Abstract:
A technique for providing device virtualization in an MS-DOS based operating environment, using an interrupt request (e.g., a non-maskable interrupt), is described. The technique includes executing an application on a processor within the MS-DOS based operating environment and, when the application attempts to address the device to be emulated, causing a processor interrupt to occur. In response to the interrupt, the processor executes code representing the virtualization of a device. The code for servicing the interrupt and emulating the device are written in protected-mode code, stored in the extended memory area, and made available by making appropriate entries into the interrupt descriptor tables (IDTs) for the protected-mode contexts which exist for the native protected-mode operating systems and for the DOS extender. The entries made into the IDT for the protected-mode context established for the DOS extender are accomplished by intercepting communications between the DOS extender and the virtual control program interface (VCPI) and patching the DOS extender&#39;s IDT with the vectors to the stored emulation code.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 08/712,363, filed Sep. 11, 1996 U.S. Pat. No. 5,790,837, issued Aug. 4, 1998 by the present assignee. The contents of that application are hereby incorporated by reference. 
    
    
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection, particularly Appendices A, B and C. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to computer software and, more particularly, it relates to the use of an interrupt request for triggering device virtualization services in a DOS-based operating system environment. 
     2. Description of the Prior Art 
     MS-DOS is Microsoft&#39;s 16-bit operating system which runs on PC-XT- and PC-AT- type personal computers (PCS). This operating system (OS) was originally developed for the Intel 8086 and 8088 microprocessors which operate in what is generally referred to as &#34;real-mode.&#34; Real-mode refers to the way the microprocessor handles memory (i.e., providing the user with a single-tasking working environment in which programs can freely access system memory and input/output devices). It is noted that real-mode, by itself, does not include features such as memory management or protection. 
     Today&#39;s personal computers are equipped with more advanced processors from Intel, specifically the 80386, 80486, and Pentium, all of which are capable of running 8086-compatible code (real-mode). These processors, however, have a more advanced 32-bit protected-mode which provides hardware support for multitasking, data security and virtual memory. Among other things, protected-mode allows access to more random access memory (RAM) than the 1 megabyte (MB) that the standard real-mode MS-DOS operating system permits (additional details described in King, A., &#34;Inside Windows 95&#34;, Microsoft Press, pp. 33-43, 1994, which is herein incorporated by reference). 
     A typical memory configuration of a personal computer is shown in FIG. 1. The memory configuration of FIG. 1 shows a memory area 110 containing 640 kilobytes (K) of conventional memory. Because MS-DOS is designed for managing this conventional memory, an additional memory manager is not required. All MS-DOS based programs require conventional memory. 
     The remainder of the standard 1 MB is referred to as the upper memory area 112 which comprises 384K of memory above the 640K of conventional memory. The upper memory area 112 is generally used by system hardware, such as a display adapter. Unused parts of the upper memory area 112 are called upper memory blocks (UMBs). On any PC, UMBs can be used for running device drivers and memory-resident programs as described below. 
     An extended memory (XMS) area 114 is memory located beyond the 1 MB boundary on computers with 80286, 80386 or 80486 processors. Extended memory requires an extended-memory manager, such as HIMEM. A high memory area (HMA) 116 is approximately the first 64K of the extended memory area 114. The HMA 116 is a small address region, slightly less than 64K, above the 1 MB boundary that can be accessed by real-mode code. The address region of the HMA is from 100000h through 10ffefh. On a computer with extended memory, the setup program can install MS-DOS to run in the HMA 116. This provides more conventional memory to be available for other software programs. 
     Another type of memory available, as shown in FIG. 1, is known as expanded memory. An expanded memory board (EMS) 118 contains additional memory that some MS-DOS based applications can use. Most personal computers accommodate expanded memory. Expanded memory is installed on an expanded memory board 118 and requires an associated expanded memory manager. Computer software programs use expanded memory 64K at a time by addressing a part of the upper memory area 112 known as the EMS page frame. Because an expanded memory manager gives access to a limited amount of expanded memory at a time, using expanded memory is slower than using extended memory. A conventional software program such as EMM386 can simulate expanded memory using extended memory for programs that require it. 
     As MS-DOS matured through the 1980s, much of its functionality was added in the form of device drivers. As set forth in the Encyclopedia of Computer Science, Third Edition, IEEE Press (1993), which is hereby incorporated by reference, a device driver is a program or subprogram that is written to control either a particular hardware device or another software routine. This low-level software program is loaded into memory after the operating system boots and remains in memory for the duration of the operating session. As mentioned, the device driver can deal directly with the hardware of a particular device. The device driver is capable of responding to hardware and software interrupts and interfacing to the operating system through an established MS-DOS device driver protocol in order to serve as an interface between such hardware or software and the operating system (described in &#34;Microsoft MS-DOS 6 User&#39;s Guide&#34;, Microsoft Corporation, pp. 87-89, 1993, and which is also hereby incorporated by reference). It should be noted that device drivers are generally limited in size to 64K bytes. 
     In addition to device drivers, MS-DOS also uses memory-resident software programs, often referred to as terminate-and-stay-resident programs (TSRs). These programs are executed like normal software programs, but after termination, a portion of the program remains installed in memory in order to monitor and/or service hardware or software interrupts. TSRs can also interface with the operating system in a limited sense, but they do not use the MS-DOS device driver protocol. 
     Both device drivers and TSRs are typically real-mode code and reside in memory below the MS-DOS 1 MB limit. In many cases, these resident software programs reside within the conventional 640K of memory which is normally used by MS-DOS to load and execute programs. As mentioned above, they may reside above the conventional 640K in the upper memory area 112 when, for example, a memory optimizing routine is executed such as MemMaker (additional details are provided in MS-DOS User&#39;s Guide, beginning on page 131). In this case, the driver or TSR would be placed into the upper memory area 112 with a link to its location placed in the 640K conventional memory 110 to indicate its memory location in the upper memory area 112. Even so, as more and more device drivers and TSRs are loaded into a system, the maximum allowable memory area for MS-DOS programs, drivers and TSRs is rapidly reduced. 
     To further illustrate the functional inter-relationships of MS-DOS, device drivers, hardware, etc., FIGS. 2 through 4 illustrate the evolution, from a functional perspective, beginning with the original MS-DOS real-mode environment up to present day environments allowing for protected-mode and DOS extenders. It is noted that the functional interrelationships shown in FIGS. 2 through 4 are well known to those of ordinary skill in the art and well documented as well; therefore, each figure is only briefly described. A more comprehensive treatment of DOS extenders and their evolution is provided in Duncan et al., EXTENDING DOS, A Programmer&#39;s Guide to Protected-Mode DOS, 2nd Edition, Addison-Wesley (1992), which is herein incorporated by reference for its teaching of DOS extenders and associated aspects relevant herein. 
     FIG. 2 shows a block diagram of a conventional DOS operating environment with no memory management services. This operating environment is similar to that used in the 8086- and 8088-based PCS. The hardware 210 sends an interrupt request, represented by the dotted line 211, to the DOS operating system 212 which is initially serviced by the DOS Interrupt Vector Table 213. Based on the entries in Interrupt Vector Table 213, the interrupt request can be directed to the DOS OS 212, device drivers 214, TSRs 216, etc. Similarly, software interrupts generated by programs 218 are serviced by the DOS Interrupt Vector Table 213 and directed as appropriate. 
     As mentioned above, with the release of more powerful Intel processors in the late 1980s, special &#34;extensions&#34; to the standard MS-DOS operating system were developed to allow MS-DOS programs to easily access computer memory that would not otherwise be addressable using real-mode code. These extensions (i.e., device drivers) to the operating system take advantage of the protected-mode features of the 8OX86 processors while allowing MS-DOS to continue executing in real-mode. These device drivers implement, as a minimum, memory management services referred to as EMS and XMS services. These services allow software programs to store and retrieve large amounts of data from the memory pool above the 1 MB boundary. However, they do not allow software programs to occupy this memory for execution purposes. The well known programs provided by Microsoft to implement this functionality are called HIMEM-SYS and EMM386.EXE. 
     FIG. 3 shows a block diagram of a conventional DOS operating environment with conventional memory management services 312 installed for managing the extended memory area 114. This operating environment is similar to that used in 80286-based PCS through Pentium-based PCS. As shown in FIG. 3, a protected-mode operating system 310 executes above the DOS operating system 212. An interrupt request 311 generated in this environment passes first into the protected-mode operating system 310 allowing for higher level management. As shown, the interrupt request 311, based on entries in an Interrupt Descriptor Table (IDT, described in more detail below), may be directed to memory management services 312, to the Virtual Control Program Interface (VCPI) 314, to the default DOS service via the DOS Interrupt Vector Table 213, or the like, as in the FIG. 2 type systems. As shown, the protected-mode OS includes the IDT 318 as well as a GDT 320. Additional details of the operation of the VCPI are described in the Virtual Control Program Interface specification, Version 1.0, (1989), which is herein incorporated by reference for its teachings regarding the operation of a VCPI. 
     Concurrent with the development of these memory extensions was the development of DOS extenders. FIG. 4 shows a block diagram of a conventional DOS operating environment with memory management services 312 installed and a DOS extender 410 running. DOS extenders are low-level software components that allow programs to be executed in protected-mode under MS-DOS. The DOS extender 410 allows protected-mode applications 412 to communicate with hardware and the operating system and vice versa. 
     These programs can use the vast amount of memory above 1 MB to execute code as well as to store and retrieve data. However, the DOS extenders do not allow executable code to permanently reside in memory above the 1 MB boundary; rather, upon switching into protected-mode, the DOS extenders can temporarily copy executable code into the extended memory area for purposes of execution, but upon termination, it no longer exists in extended memory. Even so, the DOS extenders allow programs to apply a much simpler and much more powerful programming model. DOS extenders achieve their task by switching the processor into protected-mode while running the program, and switching the processor back to real-mode when the program completes. 
     The DOS extenders also switch the processor in and out of real-mode during the program execution when MS-DOS functions need to be called, or when device drivers or TSRs need to run. To do this, the DOS extender must be in complete control of the system. Thus, a new protected-mode operating environment is created by the DOS extender. As shown in FIG. 4, the newly created protected-mode operating environment created by the DOS extender 410 includes its own IDT 418 and GDT 420. It is noted, however, that when the DOS extender switches back to real mode it then uses system IDT 318 and system GDT 320. 
     In systems with memory management enabled, as shown in FIGS. 3 and 4, the device drivers that provide these services create their own protected-mode environment and already have MS-DOS running as a (virtual 8086) real-mode task. Therefore, they provide contention management services to negotiate memory allocation and mode switching between themselves and DOS extenders. This contention management is implemented by all memory managers today via the VCPI 314. If a DOS extender 410 determines that a VCPI 314 is present in a system, it uses this interface to acquire its protected-mode context and to perform its mode switching (real versus protected). In most cases, the DOS extender 410 also allocates all of its required program memory through the VCPI 314. 
     With this evolution in mind, as available memory, as well as processing power, continue to increase, it is desirable to make efficient use of these computing resources to provide device virtualization, thereby reducing the amount of hardware necessary to provide desirable features. 
     The 80386 and later processors do generally provide for hardware virtualization by allowing certain instructions to be &#34;trapped.&#34; Specifically, if a program executes an &#34;IN&#34; or &#34;OUT&#34; (I/O) instruction, a General Protection Fault handler can be invoked to decide whether or not to let the IN/OUT execute and/or whether or not to emulate the presence of some hardware. This &#34;I/O Trapping&#34; can be applied to any process running below processor privilege level (or ring) 0. Unfortunately, most DOS extenders (Rational Systems&#39; DOS4GW for example) run at ring 0, thus they are not subject to the 80386 I/O trapping mechanism. 
     In addition to the 80386 I/O trapping mechanism, at least one hardware manufacture has used a combination of external hardware interrupts and software to replace the functions of more costly hardware. Specifically, Advanced Gravis has used a technique which combines a Non-Maskable Interrupt (NMI) and a DOS TSR to virtualize hardware. Although this technique provides hardware virtualization for applications executing in real-mode, it does not provide virtualization for applications executing under a DOS extender, since there is no predetermined method for the operating system to direct the interrupt services of the DOS extender. 
     There is, therefore, a real need to provide an efficient way of performing device virtualization which is also accessible by DOS extenders. 
     SUMMARY OF THE INVENTION 
     The present invention involves a technique for providing device virtualization in a DOS based operating environment including the steps of executing an application, under a DOS extender, on a processor within the DOS-based operating environment, where the application executes within a protected-mode context created for the DOS extender and the protected-mode context includes an interrupt descriptor table (IDT). A processor interrupt is generated when the application addresses a predetermined address related to the device to be emulated and, responsive to the processor interrupt, an appropriate entry in the IDT associated with the protected-mode context of the DOS extender is referenced. In accordance with the invention, the DOS extender&#39;s IDT is patched at run-time to vector to device emulation software, thereby allowing the processor to execute computer code designed to emulate the device even though the DOS extender was not present at boot time. 
     The IDT is patched at run-time in accordance with the invention by intercepting VCPI far-calls or, in a presently preferred embodiment, by also intercepting the VCPI &#34;Switch to Protected Mode&#34; call. In particular, a new VIVO driver of the invention intercepts Int 67h handler calls from DOS virtual 8086 (real mode) software. Int 67h handler, function de0ch, is the VCPI &#34;Switch to Protected Mode&#34; call which is made by the DOS extender when it is in real mode, running under the system&#39;s protected mode context. It is a request from the DOS extender asking the VCPI to switch the DOS extender into protected mode and to apply the DOS extender&#39;s protected mode context to the system. The VIVO driver also intercepts VCPI far calls by monitoring Int 67h handler, function de01h, which is the VCPI &#34;Get Protected Mode Interface&#34; call. 
     In accordance with the presently preferred embodiment of the invention, the DOS extender makes the VCPI &#34;Switch to Protected Mode&#34; and &#34;Get Protected Mode Interface&#34; calls at system startup and many times after that during the normal execution of the protected mode application running under the DOS extender. Because the DOS extender is asking the VCPI to apply a new protected mode context, the DOS extender must tell the VCPI what that context is, and so it passes the VCPI (among others) at least the following: physical address of the Page Directory, linear address and size of the DOS extender&#39;s GDT and the DOS extender&#39;s IDT, and the like. These arguments tell the VIVO driver of the invention where the DOS extender&#39;s GDT and IDT are. By temporarily switching the processor&#39;s Page Directory, the VIVO driver can access the DOS extender&#39;s IDT and patch it with the appropriate vector to the stored emulation code. 
     In particular, the VIVO driver of the invention performs the following steps to patch the DOS extender&#39;s IDT during run-time. First, the VIVO driver intercepts Int 67h, function de0ch (switch to protected mode). The VIVO driver then switches the processor&#39;s page directory to that of the DOS extender, which was specified by the DOS extender in the VCPI call. This allows the VIVO driver to properly access the DOS extender&#39;s GDT and IDT at run-time. The VIVO driver then checks to see if its IDT entries have already been made into the DOS extender&#39;s IDT. If so, it checks to see if the selector entry in the DOS extender&#39;s IDT indicates the appropriate entry in the DOS extender&#39;s GDT, where the selector entry includes the GDT entries that get installed into the DOS extender&#39;s GDT when the VCPI get protected mode interface is called by the DOS extender. If either the DOS extender&#39;s IDT entry does not exist, or the selector entry does not match the DOS extender&#39;s GDT entry, the VIVO driver&#39;s code searches the DOS extender&#39;s GDT to find out what selector value corresponds to the VIVO emulation code and then makes the appropriate DOS extender IDT entry using the selector value. The VIVO driver then switches the processor page directory back to that of the system&#39;s protected mode context and jumps to the real VCPI switch call. The patched DOS extender IDT may now be used to emulate the device that caused the interrupt to be generated. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The above and other beneficial features and advantages of the invention will become more apparent from the following detailed description of the invention, of which: 
     FIG. 1 is a block diagram of a conventional memory configuration of a personal computer. 
     FIG. 2 is a high-level functional block diagram of a conventional DOS operating environment. 
     FIG. 3 is a high-level functional block diagram of a conventional DOS operating environment with memory management services installed. 
     FIG. 4 is a high-level functional block diagram of a conventional DOS operating environment illustrating the functional relationships of DOS with memory management services and DOS extenders. 
     FIG. 5 is a high-level functional block diagram of a DOS operating environment as it relates to an exemplary embodiment of the present invention. 
     FIG. 6 is a flow diagram of the initialization operation, at boot time, of an exemplary embodiment of the present invention. 
     FIG. 7 is a flow diagram of the operation of an exemplary embodiment of the present invention, at run time, to provide compatibility with DOS extenders by intercepting VCPI far calls. 
     FIG. 8 is a flow diagram of the operation of a presently preferred embodiment of the present invention, at run time, to provide compatibility with DOS extenders by intercepting VCPI &#34;Switch to Protected Mode&#34; calls. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Overview 
     As mentioned above in the Background section, device virtualization, in general, is known. Unfortunately, when running in the context of DOS extenders (e.g., Rational Systems&#39; DOS4GW), device drivers cannot take advantage of the I/O trapping mechanism, nor can they take advantage of Advanced Gravis technique mentioned above in the Background section. Because many present day applications use DOS extenders for various reasons, a device virtualization technique should be accessible by applications running under DOS extenders. 
     Generally, the present invention provides device virtualization, in an MS-DOS based operating environment, by way of an interrupt request (e.g. non-maskable interrupt) for applications running with or without a DOS extender. The present invention dynamically extends its device virtualization capabilities to applications running under DOS extenders by intercepting communications between the DOS extender and the VCPI at run-time. In particular, when an executing application requests the use of a particular device (e.g., requesting a predetermined I/O address), a processor interrupt request is generated. In response to the interrupt request, the device virtualization code for that particular device can be accessed and executed even for applications running under DOS extenders. It is noted that, in the exemplary embodiment, the present invention relies on the presence of a memory manager, or an equivalent thereof, to provide for a primary protected-mode operating environment including the system IDT and system GDT. 
     FIG. 5 is a block diagram illustrating the functional relationship between conventional operating system components in a typical MS-DOS environment as described in the Background section with reference to FIGS. 2 through 4 and is an exemplary embodiment of the present invention. 
     As shown in FIG. 5, the present invention functionally represents a new functional layer working in conjunction with the protected-mode operating system 310. This new layer is referred to, for purposes of this specification, as VIVO driver 510. Essentially, the VIVO driver 510 becomes an extension of the protected-mode operating system that supervises MS-DOS and DOS extenders instead of simply becoming an extension of MS-DOS itself, as is the case with standard device drivers and TSRs. In the exemplary embodiment of the present invention, in addition to VIVO driver 510 which represents a driver designed to provide the device virtualization services, a VIVO TSR 520 is provided to aid in the dynamic extension of the services by the VIVO driver 510 for use with DOS extenders 410. 
     The present invention includes loading protected-mode executable code, including an interrupt service routine and device virtualization code, into memory, and entering the associated interrupt service vector directly into the protected-mode operating system&#39;s interrupt descriptor table (IDT) 318. The use of IDTs is well known and additional details of descriptor addressing are described beginning on page 41 of the Windows &#39;95 reference incorporated herein. Additional teachings on the use of IDTs can be found in Microprocessors, Intel (1989), which is also herein incorporated by reference. Briefly, the protected-mode IDT 318 determines interrupt vectoring instead of the DOS Interrupt Vector Table 213 even when the protected-mode operating system 310 is running real-mode code. In many cases, such as MS-DOS operating system calls, the services indicated by the IDT entries in IDT 318 simply pass control to the real-mode services indicated by the DOS interrupt vector table that were installed by MS-DOS. As well as being faster and more efficient than real-mode interrupt handlers, using the protected-mode IDT 318 makes the services of the present invention unconditionally available to the system whether it is running in real- or protected-mode. 
     The present invention, by way of the VIVO TSR 520, uses the VCPI `Get Protected-mode Interface` real-mode call (Int 67h, function de01h) or the VCPI `Switch to Protected Mode` real-mode call (Int 67h, function de0ch) to intercept VCPI calls to patch the DOS extender&#39;s IDT. Initially, the strategy was to force all of the DOS extender&#39;s 410 subsequent VCPI 314 protected-mode &#34;far calls&#34; to &#34;pass through&#34; the VIVO driver 510, where the &#34;Get Protected-mode Interface&#34; was used by a DOS extender 410 at startup to acquire a protected-mode context. When the VIVO driver 510 intercepted these far calls, it was running under the context of the DOS extender&#39;s protected-mode operating system (IDT 418, GDT 420). At this point, the VIVO driver 510 installed its interrupt service vector into the DOS extender&#39;s IDT 418 making the VIVO driver&#39;s device virtualization service available to the DOS extender 410 (and its applications) until it terminates. 
     However, in accordance with a presently preferred embodiment of the invention, the strategy for making the actual patch entry into the DOS extender&#39;s IDT 418 is changed. Rather than patching the DOS extender&#39;s IDT 418 during any VCPI far call intercept, the patch is now performed at the intercept of the VCPI `Switch to Protected Mode` call (Int 67h, function de0ch). In this fashion, application programs running under DOS extenders are prevented from generating I/O related interrupts after the switch to protected mode but before any VCPI far calls are made. 
     Although the present invention is designed to operate using most any interrupt request, the exemplary embodiment of the present invention uses the non-maskable interrupt (NMI) as the interrupt request for triggering the device virtualization routine. It is noted that, historically, the NMI has been used by system motherboard logic to notify the operating system of a RAM (memory) parity error, or by I/O devices to notify the operating system of some &#34;unrecoverable&#34; error. Today, however, the RAM parity no longer exists and I/O devices do not implement NMI generation. A typical DOS extender&#39;s NMI handler, however, not knowing specifically how to handle such an event, simply re-boots the system rather than &#34;passing-down&#34; the interrupt to the currently installed DOS handler as it would do with a normal interrupt. Therefore, unlike other interrupt services, an NMI interrupt service routine installed under DOS would most likely be ignored by an application running under a DOS extender. 
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Although illustrated and described below with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. 
     During operation of the embodiment of FIG. 5, when an interrupt request 311 (e.g., NMI) occurs, the protected-mode IDT 318 directs the flow of operation to the appropriate service routine based on the entries therein. If the interrupt is not intended for the VIVO driver 510, then the interrupt is passed on to the normal flow of operation. 
     However, when the VIVO driver 510 of the present invention is present with a DOS extender 410 running, the interrupt request is not passed directly to the DOS extender 410. Rather, if the interrupt request is intended for the VIVO driver 510, the VIVO driver 510 acts on the request directly. If the request is not for the VIVO driver 510, then control is passed to the DOS extender 410, and the DOS extender 410 handles it in its normal way. It is noted that, in the exemplary embodiment of the present invention, because the NMI is used, the device virtualization can be performed immediately following the &#34;faulting&#34; instruction (i.e., the instruction which caused the NMI). 
     Turning to the generation and installation of the present invention, after the computer code embodying the present invention is written, it is compiled/assembled and linked, in the exemplary embodiment, as zero address-based, non-relocatable, flat-model, 32-bit protected-mode code. The computer code embodying the present invention is then saved as a standalone binary file. It is noted that, in preparing the computer code embodying the present invention, protected-mode code is required (at least at the NMI service entry point) since the protected mode IDT&#39;s vectoring mechanism does not allow vectoring to real-mode (or virtual 8086) code. It is also noted that, although some real-mode code is required, the exemplary embodiment of the present invention is implemented using almost all protected-mode code since it is more efficient for vectoring and execution and can be completely located in extended memory, thereby avoiding memory-hungry DOS real-mode applications. 
     Basically, the VIVO driver 510 includes the device virtualization code for emulating the particular device and it includes the code necessary to receive, at run time, the re-directed &#34;far calls&#34; and &#34;Switch to Protected Mode&#34; calls from the DOS extender 410, to make the appropriate entries in the IDT 418, and to pass control on to the VCPI 314. It also includes the Int 67h handler which is used to reflect the VCPI &#34;Get Protected-Mode Interface&#34; call return or the VCPI &#34;Switch to Protected Mode&#34; call return into the VIVO TSR 520. An exemplary implementation of a VIVO driver 510 which intercepts a &#34;Switch to Protected Mode&#34; call (Int 67h, function de0ch) and a &#34;Get Protected Mode Interface&#34; call (Int 67h, function de01h) in accordance with a presently preferred embodiment of the present invention is included as Appendix A. An exemplary implementation of a VIVO driver 510 which intercepts only the &#34;Get Protected Mode Interface&#34; call (Int 67h, function de01h) is included as Appendix A to the parent application, U.S. patent application Ser. No. 08/712,363, the contents of which are hereby incorporated by reference. 
     The installation and operation of the present invention is described with reference to the flowcharts in FIGS. 6-8. As shown in FIG. 6, at boot-time, in the exemplary embodiment of the present invention, an MS-DOS initialization program (e.g., ssinit.com) allocates a predetermined amount of the extended memory area 114 above the HMA 116 using an extended memory (XMS) interface reference, at step 610. An exemplary implementation of ssinit.com suitable for use with the present invention is included as Appendix B. The predetermined amount of memory, in the exemplary embodiment of the present invention, is the size of the 32-bit protected-mode code file. Additionally, in the exemplary embodiment, the allocated memory is below the 4 Megabyte boundary. Then, the allocated memory, using the XMS interface, is locked and the physical address is saved. 
     Subsequently, the code embodying the present invention is copied into the allocated portion of extended memory, at step 612. Appropriate entries are made into the protected-mode system IDT 318 and system GDT 320 to allow the associated hardware or software interrupt to vector directly to the 32-bit service entry point(s) in extended memory, at step 614. It is noted that since the interrupt vectoring is done at the protected-mode level, the 32-bit services are always available and can be initiated by the protected-mode operating system 310 without explicitly performing any time-consuming mode switching. 
     More specifically, steps 612 and 614 are accomplished as follows: 
     The protected-mode code file embodying the present invention is opened and a 1 k-byte portion of the code is loaded into local memory. In the exemplary embodiment of the present invention, this first portion of the code is loaded because it has variables that need to be initialized at predetermined offsets which can only be determined at this stage of loading the program (e.g., configuration parameters). 
     Next, the VCPI &#34;Switch to Protected Mode&#34; call (Int 67h handler, function de0ch) and the VCPI &#34;Get Protected Mode Interface&#34; call (Int 67h, function de01h) is invoked. Three selectors and an entry point offset for the &#34;Get Protected Mode Interface&#34; call are returned. The three selectors and the entry point offset are copied into a predetermined offset in the first portion of the protected-mode code. Also, the segment and offset of a portion of the MS-DOS initialization code is copied into a predetermined offset in the first portion of the protected-mode code. This referenced portion of the MS-DOS initialization code remains resident after the MS-DOS initialization code terminates and the protected-mode code, in the exemplary embodiment, will need to know where it is because it is essentially a shared dataspace used by the VIVO TSR 520 and the VIVO driver 510 to communicate. 
     Next, the first portion of the protected-mode code is copied into the allocated extended memory (e.g., using an XMS interface) and then the rest of the protected-mode code embodying the present invention is copied into extended memory in the same manner. 
     Next, the linear addresses for the protected-mode system IDT 318 and system GDT 320 and the physical address for the Page Directory (using standard 386 instructions) are obtained and saved. A protected-mode initialization program (e.g., a DOS4GW executable) is spawned and the physical address of the allocated Extended Memory, GDT and IDT linear addresses, as well as the Page Directory physical address, are passed thereto. An exemplary implementation of the DOS4GW executable suitable for use with the present invention is included as Appendix C. 
     The functions of the protected-mode initialization program, in the exemplary embodiment of loading the present invention, are to: 
     1) Make three entries at the top of the protected mode operating system&#39;s GDT 320. The first entry is a 32-bit code selector with a linear address based at the beginning of the allocated Extended Memory. The second entry is a data selector to alias the first entry. The third entry is a 32-bit data selector that is based at linear address 0 and has a maximum size (or limit). 
     2) Save the selector base of the entries made in step 1. 
     3) Make an entry into the protected mode operating system&#39;s IDT 318 for vector 02h (the NMI vector). This vector uses the selector base saved above with an offset of zero (in the exemplary embodiment, the NMI entry point in the 32-bit code). 
     4) Save the original Int 67h handler vector. 
     5) Make an entry into the protected mode operating system&#39;s IDT 318 for vector 67h (the Int 67h vector). This vector uses the selector base saved above with an offset of eight (the Int 67h handler intercept entry point in the 32-bit code). 
     6) Then, return control to the MS-DOS initialization program (Appendix B). 
     Finally, the MS-DOS initialization program terminates, leaving a relatively small program in memory--referred to above as the VIVO TSR 520. The general technique of leaving a TSR in memory upon initialization program termination is a well known technique to those of ordinary skill in the art. 
     By way of the Int 67h handler entry, the VIVO driver 510 is linked into the VCPI service/control chain via the IDT 318 for Interrupt 67h so that the hooks are present for the present invention to extend its functionality to DOS extenders 410 when they initialize. It is noted that Interrupt 67h is a control function interface to the EMS 118 and VCPI 314. This Interrupt 67h handler (VIVO driver 510) and VIVO TSR 520 act upon VCPI functions, and then pass control to the normal Interrupt 67h handler installed by the protected-mode operating system. 
     At this point, the protected-mode operating system 310 is configured such that hardware virtualization is available to all real-mode programs and drivers. There are also now hooks in place to allow for the extension of the device virtualization service to applications running under DOS extenders. It is noted that, in the exemplary embodiment of the present invention, although the hooks are put in place during boot time, the extension of the device virtualization services to applications running under DOS extenders is completed at run-time. This is an important aspect of the present invention becomes it allows the present invention to avoid many of the drawbacks of the prior art techniques. 
     It is noted that, although the above-described initialization process is performed using a combination of the MS DOS initialization program and the protected-mode initialization program, in an alternate embodiment, all of the initialization functions performed by the MS DOS initialization program can be performed by the protected-mode initialization program. To do so, the MS DOS initialization program (e.g., Appendix B) needs to pass an extra parameter to the protected-mode initialization program. The extra parameter is the real-mode address of the shared data space within the DOS initialization program. 
     The way in which the VIVO code of the present invention extends its functionality to DOS extenders via the Interrupt 67h handler is described with reference to the flowcharts of FIGS. 7 and 8. 
     In a first embodiment of the invention, as shown in FIG. 7, after having established the VIVO TSR 520, it is triggered when, at run time, the DOS extender 410 calls the VCPI &#34;Get Protected-Mode Interface&#34; function, at step 712, via Int 67h. At this point, the DOS extender 410, initially in real-mode, expects to acquire the protected-mode far-call entry point to the VCPI 314. This entry point is used by the protected-mode operating system created by the DOS extender 410 to communicate directly with the VCPI 314 without invoking software interrupts and without switching to real mode. However, the VIVO TSR 520 of the present invention, having been passed control indirectly, by the VIVO driver 510, at this point gives the DOS extender 410 an entry point to the VIVO driver 510, at step 714. It is noted that, in this embodiment of the present invention, the VIVO TSR 520 relies on the presence of a VCPI 314, or equivalent thereof, for cooperation with DOS extenders 410. 
     By providing DOS extender 410 with an entry point into the VIVO driver 510 rather than the actual VCPI entry point, the DOS extender 410 will now pass through the VIVO driver 510 on subsequent &#34;far calls.&#34; When the DOS extender 410 makes these VCPI &#34;far calls,&#34; the system is in the DOS extender&#39;s IDT/GDT protected mode context As such, when the VIVO driver 510 intercepts the VCPI far-calls, it makes its own interrupt vector entries into the DOS extender&#39;s protected-mode operating system IDT 418, at step 716, similar to that which was done for the native protected-mode operating system 310. After making the appropriate entries, the VIVO driver 510 then passes control to the actual VCPI 314 via the previously saved VCPI protected-mode far-call entry point, at step 718. 
     More specifically, steps 714, 716 and 718 are accomplished as follows: 
     The VIVO driver 510 intercepts the Get Protected-Mode Interface call at the Int 67h entry point (all other Int 67h calls are passed through to the default handler). The VIVO driver 510 saves the real-mode return address for the DOS extender 410 into the shared data space. It then changes the real-mode return address (stored on the processor stack) so that, when the actual VCPI service completes, control will pass to the VIVO TSR 520. So, essentially, all Int 67h calls still get processed by the default handler except a &#34;Get Protected-Mode Interface&#34; call causes the change of the real-mode return address, by the VIVO driver 510, prior to being processed by the default handler. 
     It is noted that, in an alternate embodiment, the function of intercepting the initial Int 67h call from the DOS extender 410 may be performed by VIVO TSR 520. This may be problematic, however, because when the present invention runs under a memory manager, such as QEMM by Quarterdeck Office Systems, the memory manager does not pass the VCPI Int 67h calls through the DOS Interrupt Vector Table, but rather, fields all VCPI Int 67h calls in protected mode via the IDT 318. 
     As shown in FIG. 7, in the case of the &#34;Get Protected-Mode Interface&#34; call, control is then passed to the VCPI 314 which performs its service. The VCPI 314 returns control, via the new return address, to the VIVO TSR 520. At this point, a page table context has been setup in the DOS extender&#39;s data space. The VIVO TSR 520 then makes additional page entries so that the VIVO driver 510 is valid within the DOS extender&#39;s protected-mode context. The VIVO TSR 520 then copies the three selectors being passed back to the DOS extender 410 into the shared data space as well as the VCPI far call entry offset. Next, the VIVO TSR 520 copies its own three selectors (described above) into the DOS extender&#39;s return data space. It then alters the VCPI entry offset (in register ebx) to 16, which is the offset into the VIVO driver 510 for the VCPI far can intercept. Finally, it returns control to the original real-mode return address (the DOS extender 410) saved above in the shared data space. 
     At this point, all VCPI far calls made by the DOS extender 410 now pass control to the VIVO driver 510 within the context of the DOS extender&#39;s new protected-mode operating system. These calls include allocating extended memory for the DOS extender application and (temporarily) switching back to real-mode to service DOS interrupts and DOS operating system calls. Whenever a VCPI far call is made, an interrupt entry is made in IDT 418 by VIVO driver 510. 
     With this completed, device virtualization (e.g., hardware virtualization) services are available to the DOS extender&#39;s protected-mode application program as well as to the normal DOS real-mode programs (i.e., VIVO driver 510 is accessible by programs running under DOS extender), at step 720. 
     In a presently preferred embodiment of the invention, the strategy for making the actual patch entry into the DOS extender&#39;s IDT 418 is changed from that just described with respect to FIG. 7. Instead of patching the DOS extender&#39;s IDT 418 during any VCPI far call intercepts, the patch is instead performed at the intercept of the VCPI &#34;Switch to Protected Mode&#34; call (Int 67h, function de0ch). In the FIG. 7 embodiment, the VIVO code always intercepts Int 67h calls to monitor VCPI startup activity by intercepting the &#34;Get Protected Mode Interface&#34; call and passing all other calls directly to the default VCPI handler. In accordance with the presently preferred embodiment (FIG. 8), however, the VIVO code also watches for the &#34;Switch to Protected Mode&#34; call. If this call is made, the DOS extender IDT 418 is patched then, and control is passed to the default VCPI handler. 
     In the FIG. 7 embodiment, the DOS extender&#39;s IDT 418 was patched during VCPI far calls since it was known that the DOS extender&#39;s protected mode context would be invoked at the time of the call. Unfortunately, the inventor has found that, under some circumstances, the DOS extender&#39;s application can execute code after the switch to protected mode but before any VCPI far calls are made. As a result, the DOS extender&#39;s application could generate an I/O related interrupt event before the VIVO code makes its dynamic DOS extender IDT patch in accordance with the invention. Accordingly, in accordance with the embodiment of FIG. 8, the VIVO code makes its dynamic DOS extender IDT patch before the first switch to protected mode. 
     It will be appreciated by those skilled in the art that the DOS extender must make a &#34;Switch to Protected Mode&#34; call at least once before any of the DOS extender&#39;s application code runs. When the DOS extender makes this call, it is executing in virtual 8086 mode and under the system&#39;s protected mode context (IDT 318, GDT 320). To make the switch to the DOS extender&#39;s protected mode, the DOS extender calls the VCPI interface via Int 67h (function de0ch), at step 812, and provides the VCPI interface with a list of parameters that indicate what the DOS extender&#39;s protected mode context is to be and where to begin executing protected mode code under the DOS extender&#39;s protected mode context, at step 814. More specifically, the DOS extender provides to the VCPI its Page Directory address, GDT and IDT addresses, LDT address, TSS (Task State Segment), and protected mode program execution address. This is all the information that the VIVO Int 67h intercept code needs to be able to successfully patch the DOS extender&#39;s IDT 418 at step 816 before it ever gets into protected mode. The real VCPI switch handler is only given control at step 818 after the VIVO Int 67h intercept code is sure that the DOS extender&#39;s IDT is properly patched. With this completed, device virtualization (e.g., hardware virtualization) services are available to the DOS extender&#39;s protected-mode application program as well as to the normal DOS real-mode programs (i.e., VIVO driver 510 is accessible by programs running under DOS extender), at step 820. 
     In the embodiment of FIG. 8, because the DOS extender is asking the VCPI to apply a new protected mode context, the DOS extender must tell the VCPI what that context is. Accordingly, it passes the VCPI (among others) at least the following: physical address of the Page Directory, linear address and size of the DOS extender&#39;s GDT and the DOS extender&#39;s IDT, and the like, at step 814. These arguments tell the VIVO driver of the invention where the DOS extender&#39;s GDT and IDT are. By temporarily switching the processor&#39;s Page Directory at step 816, the VIVO driver can access the DOS extender&#39;s IDT and patch it with the appropriate vector to the stored emulation code. 
     In particular, the VIVO driver of the FIG. 8 embodiment performs the following steps at step 816 to patch the DOS extender&#39;s IDT. Once the VIVO driver intercepts Int 67h, function de0ch (switch to protected mode), the VIVO driver then switches the processor&#39;s page directory to that of the DOS extender, which was specified by the DOS extender in the VCPI call. This allows the VIVO driver to properly access the DOS extender&#39;s GDT and IDT at run-time. The VIVO driver then checks to see if its IDT entries have already been made into the DOS extender&#39;s IDT. If so, it checks to see if the selector entry in the DOS extender&#39;s IDT indicates the appropriate entry in the DOS extender&#39;s GDT, which is the GDT entry that gets installed into the DOS extender&#39;s GDT when the VCPI get protected mode interface is called by the DOS extender. If either the DOS extender&#39;s IDT entry does not exist, or the selector does not match the DOS extender&#39;s GDT entry, the VIVO driver&#39;s code searches the DOS extender&#39;s GDT to find out what selector value corresponds to the VIVO emulation code and then makes the appropriate DOS extender IDT entry using the selector value. The VIVO driver then switches the processor page directory back to that of the system&#39;s protected mode context and jumps to the real VCPI switch call (step 818). The patched DOS extender IDT may now be used at step 820 to emulate the device that caused the interrupt to be generated. 
     Thus, in the embodiments of FIGS. 7 and 8, the startup of a DOS extender is dynamically detected by monitoring the VCPI calls and dynamically patching the DOS extender&#39;s IDT when such startup is detected. 
     In accordance with the embodiment of FIG. 8 (Appendix A), the VIVO driver continues to receive the VCPI far calls even though it does not patch the DOS extender&#39;s IDT 418 at the time of the far call. This is necessary because the DOS extender gets the selectors (GDT entries) when the DOS extender makes the VCPI &#34;Get Protected Mode Interface&#34; call. A far call is still necessary so that the selectors are guaranteed a &#34;home&#34; in the DOS extender&#39;s GDT 420. If the selectors could not be placed in the DOS extender&#39;s GDT 420, the DOS extender&#39;s application could never vector to the stored device emulation code since these selectors are the protected mode &#34;pointers&#34; to the device emulation code. Since the DOS extender is given these entries during the &#34;Get Protected Mode Interface&#34; call, the DOS extender cannot be prevented from calling the VIVO driver when it thinks it is making VCPI far calls. Hence, in the preferred embodiment, the VIVO TSR 520 monitors both the &#34;Switch to Protected Mode&#34; and &#34;Get Protected-Mode Interface&#34; calls. 
     The VIVO driver 510 of the present invention, like other device drivers and TSRs, implements software services for hardware interrupt events. However, the VIVO driver 510 in accordance with the present invention is different than standard MS-DOS device drivers and TSRS. For example, the memory-resident, executable code of the VIVO driver 510 of the present invention can permanently reside above the 1 MB memory boundary and above the HMA 116. As such, the VIVO driver 510 does not compete for valuable memory space with standard MS-DOS programs and other device drivers and TSRs. Moreover, the VIVO driver 510 is not limited to the 64K restriction of a typical device driver allowing for applications significantly larger than 64K which is particularly useful when, for example, one desires to emulate hardware with software. 
     Commercial Embodiment 
     A commercial embodiment in accordance with the present invention is the Ensoniq® Soundscape™ sound card VIVO drivers. In this commercial embodiment, a VIVO driver is used to perform the function of hardware which previously existed on a sound card. By replacing the hardware with a VIVO driver, the space consumed and cost of the sound card are significantly reduced. In operation, when an application requests access to the sound card, the request is directly processed by the VIVO driver and the functionality of that hardware element is performed in software rather than hardware. This is possible, in part, because present generation microprocessors (e.g., Pentium) are so powerful that they typically have considerable idle time which can be used to execute the VIVO driver, which appears like an application, without a noticeable delay in other necessary processing. This is only one example of how the use of a VIVO driver provides substantial advantages to not only optimizing conventional memory space but also optimizing the size and cost of an auxiliary computer card. 
     In particular, in the commercial embodiment, if an application attempts to address the sound card in the processor I/O address space, normally 220h through 22fh, an NMI is generated. As a result, the VIVO driver performs functional emulation of various hardware previously residing on the card (e.g., the Sound Blaster) in response to I/O writes and, during certain I/O reads, the handler alters the hardware I/O return value (in the Intel 386, register EAX) in order to emulate or virtualize the presence of the hardware in the system. Since the commercial embodiment of the present invention optionally alters the processor register state, it is necessary that the NMI service routine execute immediately following the faulting I/O instruction. 
     Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. For example, the invention may be triggered by hardware or software interrupts. ##SPC1##