virtual memory for untrusted applications. Most services will terminate themselves when they are no longer needed. 
   Drivers and virtual memory can not be used when the hardware to support them is not present. An application that tries to use them will look them up in the demand-loading namespace. The lookup operation fails, either because the driver is absent or it returns a NULL pointer. 
   Execution Model 
   Components have code, static data, a stack and a number of dynamic objects. A heap object allows dynamic memory allocations. The stack is pointed to by the stack pointer register; it is allocated from the heap. In a physical memory system the initial size of the stack is also the maximum size of the stack; every byte has to be paid for by real memory. Thus in an embedded application the stack size must be chosen carefully. Most compilers can generate stack checks at function entry, to guard against stack overflows. In a virtual memory system, the stack does not have to be backed by real memory, which can be allocated on demand. The stack only consumes virtual address range and can thus be allocated liberally. A Real-Time application might still want to pre-allocate all memory in order to avoid run time fluctuations. In this case the existence of virtual memory does not affect the stack. 
   Memory for code and static data is also allocated from the heap. Code can be placed anywhere in memory if it is either position-independent (pc-relative) or relocatable. The Microsoft Visual C++ compiler, for instance, creates a compressed relocation table that the runtime loader uses to fix any references if the executable was placed in a different place in memory than it was linked for. All compilers for embedded use provide similar functionality, although the specific image formats and relocation schemes differ. 
   On the other hand, it is often found that most compilers do not support reentrancy. If the code in an image is not reentrant, it is still possible to execute multiple instances of the same image in the same address space. The code and data are simply loaded multiple times, each time relocated differently. 
   If the relocation information is not present, and a component virtually overlaps with another component it cannot be executed in the same address space. In this case a new address space is required, which in turn requires virtual memory. 
   System Components 
   An exemplary base set of system components in a preferred embodiment of the invention is now described. 
   Referring to  FIG. 1B , an exemplary operating system in accordance with an embodiment of the invention has a kernel or link-time component  202  and a set of run-time loadable resources  204 . The kernel  202  includes a set of software resources including, preferably, a HEAP (physical memory manager)  302 , a loader  304 , a support library  306 , a timer  310 , an interrupt control unit  312 , a scheduler  314 , thread support  316  including synchronization primitives  318 , NameSpace  320 , filesystem  322  and a startup program  324 . The set of run-time loadable resources  204  are available to the system through the loader  304 . The resources  204  include, preferably, a virtual memory manager (VMM)  362 , inter-process communication  364 , drivers  366 , applications  368  and a network program  370 . A minimal multi-threaded kernel may be provided in accordance with the present invention having only the thread support  316 , the scheduler  314 , the library  306 , the timer  310  and the startup  324 . If multi-threading is not desired, the kernel may be further minimized to include only the library  306 , the timer  310  and the startup  324 . 
   As illustrated in  FIG. 1B , the ICU (interrupt control unit)  312  preferably includes the following software methods at link time: install VMM (virtual memory manager) trap handler  372 , install IPC (inter-process communication) trap handler  374 . These resources are preferably included in the interrupt control unit  312  because it is possible for such a system to take a VMM trap or an IPC trap or a page fault whether or not a VMM or IPC has been loaded. 
   Any one of the components contained in the set of loadable resources  202  may be fetched by the loader  304  and loaded into the operating system on a demand or as-needed basis during run time. The loader  304  registers the name of any such component that has been so loaded in NameSpace  320  so that all users in the system can find the component by querying NameSpace  320 . 
   In particular, the VMM  372  is loadable upon demand into the operating system and may be unloaded when all demand for it disappears. 
   Different implementations of a virtual memory manager may be selected for different purposes during run time from a VMM library  380  storing a set of VMMs, as shown in  FIG. 1B . 
   Heap 
   The Heap  302  implements (physical) memory management, allowing dynamic memory allocations with specifiable alignments. The constructor allows creating nested heaps or heaps over virtual memory. 
   Loader 
   The Loader  304  is used to load additional components into a running system. Most embedded systems do not provide a loader, and if not needed it can be eliminated at link time from this system as well. Multiple image formats are supported. The loader loads images into the same address space, or given a flag and a virtual memory system, it creates a new address space and loads the image in there. 
   No particular distinction is made herein between executables and DLLs (shared libraries). An executable is simply a DLL that exports no entry points besides main ( ). 
   Support Library, Machine Initialization 
   The library  306  is a shared support library and includes common base utilities like memcpy and other compiler support routines, AtomicAdd, CurrentThread, etc. It is used by many system components and is available to applications. 
   Basic machine initialization code is used at startup and system reset. Most of the machine dependent code of the componentized operating system of the invention resides here. 
   Timer and Interrupt Drivers 
   A driver for the timer  310  is used by the scheduler  314  to keep track of time and for thread pre-emption. A driver for the Interrupt Control Unit (ICU)  312  dispatches interrupts and keeps a registry of interrupt routines, which can be installed and removed by other components. The system has no particular notion of a “device driver” per se. It does enforce strict limits as to what an interrupt routine can do: wakeup a thread. 
   Scheduler 
   The scheduler  314  is a policy module that determines which thread should run at any given time. Low-level management of blocking and switching between threads is handled by the thread and synchronization components  316 ,  318 . 
   The timer interrupt and thread and synchronization modules call into the scheduler  314 , possibly passing callback functions as arguments. 
   Three example schedulers have been implemented: the null scheduler, a round robin scheduler, and a constraint based Real-Time scheduler. The null scheduler is for systems that use only one thread. The round robin scheduler provides time-sharing, it can easily be extended to handle dynamically changing priorities. Constraint scheduling is for consumer Real-Time applications and is described in [Jones97]. The existence of these schedulers proves that the scheduling interface is necessary and sufficient to implement all of the prior art scheduling policies. 
   Threads and Synchronization 
   The thread support and synchronization components  316 ,  318  provide basic thread support and synchronization primitives. Each thread is run in one address space. A thread is usually created in the address space of the component in which it is started. If there is no virtual memory, the address space is always the physical address space. Threads can be created in remote components as well as in local components that are part of the same address space. Threads can block on mutexes and conditions. They can inform the scheduler of their time constraints, but these calls will fail if the scheduler is not a constraint scheduler. The constraint scheduler performs priority inheritance when threads block on mutexes. Preferably, the thread support  316  and scheduler  314  are separated so that the scheduler  314  and be changed while maintaining thread support. So, a third party could change the scheduler  314  without affecting applications, so that the applications and the scheduler are isolated. 
   NamespacesA simple boot namespace where applications register objects may be provided. The Namespace  320  is a namespace that cooperates with the loader  304  in demand-loading and caching of components. A namespace may be used for displaying the status (e.g. running threads) and performance parameters (e.g. execution times) of a system during development. Filesystems are also namespaces. 
   Filesystem 
   Filesystem  322  is used to load additional components during runtime, and as permanent data repository. RomFS is a filesystem for read-only in-memory images (arbitrary files and the system can be merged into one image) and FatFS is for reading/writing disks. NetFile is a network filesystem client built on top of sockets. 
   Startup Program 
   Startup  324  is a program that is started once the system has been initialized. It can be a simple command interpreter that configures the system and launches applications, or the (only) application itself. 
   Network 
   The Network Program  370  provides the entire BSD4.4Lite network protocol code, with minor adaptations. The interface is a COM interface that provides sockets. The protocols operate the network drivers through another interface. 
   Other Components 
   A small Win32 compatibility library may be provided to make it easier to use WindowsNT code in some of the drivers and applications. 
   How Namespace is Used to Manage the Loading of Objects 
   Namespaces are used to let applications gain access to objects provided by other components. A namespace is like a filesystem directory tree, except it can hold any kind of objects, not just files. Namespaces can themselves be implemented by different components, including a filesystem that exports its directories as sub-namespaces, and files as registered objects. Namespaces can be registered into other namespaces, extending the directory tree. Location transparency of all objects automatically makes namespaces distributed. Therefore it is easily possible to view some other machine&#39;s NameSpace as a sub-Namespace of one&#39;s own machine. Namespaces can be filtered for access control or for providing different views for different applications. There is no limit as to the number of namespaces. A component can gain access to its namespace through a call to CurrentNamespace( ). In a minimal system, all applications share the same boot namespace. 
   When an application looks up a name in the namespace, it obtains a reference to the object: a local direct reference in case the object is local, or an automatically created proxy if the object is remote. (For remote objects, the interprocess communication (IPC) system described below in this specification is responsible for creating proxies, handling the delegation to remote objects, and reference counting.) A namespace is free to only give access to previously registered objects or to create objects on demand, as it sees fit. The namespace only handles the IUnknown interface. It is up to the application to obtain the proper interface from the object, using the QueryInterface method. 
   The interface of Namespace includes a method called Bind. The Bind method is used to request an object. The Bind method finds whether the requested object has already been loaded and, if not, Bind obtains the IUnknown interface of the requested object and returns it as an argument. Bind returns a pointer to the IUnknown&#39;s pointer to the requested object. Bind is the method that looks up the requested object in Namespace while Register is the method for registering the object in Namespace. After the object has been loaded, the Query Interface method may be used to query the object. 
   Objects can be made available to other components by registering them in a namespace. Every COM object has a virtual method table and at least the three methods derived from the base interface (the IUnknown) used with Namespace: QueryInterface for agreeing on the interface protocols, and AddRef and Release for reference counting. Specific interfaces have additional methods to do the actual work. In addition, a constructor is usually provided. 
   Garbage collection is done through reference counting. When Release has called for the last reference, the implementation can finalize and deallocate the object. Even if reference counting has limitations, it is convenient in a system environment due to its simplicity. 
   Interaction with objects using other garbage collection models can be achieved through proxies that intercept the IUnknown methods to update their root sets. 
   Loadable Virtual Memory Manager 
   Virtual memory provides virtual address spaces. Threads run in either a virtual address space or in the physical memory space. Components can be loaded into any address space. Component code may be shared between various address spaces like shared libraries. For instance, any code loaded into the physical memory space is made visible to applications regardless of their address space. There is nothing secret in the system&#39;s code, so there is no security problem. Virtual memory can be used to protect sensitive application code, for instance to defend it against disassembly and theft of intellectual property. Unlike most existing operating systems, in the preferred embodiment of the invention the support for virtual memory is not an integral part of the system. The system can function with or without it, and it executes the same executable binary images. The virtual memory manager is a component like any other, and is loaded dynamically on demand. 
   Loading or unloading of the virtual memory system does not interfere with applications already running, or started later on in the physical memory space. Once the virtual memory system has been started, new components can be loaded into any address space. Component code may be shared between different address spaces, as is the case with shared libraries. 
   Virtual memory can be used for:
         Security reasons, when a program is not trusted. The virtual memory system implements firewalls between applications.   Covering for common programming errors such as NULL pointer references and memory leaks.   Creating a sparse address space. This often leads to better memory utilization with fragmented heaps.   Paging. This provides more memory than available, working set adaptation, and mapped files.   Safe and flexible memory sharing: Copy-on-write for libraries, shared memory windows.   Running non-relocatable executables as described above.   Implementing garbage collection and other protection mechanisms.       

   In accordance with preferred embodiments of the invention, the virtual memory manager is a loadable component that provides multiple virtual address spaces. It can be viewed as a driver for MMU hardware. It creates virtual memory mappings using physical memory and MMU hardware. Loading and starting the virtual memory manager executable does not interfere with applications already running. Unloading can be done once all references to objects provided by the manager are released. A new one can be started if needed. 
   A virtual memory space looks like the physical memory space, except it can be larger, doesn&#39;t have to be contiguous, can be paged, protected, replicated, and can be (recursively) mapped to other objects. 
   The virtual memory manager exports a number of control interfaces that are used to create new address spaces (VMSpace), to map address spaces or files to address spaces (VMMap), to link threads to address spaces (VMView), to control state and protections (VMSpace), and to create instances of virtual memory objects (VMFactory). 
   Realistically, any MMU driver will need exclusive control of the MMU hardware. However, other objects implementing the virtual memory interfaces can be interposed between an application and the MMU driver. In this way, logical virtual memory systems can be arbitrarily composed or, for instance, stacked as in the Exokernel. A stacked virtual memory system relies on another one for its base functionality, but add some specific functionality of its own. For example, the bottom virtual memory manager in the stack could control the local or host machine, while virtual memory managers stacked in higher layers would control other machines thereby providing an application running on top of multiple machines the illusion of running on a single shared memory multiprocessor system. What the present invention facilitates is multiple levels of virtual memories which are recursively stackable. Furthermore, the invention provides transparency. The Exokernel approach was to have a library that implements a virtual memory, so that there is in effect only a single level, since one library cannot be stacked over another library. In the present invention, the different virtual memories exist in different address spaces, so that there is no corresponding limitation on stacking multiple levels, and the stack is remotable. For example, referring to  FIG. 1C , the invention can be used to make a collection of different machines (machine-A and machine-B) appear as a single address space by employing a local virtual memory (VM- 1 ) while other virtual memories (VM- 2  and VM- 3 ) relating to the address spaces of the other machines but having the same interface as the local virtual memory are stacked over it. 
   Design 
   The virtual memory interfaces were designed for flexibility and simplicity:
         VMSpace. Methods: Reserve, Delete, Map, Protect, CacheControl, QueryVM, CreateShadow.       

   Reserve( ) reserves regions in the VMSpace, Delete( ) deletes them. Map( ) maps VMMap objects to the space. Protect( ) sets permanent attributes (e.g. read-only), CacheControl( ) controls transient attributes (e.g. dirty) and paging state (e.g. present). QueryVM( ) returns information about ranges, CreateShadow( ) creates a new mapping, atomically moves ranges from the current mapping to the new one and maps the new one into the current one. It facilitates symmetric copy-on-write. A VMSpace also implements the VMMap interface, returned by QueryInterface( ).
         VMMap. Methods: Read, Write, Share, QueryAccess, GetSize, Clone.       

   Read( ) and Write( ) are used for copy-paging. Share( ) is used to establish shared pages between two VMSpaces and to return pages to VMView::Fault( ). QueryAccess( ) and GetSize( ) return information about the mapping. Clone( ) creates a restricted mapping from the given VMMap. The constructors are used to turn files into mappings and to create memory with no backing store.
         VMView. Methods: SwitchTo, Fault, SetMapping, GetMapping.       

   SwitchTo( ) is called by the context switch path. 
   Fault( ) is called at page fault time. 
   SetMapping( ) changes the associated VMMap (and indirectly VMSpace). 
   GetMapping returns the current associated mapping.
         VMFactory. Methods: CreateVmView, CreateVmSpace, CreateVmMappingFromFile, CreateVmMappingFromZero. VMFactory is an object that contains the constructors of other VMM objects.       

   Process creation works as follows. A new VMSpace and a new VMView are created. The VMView is bound to the VMSpace. A temporary file is created as backing store for zero-fill memory allocations. A VMMap is constructed with CreateFromFile( ), and it is mapped into the VMSpace. A Heap is created from the resulting memory. The file for the executable is looked up from a namespace. A VMMap object is constructed from that file. The VMMap is mapped into the VMSpace copy-on-write, with backing store for any copies coming again from the temporary file. The loader relocates the code if necessary. An IPC channel to the newly loaded component is created. A thread is created with the VMView associated to it. The thread is started by handing it to the scheduler. 
   Page fault handling works as follows. VMView::Fault( ) is invoked. This in turn calls Share( ) on the associated VMSpace (which also exports VMMap interface). Assuming the page is not present, Share first calls VMSpace::CacheControl( ), which calls VMMap::Read( ) on the file&#39;s VMMap, which in turn calls File::Read( ) on the file object. The VMSpace then adds the returned data to its page list and returns a reference to it to the VMView, which adds it to the VTLB (virtual translation look-aside buffer), which makes it available to the hardware. 
   Memory wiring works as follows. The application invokes VMSpace::Protect( ) with the wire flag set. Protect first calls VMSpace::CacheControl( ) to page in all the pages. VMSpace::CacheControl( ) may fail if the physical memory is exhausted. In this case Protect fails without wiring any pages. Otherwise it marks the pages wired and returns successfully. Any operation affecting permanent attributes is atomic. Those affecting transient attributes are not. 
     FIG. 2  illustrates how page table entries are used in providing translation between virtual memory addresses and physical memory addresses by the run-time loadable VMM  362  of  FIG. 1B . Each VMSpace object stores a pointer value which is loadable into a page directory register  400  provided in the host computer&#39;s microprocessor. The pointer value, once loaded into the page directory register  400 , points to a particular page table  402  which is one of many page tables available to the system. Each page table provides a particular set of address translations between virtual memory addresses and physical memory addresses. Different objects may be stored in different memory locations and therefore may require different address translations. The individual page table  402  has a set of virtual addresses  404  and physical addresses  406  correlated to corresponding ones of the virtual addresses, the addresses constituting the page table entries of the page table  402 . 
   Certain advantages of virtual memory are realized by controlling the allocation of physical memory. For example, in order to protect certain areas of memory from being written, the page table entry for a particular physical address may include an indication that the address is available for read-only. In order to hold a certain area in reserve or to delay the allocation of physical memory until an actual need arises, the page table entry for a particular physical address may include an indication that the physical address is “invalid”. A pointer to the corresponding virtual memory address will cause the system to take a virtual memory trap, as will be described below. 
   Referring to  FIGS. 3 and 4 , the virtual memory manager (VMM) includes the following interfaces: IVMSpace  610 , IVMMap  620 , IVMView  630 , IUnknown  640  and IVMFactory  650 . The IUnknown interface preferably is included in every object. The purpose of this is to give every application the ability to query the object to determine if it supports a given interface. IUnknown refers to processes for querying for a given interface (“query”), for adding a reference to the object (“add”) and for releasing a reference to the object (“release”). Thus, each of the three primary interfaces of VMM includes the three processes of IUnknown, as illustrated in  FIG. 4 . 
   VMFactory  650  has an interface INamespace  652  which it exports. Inamespace is used to enumerate all the objects that one virtual memory manager handles, regardless of whether they have also been registered in the common Namespace. 
     FIG. 4  illustrates the VMM interfaces as containing particular methods. IVMSpace contains the methods Query, Add, Release, Reserve, Delete, Map, Protect, CacheControl, QueryVM, CreateShadow. IVMMap contains the methods Query, Add, Release, Read, Write, Share, Clone, QueryAccess, GetSize. IVMView contains the methods Query, Add, Release, SwitchTo, SetMapping, Fault, GetMapping. IVMFactory contains the methods or constructors CreateVMView, CreateVMSpace, CreateVMMappingFromFile, CreateVMMappingFromZero. IUnknown contains the methods Query, Add, Release. 
     FIG. 5  illustrates the internal object architecture employed in a preferred embodiment of the invention. The object has a virtual table (V-table)  510  and a state  520 . An instance pointer  530  points to the object. The state  520  can include, for example, the page table register contents, the IUnknown reference count and pointer values. The pointers of an object will be discussed below with reference to  FIG. 7 . 
   The V-table  510  points to a particular interface  540 , which may be one of many interfaces available to the system. The interface  540  lists a number of processes or methods associated with the particular object. Each method entry listed in the interface  540  points to an implementation  550  of that method. The implementation  550  contains the code for carrying out the algorithm implementing the particular process or method listed in the interface  540 . As illustrated in  FIG. 5 , more than one object may point to the same interface. Of course, it is to be expected that different objects have their own unique interfaces in many instances. For any application calling the object, the object&#39;s interface provides a complete list of the processes or methods supported by the object. 
     FIG. 6  corresponds to  FIG. 5  and illustrates the object architecture of the VMView object of the VMM (virtual memory manager). In addition to the three universal IUnknown processes of query, add and release, VMView further includes the processes of fault and switch to. The “fault” process will be discussed below with reference to  FIG. 10  while the “switch to” process of VMView will be discussed below with reference to  FIG. 12 . 
     FIG. 7  illustrates a preferred embodiment of the Virtual Memory Manager (VMM). The VMSpace is implemented as a sequence of regions connected by a skip-list of the type well-known in the art. Each region contains a mapping, a copy mapping (for copy-on-write), a number of page lists for physical memory pages, and a set of attributes. Permanent attributes are kept in the region; any transient state is part of the page list. The VMSpace also exports a VMMap interface so that it can be directly mapped into other address spaces.  FIG. 7  illustrates mappings by VMMap of various regions of VMSpace to other objects, as will be discussed in more detail below. A VMSpace is 64 bits wide in the preferred embodiment. 
   The VMView provides a view into a VMSpace for one or more threads. It is limited by the hardware address size (e.g. 32 bits). If the VMSpace it points to is indeed larger (64 bits) then the view is a window into part of that space. A virtual translation look-aside buffer (VTLB) is attached to the VMView. The VTLB contains machine dependent mapping information for the MMU hardware and a translation cache. The VTLB interface is common across all architectures. The rest of the system is unaware of the virtual memory component, with a few exceptions. A thread must hold a pointer to its VMView so that page faults can be resolved within the correct context, and the context switch path must check for a VMView change. If the context switch path detects an address space change, it calls a VMView method to synchronize the MMU hardware with the change. The virtual memory manager registers its interrupt handler with the ICU driver. The Heap may choose to modify its behavior when running over virtual memory. The loader can only create new address spaces when a virtual memory system is present. The IPC system may utilize virtual memory mappings for data transfer. 
   Referring to  FIG. 7 , different threads  710 ,  715  can contain pointers pointing to the same VMView object  720 . The VMView object contains pointers pointing to a page of PTEs (page table entries)  725  and to a VMSpace object  730 . The VMSpace object  730  points to (defines) Region 1 , Region 2  and Region 3 , corresponding to different non-contiguous memory regions linked by the skip-list. These regions are mapped to other objects in the manner illustrated in the example of  FIG. 7  by pointers provided by the VMMap interface of VMSpace. Region 1  is reserved (no writing permitted) and points to an empty space  735  (labelled Empty). Region 2  has one pointer to a VMMap object  740  corresponding to a region in memory containing all zeroes, which is labelled ZeroVMMap. Region 2  has another pointer to a page list  745 . The page list  745  points to a memory  750  having allocated memory spaces in which writing is permitted. Region 3  has one pointer to a VMMap object  755  which is an implementation of VMMap for mapping files and is labelled FileVMMap. The FileVMMap has a pointer pointing to a file  760 . Region 3  has another pointer pointing to a VMSpace object  765  (which is different from the VMSpace object  730  discussed above). The connection through Region 3  between the two VMSpace objects  730 ,  765  supports a copy process. Region 3  has yet another pointer pointing to a PageList  770 . The PageList  770  has a pointer pointing to a second PageList  775 . The second PageList  775  points to a Memory object  780 . The linking of the two successive PageLists supports a copy-on-write function. 
   In summary, the VMMap object can map a region to another VMMap object, examples of which are illustrated in  FIG. 7  such as the pointer from Region 3  to File VMMap  755  and the pointer from Region 2  to Zero VMMap  740 . The mapping can be to a different portion of the VMMap object itself rather than another VMMap object. The VMMap object can map to the ZeroMap object providing zero-filled physical pages, as illustrated in  FIG. 7  by the pointer to the ZeroMap object  740 . The VMMap object can map a region to a VMSpace object, as illustrated by the pointer from Region 3  to VMSpace  765 . The VMMap object can map a region to a file, as illustrated by the pointers from Region 3  to the File VMMap  755  and from thence to File  760 . This may include the case of the “system pager” which handles the traditional paging file. Finally, the VMMap object can map a region to a mapping filter (“CloneMap”) which, for example, restricts the protections allowed of a mapping, as illustrated by the pointer from Region 3  to CloneMap. 
   A PageList lists what pages have been called by the object. This is useful in case this information is forgotten by the PTE, for example. 
   The state of an object, such as the virtual memory object of  FIG. 7 , consists of the pointer values of the various pointers in the object (such as the pointers illustrated in  FIG. 7 ). 
     FIG. 8  illustrates the Load VMM (load virtual memory manager) process. The first step (block  810  of  FIG. 8 ) is to install the VMM Fault Handler method. The last step (block  830 ) is to register the VMM constructors, which are part of the VMFactory interface. This step is carried out by creating an object of the IVMFactory type and registering it into the NameSpace. 
   A local constructor is a method in an object. In order to export it to another address space, the constructor must be wrapped in a factory object of the type well-known in the art. These objects are designated as “XYFactory”. For example, VMFactory can create VM objects. There is a correspondence between C++ classes and factory objects. A C++ compiler could automatically create the factory of an object. 
     FIG. 9  illustrates how a virtual memory fault is handled. The first step is to save the state of the object or thread that was running at the time the fault or trap was taken (block  910  of  FIG. 9 ). Next, the VMM Fault Handler is called (block  920 ). Then, the state of the object is restored (block  930 ). 
     FIG. 10  illustrates the operation of the VMM Fault Handler. The first step (block  1010  of  FIG. 10 ) is to call IVMView::Fault. The next step is to determine whether IVMView::Fault can provide a VM mapping (block  1020 ). If so (“YES” branch of block  1020 ), the page table entry is loaded (block  1030 ). Otherwise (“NO” branch of block  1020 ), an exception is taken (block  1040 ) and an exception handler is called (block  1050 ). 
     FIG. 11  illustrates how a VM fault is taken. The first step (block  1110  of  FIG. 11 ) is to determine whether the VM fault is due to an error. If so (“YES” branch of block  1110 ), then an exception is taken (block  1120 ). Otherwise (“NO” branch of block  1110 ), a determination is made whether the VM fault is due to a memory allocation postponement, sometimes referred to as “lazy memory allocation” (block  1130  of  FIG. 11 ). Lazy memory allocation is a feature that can be implemented to guard against premature allocation of memory otherwise caused by early requests by an application. The determination of whether the VM fault is due to lazy memory allocation is made by determining whether there is any reference to the object in the PageList. If the fault was due to lazy memory allocation (“YES” branch of block  1130 ), then VMSpace is called to perform the memory allocation (block  1140  of  FIG. 11 ). Otherwise (“NO” branch of block  1130 ), a determination is made whether the VM fault was due to a copy-on-write process (block  1150 ). This determination is made by asking whether the current PageList points to another PageList (as discussed above with reference to  FIG. 7 ). If not (“NO” branch of block  1150 ), the reference in PageList is copied to the page table or PTE (block  1160 ). Otherwise (“YES” branch of block  1150 ), the pointer to the other PageList is taken (block  1170  of  FIG. 11 ). This entails allocating a new page, copying the content of the old page to the new page, and then entering the mapping for the new page in the PTEs. 
     FIG. 12  illustrates the operation of the context switch process. First, the Scheduler (shown in  FIG. 1 ) decides to perform a context switch (block  1210  of  FIG. 12 ). This decision may be occasioned, for example, by a thread running up to a predetermined time limit. As a result, the Scheduler selects a new thread to replace the currently-running thread (block  1212  of  FIG. 12 ). In a preferred embodiment, the next step is to inspect the views of memory provided by IVMView of the current and new threads (block  1214 ) and determine whether they are different (block  1216 ). If so (“YES” branch of block  1216 ), IVMView::SwitchTo is called (block  1218 ). Otherwise (“NO” branch of block  1216 ), the system returns to the new thread (block  1220 ) without altering the contents of the page directory register  400  of  FIG. 2 . 
     FIG. 13  illustrates the SwitchTo process, consisting of the step of loading the page directory register  400  of  FIG. 2  with the new page directory value. 
     FIG. 14  illustrates the process for unloading the virtual memory manager (VMMUnload). The first step begins whenever the last thread of the last address space using virtual memory requests termination (block  1410  of  FIG. 14 ). The next step is to determine whether the reference count is zero (block  1420 ). If so (“YES” branch of block  1420 ), a THREAD 1  may make the memory appear smaller than its actual physical size, which the view of THREAD 2  may make the memory appear to have a wider address field than that of the actual physical memory. For example, the actual physical memory may have an address space that is 32 bits wide while it appears to be 64 bits wide in the view of memory provided to THREAD 2 . On the other hand, the physical memory address space may be 64 bits wide while it appears to be only 32 bits wide in the view provided to THREAD 1 . In a more complex case, the address space widths of the objects VMView (“THREAD 1 ”) and VMSpace (“THREAD 2 ”) and of the physical memory may all differ from one another, being in one example 32 bits, 64 bits and 36 bits, respectively. The address space width in VMSpace may be chosen without regard to the address space widths of the virtual memory and the physical memory. 
   The virtual memory interfaces described here loosely correspond to the Mach external memory manager scheme [Young89]. VMView and VMSpace replace Mach&#39;s task. Memory objects are replaced by VMSpaces. VMMap and VMSpace jointly implement the XMM interface, although with synchronous interfaces only. The VTLB interface is a refinement of the Pmap interface. 
   In other implementations of the invention, one could emulate Unix or Windows virtual memory APIs, and the design would easily permit it (including fork). 
   Loadable Interprocess Communication (IPC) Manager 
   An IPC system is needed if applications are to be run in separate address spaces. Otherwise the applications can not talk to each other or to system services. An IPC system allows:
         Communication between address spaces within a machine.   Communication between applications in different machines in a distributed environment.   Graceful termination and cleanup of applications even within one address space.       

   Cleanup involves releasing the memory held by an application. It also involves closing all references into and out of the application&#39;s objects. A level of indirection is needed for bookkeeping and for providing a cutoff point. This level of indirection is what an IPC system provides. 
   A preferred embodiment of the IPC system implements the COM model. It is possible, however, to replace it with another communication model for applications that expect a different model. Components implementing various communication paradigms can be loaded into the system as needed. 
   The preferred interprocess communication (IPC) manager is a run-time loadable resource residing outside of the operating system kernel, as illustrated in  FIG. 1B . The loader can load the Loadable IPC Manager at any time based upon need. The Loadable IPC Manager can also be unloaded whenever it is no longer required by any applications currently running. 
     FIG. 19  illustrates the basic components of the Loadable IPC Manager. A trap component  1910  consists of a program to install the method IPC Trap Handler  1912 , which is called whenever the system recognizes a need within the currently-running thread to communicate with another thread or a need to communicate with a thread in another machine. A copy component  1930  can be implemented with either a simple “copy” function or virtual memory or shared memory. 
     FIG. 20  illustrates the operation of the Loadable IPC Manager, which may have the file name IPC.EXE in a preferred implementation. The first step is to load the Loadable IPC Manager (block  2010 ). The next step is to install the IPC Trap Handler (block  2020 ). Finally, the IPC constructors are registered in NameSpace (block  2030 ). 
     FIG. 21  illustrates the structure of the interface between the loadable IPC and threads that use it. In the exemplary case of two threads  2110 ,  2120  which use a particular Loadable IPC Manager  2130 , each thread has its own pointer  2110   a ,  2120   a  pointing to the Loadable IPC Manager  2130 . The Loadable IPC Manager  2130  has interfaces for the following methods: Query Interface  2140 , Add Reference  2150 , Release Reference  2160  and IPC Trap Handler  2170 . As will be discussed later in this specification, each one of these methods and interfaces is replaceable. 
     FIG. 22  illustrates the intercommunication provided by the Loadable IPC Manager between threads in different address spaces. One thread (THREAD 1 ) resides in a first address space  2210 , and has a pointer to a loadable IPC Manager  2220 . At some point in the running of THREAD 1 , it causes the Loadable IPC Manager to provide communication with another thread (THREAD 2 ) in another address space  2230 . Both THREAD 1  and THREAD 2  have a pointer to the Loadable IPC Manager  2220 . The Loadable IPC Manager provides the requisite communication between THREAD 1  and THREAD 2 . For example, THREAD 1  may need to increment a counter controlled by THREAD 2  in the other address space. The magnitude of the increment may need to be communicated to THREAD 2  and the value of the count after incrementation may need to be returned to THREAD 1 , in some cases. The Loadable IPC Manager handles both of these communications, thus providing two-way communication between THREAD 1  and THREAD 2  as illustrated in  FIG. 22 . 
   The IPC process begins when THREAD 1  signifies a need to go to another address space to access some resource in that other address space. In one implementation, THREAD 1  makes a remote procedure call directed to a resource controlled by THREAD 2  in the other address space  2230 . Such a remote procedure call is transparent to the application running. Since THREAD 1  is currently running in the current address space  2210 , the thread takes an IPC trap. 
     FIG. 23  illustrates how an IPC trap is handled. First, the state of the currently running thread (THREAD 1 ) is saved (block  2310 ). Then, a call is made (block  2320 ) to the IPC Trap Handler which (unlike the Loadable IPC Manager stored outside of the operating system kernel of  FIG. 1 ) resides within the operating system kernel. 
     FIG. 24A  illustrates the operation of the IPC Trap Handler. First is the occurrence of an IPC trap (block  2410 ). The next step is to copy the arguments of the currently running thread (block  2420 ). These arguments may be, for example, the identity of a counter in another address space. The thread traps into the loadable IPC system so that its VMView is changed to the VMView that points to the new address space (block  2430 ). The thread continues running in the new address space where it invokes an object or method that can produce desired information or data such as a new incremented counter stored in a register (block  2440 ). The next step is to return the values of the new data from the thread (block  2450 ). Then, the thread switches back to the original VMView (block  2460 ). In contrast, conventional IPC systems (which were not loadable as the IPC of the present invention) typically did not move the thread from one address space to the other. Instead, the desired information in the second address space was obtained by selecting and running a different thread in that second address space. In contrast, the preferred embodiment of the loadable IPC of the invention uses a single thread and moves between among address spaces by changing its VMView. However, an alternative embodiment of the loadable IPC of the present invention could be implemented using the conventional technique employing different threads. 
   Structure of a Loadable IPC System 
   The loadable IPC system of the invention (hereinafter referred to as the “LIS”) differs in structure from a non-loadable one in the handling of the dependencies upon other subsystems (such as the scheduler and the virtual memory). The LIS can only have dynamic dependencies, and therefore cannot use any “backdoor” or make assumptions about the internals of other components. The LIS finds these components in the NameSpace, most likely at initialization time. If a specific component is not found or cannot be loaded, the LIS fails to initialize and unloads itself. 
   It is possible for the LIS to require specific interfaces from these other components, which it asks for via the Query Interface method. This could mean that not all virtual memory managers would be suitable for a given LIS, only those that implement the interface(s) the LIS requires. The converse is also true: all virtual memory managers that implement the required interfaces should be suitable for LIS use. In the preferred embodiment of the LIS of the invention, it is an interface definition error for a component to violate this rule. 
   Different LISs can depend on different sets of other components. Specifically, it is possible for a simple LIS not to require a virtual memory manager and still be fully functional. For instance, such a LIS would be useful for connecting two simple computers that do not possess a memory management unit(MMU). 
   The LIS provides two classes of services: administration services and communication services. Administrative services support the creation, linking and destruction of communication endpoints and any other ancillary service. Communication services involve the transmission of the input and output arguments of an RPC and the creation and management of proxies and method signatures. An application requests these services from the LIS in an architectural and LIS-dependent way. More specifically, a processor usually provides special instructions to request system services (“system calls”); execution of such an instruction causes a processor trap. The LIS can be viewed as the system handler for such a trap. The LIS is handed the state of the thread at trap time. It is then LIS-dependent what elements of the thread state are intended to convey information to the LIS. For example, a certain processor register might contain the index of the service the application requires. Different embodiments of the LIS could offer different services, and use different register use conventions for argument passing. 
   A thread is associated with a specific LIS. Different threads can be associated with different LIS&#39;s. If two LIS&#39;s do not communicate, then their client threads also cannot communicate. It is not preferable to split a single computer into two or more non-communicating subsystems, except for the extreme case of highly secure systems. Therefore, the most practical case is the one where multiple LIS&#39;s are able to communicate among each other. In the present invention, this can be accomplished quite naturally by loading all LIS&#39;s in the same address space, making them visible in a common NameSpace, and using normal object invocation for their interaction. 
   Interfaces and Methods 
   A preferred embodiment of LIS.EXE has five internal (not exported) interfaces: ILISFactory, IPCSpace, IEndPoint, ISignature, and IEndPointTable. 
   ILISFactory::CreateIPCSpace( ) creates an IPCSpace with one empty ExportTable and one empty ImportTable. Both tables are IEndPointTables. 
   ILISFactory::CreateEndPoint(IPCSpace,ObjectPointer) creates an EndPoint in the given IPCSpace, to represent the object at virtual address ObjectPointer. The EndPoint has the IUnknown default signature associated with it. The EndPoint receives a new IPID, which is a 128 bit universal identifier uniquely associated with the EndPoint. 
   ILISFactory::CreateSignature(IID,ParsingString[ ]) creates a type signature given an array of properly formed parsing strings. Each string describes the type signature of one method. There are as many strings as there are methods in the interface. IID is the 128 bit universal identifier for the interface. 
   IPCSpace::GetImportTable( ) returns the ImportTable associated with the given IPCSpace. 
   IPCSpace::GetExportTable( ) returns the ExportTable associated with the given IPCSpace. 
   IEndPoint::SetSignature(ISignature) changes the signature of the given EndPoint. 
   IEndPoint::GetSignature( )returns the signature of the given EndPoint. 
   IEndPoint::GetIPID( ) returns the IPID of the EndPoint. 
   ISignature::GetParsingString(MethodNumber) returns the parsing string for the given method number. 
   IEndPointTable::Add(EndPoint) adds an endpoint. 
   IEndPointTable::Remove(EndPoint) removes an endpoint. 
   IEndPointTable::Lookup(IPID) looks up the ID number of the endpoint. 
   In addition to the interfaces described above, LIS.EXE makes use of Proxies inside the application address space. A Proxy is an object like any other, but it merely acts as the representative of some other remote object. The application invokes methods on the proxy, and the proxy&#39;s implementation for all methods simply traps to LIS.EXE. Two notable exceptions are the AddRef and Release methods, which maintain a local reference count. Only the Release method traps, and only when the local reference count goes to zero. Other embodiments of LIS might not require the use of proxies. 
   Operational Overview of the Loadable IPC System 
     FIG. 24B  illustrates how instances of these interfaces are related. In the drawing, two threads belong to two different applications. Thread-B was given access to the endpoint EndP- 1 . 
   LIS.EXE handles the NameSpace specially, for three reasons. LIS.EXE is responsible for exporting primitive (‘kernel’) objects to its clients. These are the objects that are found in the boot NameSpace, but for which LIS.EXE does not have an EndPoint. Other components that are loaded in the physical address space along with LIS.EXE can create and register objects in the boot NameSpace before and/or while LIS.EXE is loaded and active. When an application looks up one such object in the NameSpace, LIS.EXE automatically creates an EndPoint in the primitive IPCSpace. Secondly, when an application calls INameSpace::Bind( ) to obtain access to a remote object, LIS.EXE must be able to intervene and create a Proxy for the remote object in the application&#39;s address space. Similarly, when an application wants to INameSpace::Register( ) an object, LIS.EXE must be able to intervene and remember which VMSpace was associated with the object. 
   Finally, when an application terminates abnormally LIS.EXE cleans up after it. Among other things, LIS.EXE is responsible for removing from the NameSpace all objects that belonged to the dead application. 
   An application thread is associated with a given LIS at thread creation time. More specifically, the LIS associates an IPCSpace instance to the VMSpace of the thread. This IPCSpace instance is the object passed to LIS.EXE at trap time. Other per-thread information that is used at trap time is the service requested, object IPID, method number, and the stack pointer register to access the arguments of the method call. 
   An application thread can find LIS.EXE in the NameSpace. An IPCSpace contains an ImportTable and an ExportTable, each table containing pointers to EndPoints. The ExportTable points to EndPoints that the IPCSpace exports to other IPCSpaces. As a result, the application associated with that IPCSpace fully implements the object the EndPoint represents. The ImportTable acts as a security filter; it only allows a thread access to those remote objects that it was granted access. The ImportTable can also be used as a renaming table: the application uses local names for remote objects that are only meaningful when used with the application&#39;s ImportTable. 
   An EndPoint has a unique ISignature. This provides an advantage in the effect of the QueryInterface method: when an application invokes QueryInterface on a remote object, it truly receives a new proxy for a new EndPoint. 
   In the example illustrated in  FIG. 24B , Thread-A creates an EndPoint EndP- 1 , passing in a Signature Signature- 1  and a virtual address Object-A. The EndPoint is entered in the ExportTable ETable-A of the thread&#39;s IPCSpace-A. A reference is taken on the thread&#39;s VMSpace-A (not shown). Thread-A now registers the EndPoint in the NameSpace. One reference is taken on the object and one on the EndPoint, to signify that the object is exported and visible in the NameSpace. 
   Generally, threads do not explicitly create EndPoints; they register objects in the NameSpace. It is LIS.EXE that automatically creates an EndPoint for the objects as part of its overriding of the INameSpace::Register( ) method. Alternatively, a method invocation might require the passing of an object as argument or as return value. Again, LIS.EXE automatically creates an EndPoint if necessary, inserts it in the ExportTable if not already in there, and inserts it in the remote party&#39;s ImportTable if not there already. 
   With reference to  FIG. 24B , Thread-B can either look up Object-A in the NameSpace, or invoke a method on some other object in VMSpace-A that returns Object-A as result. In either case, LIS.EXE finds that EndP- 1  is the associated EndPoint, and enters it in ITable-B, the ImportTable for Thread-B&#39;s IPCSpace-B. A Proxy for Object-A is created in Thread-B&#39;s VMSpace-B (not shown). In order to create the proxy, Signature- 1  is used to find the size of the necessary VTable, and for the loading (or memory mapping) of the proxy&#39;s marshalling methods. The proxy holds a copy of the EndPoint&#39;s IPID. 
   Thread-B can now invoke a method on the proxy. In this case, a pointer to the proxy&#39;s state is loaded in a register and a trap is taken. The proxy&#39;s IPID is used to find EndP- 1  in ITable-B. The remaining arguments are on the stack-A, in VMSpace-A. A new stack-B is created in VMSpace-B and is remapped in VMSpace-A. Arguments are copied from stack-A to stack-B, according to the EndPoint&#39;s signature. Thread-B&#39;s VMSpace is now changed to VMSpace-B, and the stack pointer changed to point to stack-B. The return address is set to special code that traps back to LIS.EXE. Thread-B now jumps to executing the proper method on the real Object-A. 
   The return path is symmetrical, and it includes copying the return arguments back to stack-A, and switching back to VMSpace-A. 
   If Thread-B deletes the last reference on its proxy, then a trap is taken to LIS.EXE, which removes EndP- 1  from ITable-B. A reference is Release( )d from EndP- 1 , and the trap returns. The proxy is deleted. 
   Thread-A removes Object-A from the NameSpace. LIS.EXE deletes one reference from the object itself, and one from EndP- 1 . If this was the last reference on EndP- 1 , this indicates that the object is no longer in the ImportTable of any other IPCSpace. Moreover, it indicates that the object is not in the NameSpace. Therefore, the EndPoint can be safely destroyed. 
   At application cleanup time, LIS.EXE walks the ImportTable and Release( )s all the EndPoints. It then walks the ExportTable and removes all EndPoints from the NameSpace. Each EndPoint is then Release( )d. If the reference count of one such object does not go to zero it means that some application is actively using the EndPoint. There are two equally valid alternatives for handling this case. LIS.EXE could prevent the application from terminating until all references are gone. Alternatively, it could mark the EndPoint as dead and let the application terminate. If some other application tries to use a dead EndPoint, it receives an exception. 
     FIG. 24C  illustrates the structure involved in the foregoing operations of the LIS. In the example of  FIG. 24C , a first object, Object-A, resides in a first address space, Address Space  2 , and a proxy object, Proxy-A, has been inserted by the LIS into a second address space, Address Space  1 . LIS.EXE (corresponding to the LIS.EXE illustrated in  FIG. 24B ) provides communication between the two address spaces. Proxy-A has an import index of “3” to the third entry in the import table  2472  in LIS.EXE. The import table  2472  has a pointer to an endpoint (EndPoint- 1 ) which is exported by Address Space  2 . Endpoint- 1  includes the type signature and address of (i.e., a pointer to) Object-A in Address Space A. The export table for Address Space  2  in LIS.EXE (not illustrated) would have its own pointer to the same endpoint. 
   Endpoint- 1  has a list of signatures (“iii”) which define the bits to be taken from the top of Proxy-A&#39;s stack, Stack-A, in order to obtain all the necessary arguments to be passed in the method call. Information is put onto Stack-A as part of the call to (the proxy of) Object-A.  FIG. 24C  illustrates Object-A as including a V-Table pointer and a method table, in which the second method points to the implementation code of Method  2 . Endpoint- 1  contains an “object address” field containing the address (“0X123) of Object-A (i.e., the pointer to Object-A) and a signature (“iii”) for Method  2  in Object-A&#39;s method table. 
   Proxy-A has an index value of “3” into the import table  2472 . Upon the occurrence of an IPC trap, IPCSpace points to the import table  2472  as a collection of objects that the IPC imports or exports. Stack-B in address space  2  is a free stack, ready to accept incoming method calls. Upon occurrence of the IPC trap, the IPC looks at Stack-A and finds the import index (i.e., “3”) and therefore goes to the third entry in its import table. This entry points to EndPoint- 1 , and EndPoint- 1  has the signature (“iii”) for Method  2  of Object-A. The following values are therefore copied to Stack-B as part of the call: arg  0 , arg  1 , arg  2 , method  2 , the object address 0X123, and the program counter for the code “call”. As a result, Method- 2  of Object-A is called with arg  0 , arg  1  and arg  2  as specified in Stack-B of Proxy-A, and therefore the resulting communication has the appearance of a local call. 
   The Loadable IPC Manager is not only run-time loadable from the run-time resources into the kernel by the loader, but is also unloadable. Moreover, there may be more than one Loadable IPC Manager stored among the run-time loadable resources of the operating system. For example, a very simple Loadable IPC Manager—which takes up less space in memory—may be used in cases where the communication needed is within the same machine. A more powerful Loadable IPC Manager may be called whenever it is necessary to communicate with a thread running in another machine. 
   Object Mutation by Applications External to the Object 
   An object consists of an interface, an instance pointer, an implementation, and some state. The interface is a list of methods. The instance pointers and interfaces are exposed to other objects; the state and the implementation are not. Worker threads execute implementation code that accesses and modifies the state. Once an object instance has been created, the instance pointer, interface, and implementation are traditionally immutable, only the state can be changed by method calls. 
   The preferred embodiment of the invention allows run-time changes to the ordinarily immutable part of an object, even while the object is being used The term mutation as used in this specification refers to the act of atomically changing an ordinarily constant part of an object, such as a method implementation. The thread performing the mutation is called a mutator. 
   A mutator must translate the state of the object from the representation expected by the old implementation to the one expected by the new implementation. It must also coordinate with worker threads and other mutators through suitable synchronization mechanisms. Transition functions capture the translations that are applied to the object state and to the worker thread&#39;s execution state. In order to limit the amount of metadata, execution transitions only happen between corresponding clean points in the old and new implementations. 
   A number of mechanisms can be implemented using mutation. Interposition is done via replacement of the object with a filter object that points to a clone of the original object. A dynamic software upgrade would replace the incorrect implementation of a method with the corrected one. Run-time code generation might use a stub implementation as a trigger. Mutation can be used to replace generic code with a specialized version that exploits partial evaluation by treating ordinarily non-constant state as immutable. Once the optimistic conditions are no longer true, mutation allows reverting back to the generic code. Execution profiling might indicate that a different implementation would perform better, and trigger a mutation. Object mobility is realized by turning objects into proxies and vice versa. 
   One example where mutation in accordance with the present invention was found to be useful was in device drivers. In one configuration on the x86 the invention was implemented with minimal floppy and disk drivers that called BIOS(ROM) functions to do the work. A loadable driver would later take over and mutate the BIOS driver with a real driver, transparently to the filesystem. 
   While only methods within an object can change the object&#39;s state in conventional operating systems, the present invention provides a mutation object which, during run time, can dynamically change the state of other objects as desired. The state of an object includes the object&#39;s pointers. For example, to change an implementation of a method listed in the object&#39;s interface, the pointer from the interface for that method would be changed to point to a different implementation of that method, the change in the pointer value being a change in the object&#39;s state relating to the fundamental structure of the object. 
     FIG. 25  illustrates the interface of the mutation object, IMutate, which includes the following methods: Query Interface  2510 , Add Reference  2520 , Release Reference  2530 , MutateVTable  2540  and MutateObject  2560 . The operation of the Query Interface, Add Reference and Release Reference interfaces  2510 ,  2520 ,  2530  have been described above in this specification. 
   The MutateObject method  2560  is a general method enabling the user to change any pointer or register in an object. The MutateVTable method  2540  is a special case, the method being directed specifically to changing the VTable pointer. One example of the general MutateObject method  2560  is illustrated in  FIG. 26 , in which the MutateObject method changes the interface pointer  2610  for method_i in the Object Interface  2620  from Implementation_A to Implementation_B. 
     FIG. 27  illustrates an example of the operation of the MutateVTable method. In this example, the object being altered by the MutateObject method has a VTable  2710  which can point to one of two interfaces  2720 ,  2730 . The two interfaces  2720 ,  2730  can list different methods or, if the same method is listed in both interfaces  2720 ,  2730 , then their interface pointers  2750  for the corresponding method can point to different implementations, as in the case of Method_A, or to the same implementation, as in the case of Method_C. 
   The synchronization mechanisms suitable for implementing mutation can be divided into three groups:
         Mutual exclusion: Mutation cannot happen while workers are executing methods of the object to be mutated. The implementation can be a read-write lock, disabling preemption on a uniprocessor, or a holding tank [Cowan96] with reference counting. Mutual exclusion is simple in that there is no worker state associated with the object when mutation is allowed to happen.   Transactional: Roll back the workers that are affected by mutation. Mutators and workers operate on an object transactionally and can be aborted when necessary.   Swizzling: Modify the state of the workers to reflect mutation. Instead of waiting for workers to exit the object or forcing them out, the third mechanisms just suspends them. The mutator then modifies the state of each worker to reflect the change in the object.       

     FIG. 28  illustrates how object mutation is synchronized by mutual exclusion, which is the simplest synchronization embodiment. When the Mutate Object is called, the first step is to prevent any threads from accessing the object or initiating new activity with the object (block  2810 ). Then a determination is made whether any threads or worker threads that were already running with or within the object are still running (block  2820 ). If so (“YES” branch of block  2820 ), then the object mutation process is postponed until such threads finish running with the object. If no worker threads are running (“NO” branch of block  2820 ), then the Mutate Object is allowed to mutate the object (block  2830 ). This step may be reached immediately or reached only after waiting. After the object has been mutated, threads again are permitted to access the object (block  2840 ). 
   One limitation of the synchronization by mutual exclusion is that there is a delay imposed while the system waits for all worker threads running with the object to finish. This delay can become unacceptable if one of those threads gets blocked before finishing by an event beyond its control. 
   One way of ameliorating such delays is to use transactional synchronization.  FIG. 29  illustrates how object mutation is synchronized by transactional synchronization. The first two steps of this method ( FIG. 29 , blocks  2910 ,  2920 ) are identical to that of  FIG. 28  (blocks  2810 ,  2820 ). What is different is that if there are worker threads still running (YES branch of block  2920 ), instead of waiting, the worker threads still running are rolled back to their starting points (block  2930 ) and the object mutation is performed (block  2935 ). The threads are then reactivated (block  2940 ) and access by other threads to the object is re-enabled (block  2950 ). 
   One limitation of transactional synchronization is that rolling back the threads entails a delay. Synchronization by swizzling ameliorates such a delay because it does not require rolling back any running threads.  FIG. 30A  illustrates how synchronization by swizzling operates. The first two steps (blocks  3010 ,  3120 ) are identical with the steps of blocks  2910  and  2920  of  FIG. 29 . What is different in  FIG. 30A  is that if there are threads still running (“YES” branch of block  3020 ), then the still-running threads are suspended temporarily (block  3030 ) and their states are modified to reflect the mutation of the object (block  3040 ). At about the same time, the object is mutated ( 3050 ). Then, the suspended threads are re-activated (block  3060 ) so that they continue their operations at the points at which they were suspended, except that their subsequent operations are carried out in the mutated version of the object. Access by other threads to the object is re-enabled at about this time (block  3070 ). 
   Preferably, swizzling is not attempted except at clean points in the implementation that have possible mappings, transition functions or return addresses. The definition of clean points is discussed in “Asymmetric Real Time Scheduling on a Multimedia Processor” by A. Forin, A. Raffman, J. Van Aken,  Microsoft Research Technical Report  MSR-TR-98-09, February 1998. Thus, it is preferable to pre-establish the locations of such clean points in an implementation and to pre-compute their transition functions. The computation of such transition functions is not within the scope of the present invention, and reference is made to the above-cited publication. 
     FIG. 30B  illustrates one example of structure involved in the process of  FIG. 30A  as applied to a particular thread. The thread has a stack  3080 . A stack pointer  3082  points to a Condition Wait function  3080   a  on the stack  3080  at which the thread happens to be blocked in the present example. The next item on the stack  3080  is a clean point state  3080   b , followed by a clean point  3080   c , followed by data or integers  3080   d , which in turn is followed by a pointer  3080   e  to the “old” object  3084 . The clean point  3080   b  points to a particular method  3086  of the object  3082 . In this example, the method  3086  states “add 1,2” and “call condition wait”, which is a call to the Condition Wait function  3080   a . The goal of the mutation process in this example is to replace the “old” object  3084  a “new” object  3088 . The process identifies the stack pointer  3082 . Next, the process rolls back the stack  3080  to find the bottom of the stack, which rolls back the thread. The stack  3080  is changed in place to the configuration illustrated in  FIG. 30C  by substituting a new Clean Point State  3080   b ′, a new Clean Point  3080   c ′ and a new object pointer  3080   e ′ for the old ones in the stack  3080 . The new Clean Point  3080   c ′ points to a particular method  3089  of the new object  3086  while the new object pointer  3080   e  points to the new object  3080  itself. The new method, for example, states “subtract 2,2”. 
   One advantage of object mutation is that it enables significant changes to be made in the operating system in a manner that is transparent to the application or driver that is running. This advantage is apparent in considering various applications of object mutation. 
     FIG. 31  illustrates an application of object mutation in carrying out object interposition. In the “BEFORE” portion of  FIG. 31 , an object  3110  is copied to produce a copied version  3120  of the object  3110  (in the “AFTER” portion of  FIG. 31 ) and is mutated to produce a mutated object  3130  (also in the “AFTER” portion of  FIG. 31 ). The copied object  3120  is preferably identical to the original object  3110 . The mutated object  3130  is different in that the interface points to an implementation of an interpose method, while the pointer register  3140  of the object is changed to point to the copied object  3120 . 
   The interpose method listed in the interface of the mutated object  3130  carries out the purpose of the interposition. For example, the purpose of the interposition may be to provide a filter between any external thread (e.g., the thread  3160 ) and the object  3120 . The interposition method and interposition implementation carries out the function of such a filter, so that the mutated object  3130  is the filter object. For example, the interposition method may entail determining whether the thread  3160  intends to write to the object  3120 , and, if so, to block any such write operation, so that the object  3120  is available to the external thread  3160  for read operations only. 
     FIG. 32  illustrates an application of object mutation in carrying out dynamic software upgrading. In the example illustrated in  FIG. 32 , a file manager object  3210  uses a disk driver to manage a disk drive memory, and the operating system has available two different versions  3220 ,  3230  of a disk driver. If the file manager object  3210  is using the low-speed disk driver  3220  and needs to upgrade to the high speed driver  3230 , the MutateObject  3240  changes the object pointer of the file manager object  3210  from the low speed driver  3220  to the high speed driver  3230 . Since no objects were exchanged or removed, this upgrade is transparent to the application that is running. 
     FIG. 33  illustrates an application of object mutation in carrying out run-time code generation. In the example of  FIG. 33 , an object  3310  provides, in its interface  3320 , a method or algorithm of dividing one number, x, by another number, y. Two versions  3330 ,  3340  of the algorithm are available in the object. The simplest version  3330  involves multiplying the reciprocal of y by x. The more sophisticated version  3340  involves explicitly carrying out the division of x by y. This latter version is suitable for repeated divisions of different dividends by a common divisor which can change from time to time. In such a case, it is desirable to mutate the object by moving the interface pointer  3335  from the simple version  3330  to the sophisticated version  3340 . As a result, a different object code is provided to carry out the division operation. This mutation is carried out dynamically as need at any point during run time. In the sophisticated version  3340  of the algorithm, the technique of retaining the same divisor (e.g., 2) over successive division operations is typical of a class of techniques known as “constant folding”, while the technique of retaining the same type of operator (e.g., the division operator) over successive operations is typical of a class of techniques known as “inlining”. The sophisticated version of the algorithm is applied as long as certain assumptions or parameters are true, namely that (1) each operation is a division of the operand and (2) the divisor is 2. Once these assumptions are no longer true, object mutation is applied to replace the current version of the algorithm (e.g, from the sophisticated version). For example, if the assumption that the divisor is 2 over successive division operations no longer applies, object mutation is invoked to return to the simpler version of the algorithm which makes no assumptions regarding the divisor. 
   Preferably, the implementations of an object contain already-compiled machine or object code for carrying out the corresponding method. However, in an alternative embodiment, the implementation is represented only by source code, to save space for example, and it is compiled by a compiler  3350  only as needed. 
     FIG. 34  illustrates an application of object mutation in carrying out object mobility. If the object that is the target of a method is in a different machine or a different address space, and thus can not be called directly, a proxy is interposed for delegation. Instead of calling the actual object, the client will call the (local) proxy object. The proxy marshals the parameters into a message and sends it where the actual object is located. There the message is received and dispatched to a stub object. The stub unmarshals the parameters and calls the actual method. On the return path the stub similarly marshals any return values and sends them in a message back to the proxy that in turn unmarshals and returns. 
   Aside from taking longer to execute, the remote object call through a proxy looks exactly the same as a local call directly to the actual object. Not only is the implementation of the server transparent to the client, but the location as well. 
   In the “before” portion of  FIG. 31 , threads  3110  and  3120  in Address Space  1  access an object  3130  in Address Space  2  via a proxy object  3140  in Address Space  1 . That is, the threads  3110 ,  3120  in Address Space  1  point to a proxy object  3140  in their own address space, and the proxy object  3140  points to an object in Address Space  2 . A thread  3150  in Address Space  2  points directly to the object  3130  since both the object  3130  and the thread  3150  reside in the same address space. 
   Under certain conditions, such as frequent calls by threads in Address Space  1  to the object in Address Space  2 , accessing the object  3130  through the proxy  3140  is relatively inefficient. This problem is solved by mutating the proxy  3140  into a clone of the object  3130  and mutating the object  3130  into a clone of the proxy  3140 . This, in effect, “moves” the object from Address Space  2  into Address Space  1 . As a result, calls to the object by the threads in Address Space  1  are faster because they do not pass through an intermediate proxy. Leaving a proxy in Address Space  2  from whence the object was “removed” is necessary so that the thread in Address Space  2  can find the object by pointing to the proxy in Address Space  2 . 
     FIG. 35  illustrates how a thread  3510  in Address Space  1  calling upon an object  3520  in Address Space  1  (“BEFORE” portion of  FIG. 35 ) can be diverted to an object  3530  in Address Space  2  by mutating the object  3520  into a proxy with a pointer to the object  3530  in Address Space  2 . The object  3520  in Address Space  1  has a pointer register  3560  which, as a result of the object mutation, is loaded with the address of the object  3530  in Address Space  2 . Address Space  1  and Address Space  2  may each reside in different machines, in which case the object  3530  in Address Space  2  must contain information identifying the other machine, as well as the memory location in the other machine. 
     FIG. 36  illustrates one example of a mutatable structure of the Virtual Memory Manager (VMM). For each method in the interface, namely IVMSpace, IVMMap and IVMView, there are two alternatives. For each of these alternative, there is a set of corresponding implementations. Each alternative points to the corresponding implementation. By mutating the VTable pointer (using MutateVTable), any one of the three methods may be changed between its two alternative versions. Moreover, by changing the interface pointers from the methods (using MutateObject), different implementations may be chosen for each method or for some methods or for one method only. 
   Programming Model with Dynamically Loadable Objects 
   Preferably, Namespace is a demand-loading object that supports the following new programming model. The main( ) entry point for an image is a constructor that returns the object. When an application tries to bind to a name that does not exist, the namespace invokes the loader, which looks for and instantiates a component with the given name. The loader then invokes the component&#39;s entry point, registers the resulting object in the namespace, and returns it to the application. When the application releases its last reference to the component the namespace can unload the component or choose to keep it cached. 
   In accordance with programming model of the invention, an application program or thread can access an object from any source, such as a local area network, a global network, disk or working memory, via the Namespace object. As will be shown in a working example described below, this places the loading of resources, such as plug-and-play device drivers, under the exclusive control of the application thread. With the programming model of the invention, the application thread includes calls to Namespace for each object on an as-needed basis. One advantage is that working memory space is not occupied by unneeded resources. Another advantage is that the time penalty required to load a resource, such as a device driver, is not incurred except when necessary. A further advantage is that application thread controls when to load such resources and can therefore be allowed to do so in an optimal manner designed by the programmer. In the prior art, the application thread or program did not control the loading of certain resources (such as device drivers). 
     FIG. 37  illustrates a method in accordance with a preferred programming model of the invention. An application thread currently running finds that it needs a particular object at some point. The application thread presents to NameSpace the name of the desired object (block  3710  of  FIG. 37 ). In response, NameSpace returns the IUnknown pointer of the desired object (block  3720 ). The application thread then accesses the desired object in memory using the object&#39;s IUnknown pointer to find the correct memory location (block  3730 ). The application thread may need the object because it needs to use a method provided by that object. In such a case, the next step is for the application thread to call the QueryInterface method of the object and specify the Interface corresponding to the desired method (block  3740 ). For example, the application thread may need to perform a copy method provided by the object, in which case the thread may ask for an ICopy interface. A determination is first made whether the object has the desired interface (block  3750 ). If not, an exception is taken (NO branch of block  3750 ). Otherwise, the object&#39;s QueryInterface method returns a pointer to the desired interface (block  3760 ). The application thread then invokes the corresponding interface and implementation via the pointer to the new interface (block  3770 ). 
     FIG. 38  illustrates operations carried out under control of NameSpace in support of the method illustrated in  FIG. 37 . The steps illustrated in  FIG. 38  are generally transparent to the application thread. First, NameSpace receives the application thread&#39;s request to look up the name of the desired object (block  3810 ). NameSpace determines whether that name is already registered in NameSpace (block  3820 ). If it is already registered, then the desired object is already present in working memory and an IUnknown pointer to the object is available (YES branch of block  3820 ). In this case, the next step is to return the IUnknown pointer to the application thread (block  3830 ). Otherwise (NO branch of block  3820 ), the desired object has never been loaded into working memory. In this case, NameSpace requests the Loader to load the object into working memory (block  3840 ). The Loader, in most cases, will find the desired object on disk. 
   If it is not found on disk, the Loader may be allowed to search other sources for the object, such as memories accessible on a network (local or global) for example. The Loader loads the object into working memory (block  3850 ). NameSpace registers the object&#39;s name (block  3860 ) and returns an IUnknown pointer specifying the object&#39;s location in memory to the application thread (block  3830 ). 
   One feature of this method is that the application thread is allowed to find and use objects which exist in any accessible location, such as an unsaved document that exists only in working memory, for example. This provides programming flexibility on a scale not possible prior to this invention. 
     FIG. 39  illustrates how a previously unused object stored outside of working memory (e.g., on disk) is loaded into working memory. It should be understood that the references made here to “working memory” include any other memory that may be used to supplement working memory in cases where actual working memory space is limited. For example, in some cases programs to be loaded into working memory are loaded instead into “cache” locations on the disk which are treated as part of the working memory. 
   Before the object is loaded into working memory, space must be allocated in the working memory for the object&#39;s image. The image will contain one or more VTables, Interfaces and Implementations (block  3910  of  FIG. 39 ). The image will also specify an EntryPoint, which is the constructor of the object. Once the image is relocated and loaded in main memory, the constructor is invoked (block  3920 ). Such a constructor is automatically accommodated in the C++ programming language. In a preferred embodiment, the invention is carried out in C++. The constructor allocates dynamic memory to hold the new object&#39;s state. The object&#39;s state is initialized, including the object&#39;s VTable and Interface pointers (block  3940 ). An IUnknown pointer to the object is produced specifying the memory location of the object (block  3950 ). This is the pointer that NameSpace returns to the application thread in the operation illustrated in  FIGS. 37 and 38 . 
     FIG. 40  illustrates how the method of the programming model illustrated in  FIG. 37  improves plug-and-play technology. When a device is plugged into a port of the computer (block  4010  of  FIG. 40 ), it is conventional for the system to immediately load the driver for that device or to halt operations while it tells the user that a driver cannot be found. While the automatic nature of plug-and-play technology is convenient, such interruptions are time-consuming. Even if the driver is found and loaded automatically, system time is consumed in such an operation and delays operations being performed by the user. Such delays may be unnecessary, particularly if the user is not going to use the plugged-in device immediately. In the invention, such an automatic loading operation of a plug-and-play driver does not take place, at least in the preferred embodiment. The only requirement is that at some point before the plugged-in device is called for by an application, the driver for the device is made available either on the disk or on another accessible source such as a network (block  4020  of  FIG. 40 ). Meanwhile, the system performs other tasks until an application makes a call for the plugged-in device (block  4030 ). Once an application calls for the device, the name of the device driver is handed to NameSpace (block  4040 ), and the process of  FIG. 37  is carried out. The name may be handed to NameSpace by the application itself, particularly if the application was written for the programming model of  FIG. 37 . Otherwise, another utility may be provided to determine the name of the driver for the device, although this latter mode is not preferable. 
   Examples of objects that are accessed in accordance with the foregoing programming model include NameSpace, VirtualMemoryManager (VMM), InterProcessCommunication (IPC), VMSpaceVMViewsockets, normal applications such as word processors and spreadsheets, and the like, as well as objects not stored on disk, such as objects in other machines or available on a network, or files or documents in working memory that have not yet been saved. 
   Loader Optimizations 
   The present invention optimizes the loader to compile software to run on hardware other than that for which it was designed. In a conventional system, a compiler produces an image and a linker puts it together. The image defines where each piece of code or data is loaded in main memory. With dynamically linked libraries, an image defines what pieces in the library are desired for multiple images. But, some compilers (for example, in embedded systems) do not support dynamically linked (shared) libraries. Many embedded systems do not have loaders. So, the compilers for these systems only produce a single image. As will be described later in this specification, a loader embodying the invention is optimized for use in embedded systems by connecting multiple images in a more flexible way. In part, this entails linking with a shared library after link time. For this purpose, the invention preferably includes a program to manipulate the image produced by the compiler after link time. 
   When programs are compiled and linked, they are targeted for some particular platform. Optimization decisions are made with that target in mind. If the program is run on a different platform, the optimal decisions would be different. We describe here how those decisions can be delayed and the optimization choices changed after they have been made. 
   In embedded systems, a small memory footprint is crucial and virtual memory either unnecessary for many application or completely unavailable. On a virtual memory system with multiple address spaces, the optimization goal is to maximize sharing of memory. In a system where multiple address spaces is not the norm, there is no reason to incur the cost of supporting memory sharing. The present invention eliminates this cost in such a system. 
   It is desirable to be able to run optimally the same program on multiple platforms. One alternative is to use source or intermediate representations such as bytecode. But those require compilers to be around at run-time, implying large memory and many cycles to do a good job at compiling—or slow execution through badly optimized code or interpreters. Instead, the present invention handles completely compiled runnable binaries and adapts them to the platform requirements. 
   Shared Library without Compiler Support 
   New platforms are limited as to what the build tools can do, as provided by hardware vendors. It is desirable to be able to use those tools but still have components that import and export items. The present invention accomplishes this by modifying the executable images of those components, so that they can be bound at run time. 
   The present invention does this by first using the existing build tools up to a certain point (i.e., link time) and then post-processing them with a munger program (at post-link time) and a special loader (at load and run time). 
   In order to be able to run multiple programs at once in a system that does not provide multiple address spaces, the programs have to be placed at different addresses in the same space. When memory is allocated dynamically, a program might not land at a predetermined address. But the program was previously linked for a predetermined address. The invention solves this problem by relocating the program at run time using a run-time loader of the invention. Relocation is done by means of a relocation table that lists all the locations in the program that contain location dependent pointers. The run-time loader reads the pointers from the locations indicated by the relocation table, adjusts the pointer according to the manner in which the program got relocated and puts the adjusted value back into the program. 
   The relocation entries have a type (e.g., “call”) that indicates how the locations should be interpreted. The present invention takes advantage of this feature and the existence of the relocation table by defining a new relocation type (e.g., “call-dll”) for importing from another component. When the run-time loader goes through the relocation table and relocates other relocation entries as usual, it treats the import entries specially. It uses the contents (a number) to determine the component to import from, which export table within that component to import from, and which ordinal within that table to use. It then replaces the contents with the pointer to the value found within the export table indexed by the ordinal. 
   The invention is capable of implementing dynamically linked (shared) libraries in a system whose compiler does not support dynamically linked libraries. For this purpose, it is desirable to have a dangling reference left in an image compiled by such a compiler. This dangling reference enables a loader embodying the invention to support shared libraries without support from the compiler. In order to understand how the invention accomplishes all this, it is necessary, first, to understand the current state of the art for implementing shared libraries with full support from the compiler. This will now be explained. 
   Problem Solved by the Invention 
     FIG. 41  illustrates a tutorial example of an image containing an instruction in its text section to call the function CurrentTime( ). CurrentTime is a symbol. In the example of  FIG. 41 , an instruction invoking the function CurrentTime is represented in machine language as 0X33 (“call”), 0X99 (the four-byte address of CurrentTime). In a simple system, the compiler puts this address into the instructions in the text section of the image. In a more complex system in which the compiler can support a dynamically linked (shared) library, the compiler is capable of leaving a 0 in the text section instead of the address. In this case, which is illustrated in  FIG. 41 , the compiler puts a reminder or a “relocation” entry in the relocation section or table pointing to the 0 in the text section as the place to fix the value that comes from the symbol “CurrentTime”. The relocation table is a reminder of what must change before being able to execute the image. A simple minded tool set would not be able to produce anything more than an object in which the text section has a defined address for “CurrentTime”. Such a tool set at load time would want to get rid of the relocation table and find out what the implementation for CurrentTime is, which is someplace else in the image, and would want to put the address of that in the text section. The end of result would be a non-relocatable image. The present invention overcomes such problems. 
   Conventional Linking Process without DLL&#39;s 
   A conventional process of constructing an executable image from separate files is illustrated by an example shown in  FIGS. 41 and 42 . A C-language source file called FOO.C contains a statement T=CurrentTime( ). The compiler compiles this to make an object file FOO.OBJ having a text section containing instructions including “call” (0X33) and a zero (0) (instead of the address of the symbol “CurrentTime”). FOO.OBJ also has a relocation section or table and a symbol section. Since an implementation for CurrentTime has not been provided thus far, the compiler produces the symbol section of FOO.OBJ with a location “undefined” for the symbol CurrentTime, indicating that its location is to be determined later. The relocation table has a pointer to CurrentTime in the symbol table and to the 0 in the text section. This is pending work for the linker. 
   In the present tutorial example, the symbol CurrentTime is defined in another file, namely an object file called KERNEL.OBJ. This definition includes, in this tutorial example, “0X99” as the address of the first instruction of CurrentTime. The linker puts together the different sections of FOO.OBJ and KERNEL.OBJ to produce a single file FOO.EXE, which is an executable image having no pending work for the linker. The new file FOO.EXE. has its own text section including the instructions “0X33” and “0X99”. “0X33” is (in this example) the machine language for “call” and was taken from the text section of FOO.OBJ while “0X99”, address of the first instruction in CurrentTime, was derived from the text section of KERNEL.OBJ. The linker, in linking the objects together, has changed the call for CurrentTime in the text section to 0X99, since that is where KERNEL.OBJ specifies the location of CurrentTime. Thus, the linker looks at all undefined symbols and finds their locations and definitions or instructions. 
   If the linker starts linking a file at address  0 , the virtual memory system ensures that the addresses in the file are valid at execution time (across multiple images that is). But, if virtual memory is not available, the linker must relocate (rebase) the addresses in the file before execution time, because the memory range starting at address  0  will probably not be available, and in any event would only be available for one of the images. The list of these pointers is the relocation table and is created by the compiler at the time of creating the object file. Pointers can be in the text or data sections. The linker must load an object at a different location if, in absence of virtual memory, two different objects need to be loaded in the same location/address. In this case, one of the objects is displaced from location at which it was programmed to run. In this case it is necessary to find all the pointers in the file that must be changed. 
   The linker uses the relocation section to find unknown symbols, and, from the symbol sections of the objects, FOO.OBJ and KERNEL.OBJ, resolves the unknown symbols (e.g., 0) to known symbols (e.g., 0X99) in order to produce the executable image FOO.EXE. Once this is accomplished, there is no further need for the relocation section and it therefore is not present in the final version of FOO.EXE. 
   Conventional Linking Process with DLL&#39;s 
   A conventional program that uses a shared library or DLL works by keeping certain symbols undefined in the final (“linked”) image. Later, some other program defines the symbols. But, before that, the image is not executable. Binding is the process of resolving the symbols. Deferred (late) binding involves postponing the resolving of certain symbols so that the file remains non-executable. Such a process is illustrated in  FIG. 43 . Referring to  FIG. 43 , in implementing shared libraries, the linker is provided at link time with more refined information that, instead of “KERNEL.OBJ”, refers to a file “KERNEL.LIB”. The information in KERNEL.LIB, in the present example, indicates that the symbol “CurrentTime” is in the third entry of an export table of a shared library called KERNEL.DLL. The linker copies this information to an import table of FOO.EXE. The relocation table links this entry in the import table to the undefined ( 0 ) entry in the text section. 
   Conventionally, in using a dynamically linked library, some linking is done after link time. When the library is created, the symbol table (which is large) is thrown away and a list (an export table) is created of the items to be exported by the library (which is an abbreviated version of the symbol table). In the present example, CurrentTime is in the export table of KERNEL.DLL. The import table of FOO.EXE names KERNEL.DLL, so in this example the linker refers to KERNEL.DLL and imports CurrentTime from it. The ordinal ( 3 ) is the order of the item in the stack of the library, and the linker preferably uses this rather than the symbol name since it is shorter. 
   Late Binding without Shared Library Support 
   The present invention enables the loader to be optimized for an operating system having a compiler which does not provide import or export sections and therefore provides no shared library support (e.g., a non-Microsoft compiler). The invention accomplishes this, in part, by postponing at least some of the linking or binding. This requires that the compiler can produce an object file which is not executable. Since it is not known beforehand where the object file will ultimately be allocated (to what address), the compiler must leave something for the linker to adjust. 
   In the case of an external call which is undefined, one entry in the relocation table will have two pointers, one pointing to the location of the instruction that has to be relocated, “the offset”, and the other pointing to the symbol that corresponds to that location. The entry also states the type of relocation. Thus, each entry in the relocation table has three elements: (1) the offset—“offset from the base of the file”; (2) the symbol, which is an index into this table of symbols; (3) the “type” of relocation (for example “call” or “data reference”), which indicates what type of instruction it is. 
   Symbols are either “defined” or “undefined”. Linking, including the type of linking performed in the present invention by post-link time late binding, matches defined and undefined symbols in different object files, until all symbols are defined (“resolved”). This has to be completed before execution by a compiler. The present invention has a munger program to perform the post-link time (“late”) binding—a “postlinker”. The munger transforms to go from the nonexecutable object file (FOO.OBJ) to the executable image (FOO.EXE). 
   One difference between the object file FOO.OBJ and the executable image FOO.EXE is that the object file FOO.OBJ has a text section, data section, relocation section, but not an import table, while FOO.EXE has an import table. Combining all the required objects produces the executable image FOO.EXE. The result of an executable in the dll case is that all the symbols are resolved, split into sections, including relocation and symbol sections. 
   The invention is useful in those case where the compiler is of the type that doesn&#39;t produce such a file. Some linkers get rid of the relocation table and symbols, producing a file which can&#39;t be moved. It is best if the compiler can leave some symbols undefined. If the linker refuses to deal with undefined symbols, the invention can generate or take a fake object that has the same location defined as the desired symbols, so that the undefined symbols will refer to the same location. 
   The compiler is allowed to do the “eager” binding, and the post-link-time linker of the invention, the munger, will undo it later to accommodate late binding. There are four phases, compile time, link time, load time and run time. The post link-time linking of the present invention, late binding, preferably occurs at or before load time. 
   Operation of the Post-Link Time Linker: 
   Referring now to  FIG. 44 , the munger refers to a text (ascii) file, e.g., KERNEL.DEF, which specifies the symbols to be imported from KERNEL.DLL. Files such as KERNEL.DEF have already been produced for programs running on existing machines as the precursors to the KERNEL.LIB files discussed earlier in this specification with reference to  FIG. 43 . KERNEL.DEF has a particular record (record  0 ) specifying the name (KERNEL.DLL), and a unique ID (UUID), e.g., OX123 . . . 9. (There is a conventional tool to generate the UUID.) The munger program uses KERNEL.DEF to access information to combine with FOO.OBJ to produce FOO.EXE with an import table, without requiring the compiler to support import tables, export tables or shared libraries. Record  0  of KERNEL.DEF lists exports from KERNEL.DLL (for example, CurrentTime @  1 , Last @  3 ) not listed in FOO.OBJ that must be included as entries in an import table in FOO.EXE. In  FIG. 44 , record  0  of KERNEL.DEF contains the information that “export=CurrentTime @  1 ”. The munger program looks at KERNEL.DEF and sees “export=CurrentTime @  1 ”. The munger program sees in the relocation table of FOO.OBJ that FOO.OBJ calls for CurrentTime which is undefined in FOO.OBJ (or it may bound to a fake object and needs to be redefined). The munger program does the following for that relocation: The relocation was of the type “call”, and the munger changes the type to “dll-call”, which will be described below. The munger must keep the offset, but it doesn&#39;t need to keep the symbol as the symbol is not as useful in the preferred embodiment. This is because the preferred embodiment support linking by-ordinal, not linking by name and the symbol table is therefore superfluous by this point. 
   The relocation type “dll-call” is a relocation type of the present invention which captures two pieces of information, the ordinal and the index. The ordinal refers to a particular record in the export table of KERNEL.DLL, and in the present example the ordinal is  1 . As a result, the relocation entry reads “dll call symbol  0  @  1 ”, where 0 is the index, which contains a reference to an entry in the import table. This entry in the import table contains the name of the dynamic library i.e., KERNEL.DLL and the UUID of the export table in KERNEL.DLL. The munger does not change any existing relocations, only those specified by KERNEL.DEF so the other symbols aren&#39;t shown in the drawing. KERNEL.DEF has a number of records, and record  0  names KERNEL.DLL and a UUID. The ordinal ( 1 ) references a particular entry in the export section of KERNEL.DLL, which will be described later in this specification. Other variations may be implemented depending upon the machine on which the system runs. 
   The UUID of records in KERNEL.DEF defines the specific export table present in KERNEL.DLL that is to be used by FOO.EXE. In the present invention, a dll such as KERNEL.DLL can have many different export tables, each one uniquely identified by its own UUID. Thus, the UUID enables a specific choice among multiple export tables in the dll. The text file KERNEL.DEF can be generated mathematically using existing tools or by hand by the programmer and has been produced for systems running on existing computers. Such existing files may be used to carry out the present invention. 
   The object file FOO.OBJ of  FIG. 43  lacks an import table because the compiler that produced it did not support import or export tables (in the present example). For an embedded system with very minimal compiler support, the invention decreases the time to market because there is no need to reform such a compiler. 
   The foregoing describes how to produce import tables. How to create an export section for KERNEL.DLL will now be described. The generation from FOO.OBJ of FOO.EXE with an import table has been described, and now the generation from KERNEL.OBJ of KERNEL.DLL with an export table will now be described. 
   Referring to  FIG. 45 , KERNEL.DLL must have an export table. The export table is the one that looks at both the ordinals and the names. Where the system compiler does not support import/export tables, the munger program constructs the export table as follows: It builds an array of ordinals. According to record  0  of KERNEL.DEF, the symbol “CurrentTime” has ordinal number  1  while the symbol “last” has ordinal number  3 . The munger counts ordinal number  1  as the first export table entry and therefore writes the location (“X99”) of CurrentTime in KERNEL.LIB as the second entry in the export table. In an alternative embodiment of the invention, the munger knows the location (“X99”) of CurrentTime because it is stated in record  0  of KERNEL.DEF. In the preferred embodiment, however, the munger finds this location by inspecting KERNEL.DLL. The munger counts ordinal number  3  as the fourth export table entry and therefore writes the location (“X66”) of the symbol “Last” as the fourth entry in the export table. The first and second export table entries are left undefined in this example. The munger must put the definition of the symbol “CurrentTime”, namely X99, into the export table as the second entry (ordinal  1 ) because, otherwise, the tool will fail. The symbol section of KERNEL.DLL defines the name “CurrentTime” with a pointer to the appropriate location in the text section (i.e., X99), and the munger copies this pointer (X99) to the corresponding entry (ordinal  1 ) in the new export table. 
   What goes into the relocation section of any image such as FOO.EXE or KERNEL.DLL, is machine dependent.  FIG. 45  illustrates the object KERNEL.OBJ and the new export section constructed by the munger. The munger combines the two together and thereby produces a new object, KERNEL.DLL which has a new export section, in which there are four entries: the first entry in the illustrated example is void (“ffff”), the next entry corresponds to ordinal  1  and is “X99” which is the defined symbol “CurrentTime”, and a similar entry corresponding to ordinal  3  for the symbol “last”. In resolving the unknown symbols and constructing the export table, the munger looked up the values of the exported symbols, and put them in the export section at the appropriate places (ordinals) as instructed by the definition file KERNEL.DEF. 
   Multiple Export Tables 
   Using the UUID permits the system to manage an array of multiples of export sections. In the multiple export tables illustrated in  FIG. 45 , the first entry is the UUID (123 . . . 99) and a number 4 which equals the number of ordinals, and then the four entries. A second export section has a UUID (456 . . . 999) and six as the number of ordinals. Thus, there can be different versions of the export table designated by different UUID&#39;s. Two versions of the dll within the same dll are provided by having two tables. This is done by giving the two versions of the dll two different UUID&#39;s. The loader at load time looks for the table with the correct UUID. 
   The KERNEL.DLL file resembles a COM object, analogous to performing a query interface at load time. The UUID is the interface to query for, while the export table is the V table. The loader can make the query interface available to applications, thus enabling binding at run-time. This extends prior art functionality (e.g. Windows GetProcAddress) through precise versioning and increased flexibility. 
   Short Cutting Jumps 
   In the prior art, indirectly jumping through the import section confines all of the address transformations required for sharing of memory in a virtual memory system to a single section (the import section). Such transformations allow a dll resource to land in different locations for different executables, which it typically does in virtual memory systems. In virtual memory systems, a single copy of a resource or code stored in one physical location may appear in different locations in the address spaces of different applications. The indirect jump enables each application to find the resource at the address which the application believes it to be, without having to change every call (to the DLL) that appears in the application image. 
   In the foregoing example of  FIG. 45 , the call for CurrentTime involves fetching the address to jump to from a specific entry in the import section, then jumping to that address. This is an indirect call. In an alternative implementation, the entries in the import section could be themselves instructions to jump to the DLL addresses. In this case the instructions in the image are direct calls, and the import section acts as a redirector of sorts. Both implementations obtain the same result: the call instruction in the executable image (direct or indirect that it might be) can be re-directed simply by changing the import section entries. 
   In order to increase memory sharing, those programs importing from shared libraries that are targeted for multiple address space systems do not themselves point directly to the items exported by the library. Instead they point indirectly through the import section. This way all the pointers pointing from one component to another are collected into a small area. Assuming the library lands in different locations in different address spaces, the program can still be shared. In a system where multiple address spaces is not the norm, such indirection is plainly overhead, both in space and in performance. The import section wastes memory and the indirect memory access wastes cycles. 
   This problem is solved in the present invention by providing a jump short-cutting feature in the loader. Jump short-cutting in accordance with the invention requires the loader to go through the relocation table and determine whether the contents of the location pointed to by the relocation entry points to the import section. If so, the loader finds the corresponding exported address in the exporting component. The loader looks at the machine instruction that uses the pointer, and modifies it to use a direct access to the exported address that was found. 
   For instance, on a typical 80386 or 80486 machine there is one machine instruction for an indirect jump and another for a direct jump, both followed by an address. In the present invention, the loader finds the address as it was pointed to by the relocation entry. It looks backward for the preceding machine language code that uses the address. 
   If the code is an indirect jump, it is replaced by a direct jump, otherwise it is left as it is. The address is replaced by the address found by looking at the import section entry and resolving that with regards to the exporter. Similar implementations are done for other machine instructions and other processors. 
   The jump short-cutting of the present invention operates at or after load time in systems not requiring such indirect jumping by short-cutting to a direct call by removing the intermediate jump. If the image is shared, then an indirect jump (an extra jump) is necessary and this feature of the invention is not employed. 
   As described above, the loader goes through the relocation section and finds pointers that need changing. It looks at the export section to see where they go (which symbol they go to). If there is only a small amount of memory that can be shared, the jump short-cutting feature of the invention modifies the indirect jump instruction to a direct jump instruction. This entails changing pointers and instructions and eliminates the needs for an import section. By eliminating the import table/section, memory is saved. Thus, there are two advantages: memory is saved (by discarding the import section) and speed is increased (by eliminating the middle jump). 
     FIGS. 46A  and B illustrate the jump short-cutting process as applied to instructions in the text section of an image. In  FIG. 46A , the image FOO.EXE imports a symbol from the shared library file LIBRARY.DLL by first jumping from the call in the text section to a location (0X66) in the import table (JUMP  1 ) and then from 0X66 in the import section to 0X99 in LIBRARY.DLL (JUMP  2 ). The information retrieved is then returned to the text section (JUMP  3 ). Such indirect jumping has already been described with reference to  FIGS. 43-45 . In the event FOO.EXE is to run on a system having no virtual memory, the linker bypasses the import table by changing the call in the text section to 0X99 and modifying the jump instruction to a direct jump instruction. The result is illustrated in  FIG. 46B , showing the direct jumps from the text section of FOO.EXE to LIBRARY.DLL and back again. Jump short-cutting is also applicable to data references. In  FIG. 47A , an executable, FOO.EXE, has in its text section an instruction to move, the destination being a location  77  in the import table of FOO.EXE. Location  77  has a pointer to the real destination, namely  1010  in a data section. As illustrated in  FIG. 47B , by changing the move instruction to have as its destination  1010  in the data section and modifying the move instruction to a direct move, the import table is bypassed. 
   While the jump short-cutting feature of the invention is useful for fine tuning virtual memory applications for use in embedded systems having no virtual memory, it also may be used for systems with virtual memory in those cases in which the dll resource consistently “lands” in the same location for different executables. 
   Compaction 
   In order to increase memory sharing and variable protections in a system providing multiple address spaces, programs are often linked in a way where different sections (a program file contains headers, and a number of sections containing machine instructions, data, etc.) are aligned on page boundaries. In this way, two sections never share a page, instead there is padding between sections at the end of a section. On a system that does not utilize sharing between multiple address spaces and has no need to protect different sections differently, the paddings simply amount to memory wastage. On such a system, the post-link time loader of the present invention removes the alignments pads by relocating each section separately in memory in a consecutive fashion. In general the loader is free to place each section wherever it pleases, but the compaction is particularly useful because it can result in a significant memory saving when there are multiple components loaded into memory at once. 
   Conventionally, an empty space at the end of a page is filled with zeros so that sections are aligned on page boundaries by the zero-filled padding. Compaction is a process in which the zero-filled padding is removed. Referring to a preferred embodiment illustrated in  FIG. 48 , after load time (block  4810  of  FIG. 48 ), compaction is performed (block  4810  of  FIG. 48 ). Compaction performed at or after load time is so thorough that the text and data sections may be merged (block  4830 ). A prior art linker performs compaction at link time. One embodiment of the invention also includes jump short-cutting (block  4840 ). Jump short-cutting eliminates the import section (block  4850 ) while compaction eliminates padding between images. 
   Since the invention performs compaction at load time, it uses a relocation table to adjust pointers to reflect changed locations. Prior art (Win 98 and NT) does this at link time, not load time, and as a result cannot compact text and data sections together, so that compaction was incomplete. The indirect jump eliminated at load time in the present invention permits elimination of the import section. Since the relocation table disappears at run time by resolution of all the undefined symbols (block  4860 ), and since text and data sections are merged by removing padding between them, the load-time compaction process of the invention leaves only a single section at run time, a significant advantage. 
   Synthesizing Code for Virtual Memory 
   The invention is capable of handing the reverse situation in which a program not originally written to exploit sharing of memory (via the virtual memory system) is to be loaded into a system that has virtual memory capability. The invention handles this by synthesizing at or before load time new code for the program to fully utilize the virtual memory capability of the platform on which it is loaded. 
   Imports that were optimized for direct access can be converted to indirect access at run-time. This is done by generating the otherwise unnecessary import section on the fly, filling it with pointers to the exporting entries, and modifying the jump instruction code from direct to indirect. For jumps and calls it is also possible to make jumps and calls directly, but into the generated import section instead of directly to the final target. In this case the import section contains code that jumps into the final destination. This is the reverse of the jump short-cutting process. 
   The reverse of the load-time compaction process of the invention may be performed at run time as a load-time code synthesis where memory sharing is desired. Such a load-time code synthesis would be useful where the executable was linked for non-virtual memory use and therefore lacks padding to align its section along page boundaries and lacks an import table and indirect jumping. Such load-time code synthesis would be useful to make a text section shareable (if virtual memory is provided) by providing an indirect jump and an import section that did not exist before. The post-link time linker determines from the relocation table whether to jump directly or indirectly. 
   Such load-time code synthesis is illustrated in  FIG. 49 . At load time, the post-link time munger determines from the relocation table of the linked file whether an indirect jump is required for a particular call (block  4910  of  FIG. 49 ). If so, it adds an import table to the linked file (block  4920 ), changes the direct jump to an indirect jump to the import table (block  4930 ) and adds a jump (block  4940 ) from the import table to the final destination (e.g., an export table in a shared library file). The post-link time linker further locates the boundaries (block  4950 ) between successive sections of the linked file (e.g., the boundary between the text and symbol sections) and inserts padding at the end of each section so as to bring these boundaries in alignment with the page boundaries of the memory (block  4960 ). 
   In one aspect of the invention, at link time selected code is taken out of an image and inserted into a separate dynamically linked library file. The code thus taken out of the image is now accessible to other images via the separate dll file containing that code. For example, in order to reverse the process illustrated in  FIG. 44  in which three files (FOO.OBJ, KERNEL.DLL. and KERNEL.DEF) are synthesized into a single image (FOO.EXE), the code synthesis feature of the invention starts with the single image (e.g., FOO.EXE) and extracts code therein to form a separate dll file (e.g., KERNEL.DLL), a separate object file (FOO.OBJ) and so on.