Abstract:
In a heterogeneous computer system including at least one Client CPU, at least one Server CPU and a common memory disposed therebetween for storing first and second operating systems that control operation of the Client and Server CPUs, respectively. There is provided Client and Server Programs, both of which are stored in the same common memory. The Client Program is executed by the Client CPU and the Server Program is executed by the Server CPU. A method and system is disclosed wherein the Server Program is adapted for making function calls to the Client Program and the Client Program is adapted for returning results of the called function to the Server Program. The Server Program includes Procedures for invoking a Client Program function, converting parameters and result data from a format compatible with the first operating system to one compatible with the second operating system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to the following co-pending applications, assigned to the same assignee hereof, which are incorporated herein by reference. 
     U.S. Ser. No. 08/882,639, entitled A NEW AND IMPROVED SYSTEM AND METHOD FOR PERFORMING EXTERNAL PROCEDURE CALLS IN HETEROGENEOUS COMPUTERS; 
     U.S. Ser. No. 08/882,641, entitled A SYSTEM AND METHOD FOR PERFORMING EXTERNAL PROCEDURE CALLS FROM A CLIENT PROGRAM TO A SERVER PROGRAM WHILE BOTH ARE RUNNING IN A HETEROGENEOUS COMPUTER; 
     U.S. Ser. No. 08/882,643, entitled A SYSTEM AND METHOD FOR PERFORMING EXTERNAL PROCEDURE CALLS FROM A CLIENT PROGRAM TO A SERVER PROGRAM AND BACK TO THE CLIENT PROGRAM WHILE BOTH ARE RUNNING IN A HETEROGENEOUS COMPUTER. 
    
    
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE INVENTION 
     The present invention generally relates to external procedure calls in a computer system executing a program, and in particular to a method for performing external procedure calls from a server program to a client program, wherein the server program is executing a call from the client program and both are running in a heterogeneous computer system sharing a common memory. The term “External Procedure Calls” (or “EPC”) is used herein to refer to the making of a function call from one operating environment to another of such a heterogeneous computer system. The term “heterogeneous multiprocessing system” refers to a single computer system having two or more Central Processing Units (CPUs) that operate with a shared memory and utilize two or more different operating systems. 
     BACKGROUND OF THE INVENTION 
     In general computer programs include a number of internal functions, plus computer codes which call these functions in a specific order. This approach works well when all of the necessary functions are available within a single program. However, there are times when a required function is located elsewhere. Such functions are normally referred to as remote, or external functions. 
     One way to make these remote or external functions available to a program is to incorporate them into the local program. When feasible, this is the most efficient approach. However, remote or external functions sometimes depend upon other things (e.g., data, operating systems, hardware, etc.), which may not be available to the local program. In such situations, importing the remote or external function to the local program is not possible. Hence, the only alternative is to invoke the desired function remotely. These are known as Remote Procedure Calls (RPC&#39;s), which are available for such use. RPC&#39;s operate much, much slower than internal functions, in fact they are four or more orders of magnitude slower. 
     Some systems provide a streamlined RPC mechanism for use in shared memory environments, which are referred to as Local Procedure Calls (LPC). This capability eliminates the overhead of moving a function call across a network and reduces the per call overhead to less than  1  microsecond with today&#39;s microprocessors. Local Procedure Calls, however, are only available when all of the functions are running under the control of one single operating system. In a heterogeneous multiprocessing (HMP) system, there is a desire to have two different operating systems closely cooperating to carry out certain tasks. 
     Technologies are available for carrying out this cooperation, which are variants of Remote Procedure Calls (RPCs). RPCs operate over a network transport of some sort, and serve to physically move a request from one environment to another. At best, they operate in the range of 100 microseconds overhead per call. While this overhead is acceptable for some long operations, it is excessive for short operations, making cooperation impractical. In addition, the 100 microsecond or greater overhead must be incurred by each function call, further reducing the desirability of RPCs. A function calling sequence with drastically reduced overhead is required. 
     In shared memory HMP environments, there is no need to physically move the function from the memory of one operating environment to the memory of the other. Both operating environments share a single memory. External Procedure Calls take advantage of the shared memory in an HMP system to expedite the calling sequence between operating environments, allowing overheads of less than 1 microsecond using today&#39;s Instruction Processors or as referred to herein Central Processing Units (CPU&#39;s). This overhead reduction allows EPCs to be used for cooperative processing where RPCs would be impractical. Examples of potential uses for EPCs include: direct use of DMSII verbs from NT applications and direct use of NT security and encryption algorithms from MCP environments. 
     SUMMARY OF THE INVENTION 
     In accordance with the above and other objects, features and advantages of the present invention, there is provided a method and system in a heterogeneous computer system including at least one Client CPU, at least one Server CPU and a common memory accessible by each of the CPU&#39;s and disposed for storing first and second operating systems that control operation of the Client and Server CPUs, respectively. Client and Server Programs are both stored in the same common memory. The method and system comprise a Client Program executed by the Client CPU and a Server Program executed by the Server CPU. The Server Program is adapted for making function calls to the Client Program and the Client Program is adapted for returning results of the called function to the Server Program. 
     The Server Program includes procedures for invoking a Client Program function, and for converting parameters and result data from a format compatible with the one operating system to the other. 
     An object of the present invention is to provide a direct calling sequence from one operating system environment to another. This includes the direct mapping of parameters and return values. 
     A feature of the present invention is that a function being called is written in standard MCP languages. The functions need not be aware that they are being called from a foreign environment. 
     An advantage of the present invention is that the called functions are packaged in the same way they would be for use in the native environment, and are fully usable in the native environment. For MCP, functions are packaged in a standard MCP library format, and are fully usable by both MCP based applications and (once set up) by Windows NT programs as well. In both cases, this dual usage can be concurrent. That is, both native and foreign programs can be calling the same routines concurrently. MCP is the name of the operating system for A Series and ClearPath/NX Computers available from Unisys Corporation of Blue Bell, PA, assignee of this patent application. Windows NT is an operating system available from Microsoft Corporation of Redmond, Washington. It is noted at this juncture of the description that the term NT or NT Operating System is used to mean the Windows NT Operating System. 
     Another advantage of the present invention is that the mechanism supports all parameter types that can be readily mapped from the initiating environment to the target environment. This is a subset of all possible parameter types. Simple on-stack parameter types are supported (word values, such as int, long, unsigned, float, INTEGER, REAL are all supported. Simple off-stack arrays are also supported (char*, int*, long*, INTEGER ARRAY, REAL ARRAY, EBCDIC ARRAY). Array contents can either be translated, or left in native format, at the option of the developer. 
     By use of the present invention, modern computers can execute in excess of 1,000,000 External Procedure Calls per second, using the ClearPath HMP systems available from Unisys Corporation, assignee of this patent application. These speeds are well within one order of magnitude of local function calls, and are a full three orders of magnitude faster than the prior art Remote Procedure Calls. This speed allows programmers to use remote or external functions for many more purposes than were possible using standard Remote Procedure Calls. 
     Still other objects, features and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein there is shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive, and what is intended to be protected by Letters Patent is set forth in the appended claims. The present invention will become apparent when taken in conjunction with the following description and attached drawings, wherein like characters indicate like parts, and which drawings form a part of this application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the software modules of the prior art remote procedure call schema. 
     FIG. 2 is a block diagram that illustrates software modules depicting the sequence of steps necessary to develop programs that could make use of the method of the present invention. 
     FIG. 3 is a detailed block diagram of the Client Program and the Server Program during run time. 
     FIGS. 4A through 4M (intentionally excluding the letter I so as to avoid confusion) combined form a flow chart illustrating the method of the present invention. 
     FIGS.  5 A through  5 BY (intentionally excluding the letters I and O to avoid confusion) combined are a detailed diagram of the steps of the method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a block diagram illustrates the computer system and software modules that form the prior art remote procedure call schema. Remote or external functions are first identified through an Interface Definition Language (IDL block  101 ). This input is processed by an IDL compiler  102  to produce two program skeletons for exemplary functions B and C: i.e., a Server Program skeleton  103  and a Client Program skeleton  104 . The Server Program skeleton  103  contains declarations for the remote or external functions (Functions B and C in this example), plus some Generated Functions to allow the remote or external functions to be accessed remotely. A computer programmer adds computer codes to the Server Program to provide the actual workings of each remote or external function (e.g., Functions B and C). The computer programmer may also add additional functions which are not remotely or externally callable (e.g., Function A). The resulting program is then compiled with a server language compiler  105  to produce an executable Server Program  106  stored in a memory  107  of a computer  108  having a CPU  109  coupled thereto. An operating system O/S II for the CPU  109  is also stored in the memory  107  and controls operation of the CPU  109 , the memory  107  and the Server Program  106 . 
     The Client Program skeleton  104  contains declarations for the remote or external functions as well, with indications that they are remote or external (again, Functions B and C). The Client Program skeleton  104  also contains Generated Functions to allow the remote or external functions to be accessed remotely. A computer programmer adds codes to the Client Program skeleton  104  to provide the actual workings of the program itself, including calls to the remote or external functions (for example, statements CALL B and CALL C). The computer programmer may also add additional functions that are accessed locally (e.g., Function F). The resulting program is then compiled with a client language compiler  110  to produce an executable Client Program  111  stored in a memory  112  of a computer  113  having a CPU  114  coupled thereto. An operating system O/S I for the CPU  114  is also stored in the memory  112  and controls operation of the CPU  114 , the memory  112  and the Client Program  111 . 
     At this stage, the programs are ready for execution. The Client Program  111  is started on the computer  113  under the control of O/S I. The Server Program  106  is started on the computer  108  under the control of the operating system O/S II. The mechanisms for starting each program are specific to the computers on which they run; and, since such mechanisms are well known in the art, will not be further described herein. It is pointed out that each computer could be using entirely different operating systems, comprising entirely different hardware, utilizing entirely different CPUs and instruction sets, and having access to entirely different hardware and software resources. 
     When the Server Program  106  calls one of the remote or external functions (e.g., CALL B), the Generated Functions intercept the CALLS and transport them to the computer  113 , via a network  115 , where the Client Program  111  is running. Then, using the Generated Functions of the Client Program  111 , the remote or external function is invoked in the Client Program. When the invoked function is finished, any results are returned to the Server Program  106  in a like manner. The Generated Functions are obliged to communicate over the network  115  between the two computers; and, are obliged to perform any data translations and network protocols necessary to transport call requests, parameters and responses from one computer to the other. 
     There is a problem with the previously described solution. When a function is called locally within a program, the overhead necessary to make the function call is several CPU instructions in length. Today&#39;s CPUs can execute tens of millions of function calls per second. When a function is remote or external, the Generated Functions have much work to do. This work becomes an integral part of the function invocation. Such an overhead takes thousands of CPU instructions, plus transmission delays. The result is that today&#39;s computers can execute on the order of 50 to 1,000 remote or external function calls per second. Thus, remote or external function calls operate around 4 orders of magnitude slower than local function calls. This makes remote or external function calls impractical for many applications. 
     Referring now to FIG. 2, a block diagram illustrates software modules depicting the sequence of steps necessary to develop programs that could make use of the method of the present invention. External Procedure Calls begin with the same basic approach as Remote Procedure Calls. The same Interface Definition Language  101  is used, and the same IDL compiler  102  generates a Server Program  103 , and a Client Program  104 . There are differences however in the generated functions, and in the location of the defined functions, but these differences are transparent to the computer programmer. The computer programmer performs the identical steps of completing each program, and compiling it using the same appropriate language compiler  105  or  110 . Note, however, that the program must put the actual codes for the defined functions (Function B and Function C) in the Client Program, and that calls to these functions (CALL B and CALL C) are programmed as part of the Server Program. The resulting programs  106  and  111 , respectively, are then run on a special computer system  119  having two operating system environments (MCP and NT) controlling the two CPU&#39;s (CPU  114  and CPU  109 , respectively) connected in a very closely coupled way by means of a bus  120 . A suitable example of such a system is the ClearPath HMP systems delivered by Unisys Corporation, assignee of this patent application. The only special requirements for such a computer system are that it must incorporate a shared memory  118  between the two Operating Systems, and there must be a high-speed CPU-to-CPU signaling bus available, such as the bus  120 . 
     When the Server Program  106  calls a remote or external function using External Procedure Calls, the generated functions operate to invoke the appropriate Client function at very nearly the same speed as if the function was a local one. This is accomplished while still allowing the Operating Systems involved to be different, allowing entirely different hardware, utilizing entirely different CPUs and instruction sets, and having access to entirely different hardware and software resources. Only the two special requirements of a common memory and the high-speed CPU-to-CPU bus  120  need be satisfied. 
     In one embodiment of the present invention, the Server Program  106  running in the Windows NT environment makes a call on the Client Program  111  running in the MCP environment. The External Procedure Call implementation will make use of the Microsoft Interface Definition Language, with extensions, to define the protocol between the Client and Server Programs. This IDL will be compiled by an EIDL (E-Mode Interface Definition Language) compiler, which parses the IDL input file and generates the following: 
     Proxy Procedures: Specific examples of the Generated Functions referenced in both the Server and Client Program skeletons  103  and  104 , respectively. These are Server Program procedures that the Server program calls when it wishes to invoke a Client Program function. They have the same parameters and procedure types as the procedures defined in the IDL specification. The Proxy Procedure is responsible for notifying the MCP Operating System that a function in the Client Program is to be called. 
     Stub Procedures: Specific examples of the Generated Functions referenced in both the Server and Client Program skeletons  103  and  104 , respectively; and, are Server and Client Program procedures that convert the parameter and result data between the MCP and NT formats and call the appropriate Client and Server Program Procedures. 
     Server Skeletons: These are depicted as Server Programs in FIGS. 1 and 2 and contain the calls to the MCP Client Procedures. 
     Client Skeletons: These are depicted as Client Programs in FIGS. 1 and 2 and contain the MCP Client Procedure headings and empty bodies. 
     MCP Client Procedure headings and empty bodies are shown in FIGS. 1 and 2 as FUNCTION B and FUNCTION C, respectively, inside the Client Program skeleton  104 . The application programmer is responsible for providing the logic within the procedure bodies, which is then referred to as a Client Procedure. 
     In one embodiment of the present invention, an additional type of procedure is provided in the Generated Functions for use by Proxy and Stub Procedures referred to herein as an FCW Protocol Procedure. The MCP Client program is responsible for initiating the connection between the MCP Operating System and the Windows NT programs, which connection is established by calling the Open_FCW Protocol Procedure. 
     Referring now to FIG. 3, there is illustrated a block diagram of the software modules of the schema of the present invention stored in the common memory  118 . This diagram represents a run-time composition of the Server Program  106  and the Client Program  111  for External Procedure Calls, as well as the sequence of operations for procedure calls from the Server Program to the Client Program. For purposes of illustration only of a specific embodiment of the method of the present invention, the Client Program  111  comprises an EPC program (i.e., External Procedure Call) executable by an MCP Operating System running on a Unisys ClearPath/NX HMP computer. An exemplary Server Program  106  is shown as a DLL (i.e., Dynamic Link Library) executable by the Windows NT Operating System  122 . 
     In operation, when a Windows NT calling program wishes to call a Client Program  111  function (for example, CALL B), the calling program instead calls the corresponding Server Program Proxy Procedure for Function B (CALL B). This Server Program Proxy Procedure first notifies the MCP Operating System that a Client Program  111  function is about to be called, via an Interrupt. The MCP then causes an event associated with the EPC Client Program. The EPC Client Program then resumes execution in a Client Program Proxy Procedure. This Client Program Proxy Procedure then calls a Server Program Stub Procedure (using a mechanism disclosed in detail in the above-cited co-pending patent application entitled A METHOD FOR PERFORMING EXTERNAL PROCEDURE CALLS FROM A CLIENT PROGRAM TO A SERVER PROGRAM WHILE BOTH ARE RUNNING IN A HETEROGENEOUS COMPUTER). 
     The called Server Program function calls the Server Program Proxy Procedure previously referenced. This Server Program Proxy Procedure then performs any parameter translation necessary, and again notifies the MCP Operating System that a Client Program  111  function is about to be called. The MCP invokes the corresponding Stub Procedure in the Client Program  111 . The Stub Procedure then calls the actual function B. When the Client Program Server Function B finishes, the Stub Procedure informs the Windows NT Operating System to return control to the Server Program  106 . The Client Program then waits until control is returned from the Server Program. The MCP notifies the Windows NT Operating System to resume operation of the Server Program  106 . The Server Program  106  Proxy Procedure then resumes execution, performs any parameter and return value translation necessary, and returns to the Server Program  111  Stub Procedure. 
     The Server Program Stub Procedure then exits returning any results to the NT calling program (in the manner described in the above-cited co-pending patent application). This returns control to the MCP Client Program Proxy Procedure, as well as returning control to the Windows NT calling program. The Windows NT calling program then continues processing, making use of the parameters and result values from the Client Program  106  Function B. In the meantime, the MCP Client Program Proxy Procedure waits for the MCP event to again be caused. The details of this process are amplified in the description to follow and with reference to the flow chart illustrated in FIGS. 4A through 4M, and with reference to the illustrations in FIGS.  5 A through  5 BY. 
     Referring now to FIG. 4A, the first of a ten-part diagram illustrating the steps of the method of the present invention is shown. A convention used in organizing these figures is to illustrate the steps performed by the MCP Operating System  121  on the left side of each sheet and the steps performed by the Windows NT Operating System  122  on the right hand side of the sheet (unless otherwise noted). Likewise referring now to FIG. 5A, the first of a 71-part diagram illustrating the steps of the method of the present invention is shown. A convention used in organizing FIGS.  5 A through  5 BY is to illustrate the steps performed by the first operating system (“MCP”) on the left-hand side of each sheet of the drawing (the part of the illustration without a border) and the steps performed by the second operating system (“NT”) on the right-hand side of each sheet (the part of the illustration contained within a border), unless otherwise noted. The steps shown in FIGS. 4A through 4M and FIGS.  5 A through  5 BY are two ways to look at the same steps, and the description which follows applies to both illustrations. The method or process begins in the MCP Operating System  121  with a start bubble  130  followed by a step performed by the Client Program  106  of calling the Open_FCW Protocol Procedure (block  131 / 331  FIG.  5 A). The complete name of the NT Server DLL is passed in an ASCII null-terminated string. Entry into the Open_FCW Protocol Procedure transfers control to the CPU  109  (block  132 ). The Windows NT Operating System attempts to initiate the DLL identified in the passed parameter (block  133 ). Next, an inquiry is made as to whether or not the initiation by the NT Operating System was successful (diamond  134 ). If the answer to this inquiry is yes, then the Windows NT Operating System assigns an identifying number to the DLL, stores it in the DLL_Number parameter to the Open_FCW Protocol Procedure and leaves a value of zero (0) on top of the MCP Client Program stack (block  135 / 335  FIG.  5 B). On the other hand, if the initiation by the Windows NT Operating System was not successful, it leaves a value of one (1) on the top of the MCP Client Program stack (block  136 ). 
     The zero (0) or one (1) will be used as a result by the Client Program to indicate success or failure of the attempt to initiate the NT Server DLL. In the case of failure, further EPC execution is not possible (although the Client Program can continue performing other functions). The Client Program  111  can take any appropriate action, such as correcting the name of the NT Server DLL or perhaps terminating operation. Following the step depicted by the block  136 , the Client Program may not make calls to the Server Program (block  137 ). Hence, the Client Program continues with other processing, possibly making subsequent calls to the initialization function (block  138 ). At this point a branch is taken to the end of the process, which will be described hereinafter, at a connector Y in FIG.  4 M. 
     Following the step depicted by the block  135 , the process illustration continues in FIG. 4B at a connector J wherein the CPU  114  under control of the MCP cuts the Client Program stack back to the activation record below the Open_FCW Protocol Procedure (block  140 , FIG. 4B via connector J), and the Client Program resumes processing. The Client Program next calls the Call-back Registration Proxy Procedure for passing an SIRW (Stuffed Indirect Reference Word) to the zeroeth Server Procedure (block  141 ). An SIRW is an MCP-specific hardware construct used as an indirection to reference another location. The purpose for doing this is to provide the Proxy Procedures in the NT Server Program with a way to reference functions in the MCP Client Program. The SIRW can be thought of as a jump table. Indexing the jump table allows reference to a specific MCP Server function. The Windows NT Proxy Procedures know the appropriate index for each function, which is part of the Generated Functions that were supplied when the Windows NT Proxy Procedures were generated. 
     The Server Registration Proxy Procedure next calls the Call_Exit_FCW Protocol Procedure (block  142 ). Entry into the Call_Exit_FCW Protocol Procedure transfers control to the Windows NT Operating System (block  143 ). Running on CPU  109 , the Windows NT Operating System calls the Server Registration Procedure in the Server Program DLL. The Server Program DLL and the Stub Procedure are identified by the two parameters to the Protocol Procedure (block  144 ). Next, the Server Registration Stub Procedure moves the SIRW to the NT section of the common memory  118  and calls the Server Registration Server Procedure (block  145 ). Following this, the Registration Server Procedure saves the SIRW in the DLL memory (block  146 / 346 , FIG.  5 C). The process illustration continues in FIG. 4C as depicted by a connector K, where. the Windows NT Operating System then returns control to the MCP Operating System. 
     Referring now to FIG. 4C at the connector K and under control of the MCP, the CPU  114  cuts the MCP Client Program stack back to the Server Registration Proxy Procedure activation record (block  148 ). The Call-Back Registration Stub Procedure exits to NT, which returns control to the MCP operating system (block  148 ). The Call-Back registration Proxy Procedure then exits (block  149 ). 
     Next, the Client Program calls the RPCEvent Procedure for passing thereto an event to be registered (block  150 / 350 , FIG.  5 D). Following this the RPCEvent Procedure stores an SIRW to the event in a global MCP Operating System array and returns the index used in the array to the Client Program (block  151 / 351 , FIGS.  5 E and  5 F). The reason for this will become apparent hereinbelow when the Windows NT Server Program actually makes a call to an MCP Server function in the MCP Client Program. In brief, a way is needed for the NT Server Program Proxy Procedure to notify the MCP Client Program that a call is about to take place. This is necessary in order to cause a context switch to get the CPU  114  to begin running the Client Program. The Client Program then calls the Event Registration Proxy Procedure for passing the index returned by the RPCEvent to the Windows NT Server Program Proxy Procedures (block  152 / 352 , FIG.  5 G). The process illustration continues in FIG. 4D as denoted by a connector L. 
     Referring now to FIG. 4D at the connector L, the Event Registration Proxy Procedure calls the Call_Exit_FCW Protocol Procedure (block  153 / 353 , FIG.  5 H). Then, entry into the Call_Exit_FCW Protocol Procedure transfers control to the Windows NT Operating System (block  154 / 354 , FIG.  5 J). 
     Within the Windows NT operating system running on the CPU  109 , NT calls the Event Registration Stub Procedure in the Server Program DLL. The Server Program DLL and the Stub Procedure are identified by the two parameters to the Protocol Procedure (block  155 / 355 , FIG.  5 J). Next, the Event Registration Stub Procedure converts the event number to NT format and calls the Event Registration Server Procedure (block  156 / 356 , FIG.  5 K). The Event Registration Server Procedure then saves the event number in the DLL section of the memory (block  157 / 357 , FIG.  5 K). The Event Registration Stub Procedure exits and waits for calling program calls (block  158 / 358 , FIG.  5 L). At this point, External Procedure Calls are initialized, and Windows NT programs can begin calling MCP Client Program Server Functions. Following this and under control of MCP, the CPU  114  cuts the MCP Client Program stack back to the Event Registration Proxy Procedure activation record (block  159 / 359 , FIG.  5 M). The process illustration continues in FIG. 4E as denoted by a connector M. 
     Referring now to FIG. 4E at the connector M and under control of the MCP operating system, the Event Registration Proxy Procedure exits (block  259 / 459 , FIG.  5 N). Next, the Client Program waits on the event (block  160 / 360 , FIG.  5 P). At this point, the External Procedure Call mechanism is initialized and Windows NT programs can begin to actually call the MCP Client Program Server Functions. However, the MCP Client Program is an active program, and needs to do something when it isn&#39;t being called. It is thus necessary to wait for the event. The event will be caused when the Windows NT Server Program actually makes a function call. 
     At some time later, under control of the Windows NT Operating System, a Windows NT program ha a need to invoke an MCP Client Program Server Function. When this happens, the Windows NT program invokes the appropriate Server Proxy Procedure in the Windows NT Server Program DLL. The Proxy Procedure notifies the CPU  114  that a call is needed to an MCP Client Program Server Function. An example of this is a return from an inquiry made later as to whether or not there are more Server function calls to be made (bubble  161 ). Next, the calling program calls the Server Program Proxy Procedure (block  162 / 362 , FIG.  5 Q). Following this, the CPU  114  causes an MCP asynchronous interrupt passing in the event number as the P 2  parameter (block  163 / 363 , FIG.  5 R). The hardware interrupt handler in the MCP Operating System causes the event identified by the P 2  parameter (block  164 / 364 , FIG.  5 S). 
     A t a later time, the Client Program is scheduled to run on the CPU  114  by using its scheduling mechanisms (see FIGS.  5 U and  5 V). This happens because the event that the Client Program has been waiting on has happened. The Client Program immediatel y calls the Dispatch Proxy Procedure (block  165 / 365 , FIG.  5 W). The Dispatch Proxy Procedure calls the Call_Exit_FCW Protocol Procedure (block  166 / 366 , FIG.  5 X). Entry into the Call —Exit_FCW Protocol Procedure transfers control to the NT Operating System (block 167/367, FIG. 5Z).    
     Under control of the Windows NT Operating System running on the CPU  109 , the Windows NT Operating System calls the Dispatch Stub Procedure in the Server Program DLL. The Server Program DLL and Stub Procedures are identified by the two parameters to the Protocol Procedure (block  168 / 368 , FIG.  5 Z). Following this, the Dispatch Stub Procedure calls the Server Program Proxy Procedure (block  169 / 369 , FIG.  5 AA) that was previously called by the Windows NT calling program. The process illustration continues in FIG. 4F as denoted by a connector N. 
     Referring now to FIG. 4F at the connector N and under control of the Windows NT, the Dispatch Server Procedure calls the Stub Procedure for the Call-Back Procedure (block  169 / 369 , FIG.  5 AA). Next, the Server Proxy Procedure converts the input parameters from Windows NT format to MCP format and builds a stack activation record on top of the Call_Exit_FCW Procedure (block  171 / 371 , FIG.  5 AB). Under control of the MCP Operating System, the stack activation record is entered, and the Client Program Server Function begins executing. The Client Program Server Function performs functions (which may include calling into the NT Server DLL again). Eventually, the Client Program Server Function exits or returns. This exit or return is noticed by the CPU  114 , which returns control back to the Windows NT Operating System (block  172 / 372 , FIG.  5 AC). The Server Program Proxy Procedure then converts the output parameters from MCP format to NT format as data is moved from the MCP section of the memory  118  to the NT section of Memory (block  173 / 373 , FIG.  5 AC). The Server Program Proxy Procedure also converts any result value from MCP format to NT format as the result (if any) is moved from the MCP section of the memory  118  to the Windows NT section of memory (block  173 / 373 , FIG.  5 AC). The Server Program Proxy Procedure then exits (block  174 / 374 , FIG.  5 AD). The process illustration continues in FIG. 4G as denoted by a connector P. However, at the same time the Dispatch Stub Procedure exits which returns control back to MCP (block  178 / 378 , FIG.  5 AE). Next, under control of CPU  114  and the MCP operating system, the Call Exit FCW procedure exits (block  195 / 395 , FIG.  5 AF). After this, the Dispatch Proxy procedure exits (block  196 / 396 , FIG.  5 AG). This branch of the process continues at the connector Q in FIG. 4E (see FIG.  5 AH), which was described hereinabove. 
     Referring now to FIG. 4G at the connector P and under control of the Windows NT, the results are returned to the calling program, which resumes normal execution (block  180 / 380 , FIG.  5 AC). Next an inquiry is made as to whether or not there are any more Server Function Calls (diamond  181 ). If the answer to this inquiry is yes, then a branch is taken back to FIG. 4A at the beginning block  131  (as denoted by a connector Z). On the other hand, if the answer to this inquiry is no, then the Calling Program calls the Server Program Close Proxy Procedure (block  182 / 382 , FIG.  5 AJ). Next, the Server Program Close Proxy Procedure causes a CPU asynchronous interrupt, thereby passing an event number as the P 2  parameter (block  183 / 383 , FIG.  5 AJ). 
     Under control of the MCP Operating System, the asynchronous interrupt in the MCP causes the event identified by the P 2  parameter (block  184 / 384 , FIG.  5 AJ). Following this, the Client Program wakes up on the event and calls the Dispatch Proxy Procedure (block  185 / 385 , FIGS.  5 AL,  5 AM and  5 AN). The Dispatch Proxy Procedure then calls the Call_Exit_FCW Protocol Procedure (block  186 / 386 , FIG.  5 AP). Entry into the Call_Exit_FCW Protocol Procedure transfers control to the NT Operating System (block  187 / 387 , FIG.  5 AR). The process illustration continues with reference to FIG. 4H at a connector S. 
     Referring now to FIG. 4H at the connector S, the CPU  109  calls the Dispatch Stub Procedure in the DLL. 
     The DLL and Stub Procedures are identified by the two parameters to the Protocol Procedure (block  188 / 388 , FIG.  5 AR). Next, the Dispatch Stub Procedure calls the Server Program Close Proxy Procedure (block  189 / 389 , FIG.  5 AS). The Server Program Close Proxy Procedure then calls the Client Program Close Proxy Procedure (block  190 / 390 , FIG.  5 AT). This transfers control to CPU  114  under control of the MCP operating system. The Client Program Close Proxy Procedure calls the Event Unregistration Proxy Procedure (block  197 ). The event Unregistration Proxy Procedure calls the Call FCW Exit protocol procedure (block  198 ). The call FCW protocol procedure transfers control to CPU  109  under control of the Windows NT Operating System (block  199 ). The process illustration continues with reference to FIG. 4J at a connector T. 
     Referring now to FIG. 4J at the connector T and under control by the Windows NT Operating System, the CPU  109  calls the Event Un-Registration Stub Procedure in the DLL (block  201 / 401 , FIG.  5 AW). The Event Un-Registration Stub Procedure calls the Event Un-Registration Server Program Procedure (block  202 / 402 , FIG.  5 AW). Next, the Event Un-Registration Server Program Procedure unregisters the event by erasing the reference to it from the NT DLL part of memory (block  203 / 403 , FIG.  5 AX). The Event Un-Registration Server Program Procedure exits to the Event Un-Registration Stub Procedure, and the Event Un-Registration Stub Procedure then exits (block  204 / 404 , FIG.  5 AY). Control is returned to the MCP Operating System where the CPU  114  cuts the MCP stack back to the Event Un-Registration Proxy Procedure (block  205 / 405 , FIG.  5 AZ). The Event Un-Registration Proxy Procedure then exits (block  206 ). Next, the Client Program calls RPCEvent to un-register the event (block  207 / 407 , FIG.  5 BA). The RPCEvent removes the SIRW from the global MCP Operating System array (block  208 / 408 , FIG.  5 BB). The Client Program then calls the Call-Back Un-Registration Proxy Procedure (block  209 / 409 , FIG.  5 BE). Following this, the Call-Back Un-Registration Proxy Procedure calls the Call_Exit_FCW Protocol Procedure (block  210 / 410 , FIG.  5 BF). Entry into this Procedure transfers control to the NT Operating System running on the CPU  109  (block  211 / 411 , FIG.  5 BG). Under control of the Windows NT Operating System, NT calls the Call-Back Un-Registration Stub Procedure in the DLL (block  212 / 412 , FIG.  5 BG). The process illustration continues with reference to FIG. 4K at a connector V. 
     Referring now to FIG. 4K at the connector V and under control of the Windows NT Operating System, the Call-Back Un-Registration Stub Procedure calls the Call-Back Un-Registration Proxy Procedure (block  214 / 414 , FIG.  5 BG). Next, the Call-Back Un-Registration Server Procedure un-registers the Call-Back Procedures (block  215 / 415 , FIG.  5 BH). The Call-Back Un-Registration Stub Procedure then exits (block  216 / 416 , FIG.  5 BJ). Control is returned to the MCP Operating System wherein the CPU  114  cuts the MCP stack back to the Call-Back Unregistration Proxy Procedure (block  217 / 417 , FIG.  5 BK). Next, the Call-Back Un-Registration Proxy Procedure exits (block  218 / 418 , FIG.  5 BL) and the Close Procedure exits (block  219 / 419 , FIG.  5 BM). The Close Stub Procedure then exits, returning control to the Windows NT operating system (block  220 / 420 , FIG.  5 BN). The process description continues with reference to FIG. 4L at a connector V. 
     Referring now to FIG. 4L at the connector V and under control of the Windows NT Operating System, the Close Proxy Procedure returns control to the NT calling program (block  221 / 421 , FIG.  5 BP). Next, the Close Proxy Procedure exits, returning control to the Dispatch Stub Procedure (block  222 / 422 , FIG.  5 BP). After this, the Dispatch Stub Procedure exits (block  223 / 423 , FIG.  5 BQ), returning control to the MCP Operating System where CPU  114  cuts back the stack to the Dispatch Proxy Procedure (block  224 / 424 , FIG.  5 BR). Next, the Dispatch Proxy Procedure exits returning control to the Client Program (block  225 / 425 , FIG.  5 BS). The Client Program then calls the Close_FCW Protocol Procedure for passing thereto the DLL number assigned during the Open_FCW (block  226 / 426 , FIG.  5 BT). The process illustration continues with reference to FIG. 4M at a connector W. 
     Referring now to FIG. 4M at the connector W and under control of the Windows NT Operating System, the CPU  109  terminates the specified DLL (block  228 / 428 , FIGS.  5 BU,  5 BV and  5 BW). Next, an inquiry is made as to whether or not the termination was successful (diamond  229 ). If it was successful, then the CPU  109  leaves a zero (0) on top of the MCP Server Program stack (block  230 ). On the other hand, if the termination was not successful, then the CPU  109  leaves a one (1) on top of the MCP Server Program stack (block  231 ). Following either the step depicted by the block  230  or the block  231  the Close Procedure exits and control is returned to the MCP operating system (see FIG.  5 BX) where the CPU  114  cuts the MCP stack back to the activation record below the Close_FCW Procedure ( 232 / 432 , FIG.  5 BY). At this point the Client Program may no longer make function calls to the Server Program (block  233 ). The Client Program then continues with other processing, possibly including subsequent calls to the initialization function (block  234 ). Next, an inquiry is made as to whether or not there are more Server Programs to be called (diamond  235 ). If the answer to this inquiry is yes, then a return is made back to the beginning block  131  as denoted by a bubble  236 . On the other hand, if there are no more Server Programs to be called, the Client Program continues with other processing (bubble  237 ). 
     Referring now to FIGS.  5 A through  5 BX (intentionally excluding letters I and O), the individual steps of the method of the present invention are illustrated graphically. It should be noted that the diagrams in the FIGS.  5 A through  5 BX represent parts of the Programs stored in the memory  118  and are divided in a similar manner to that used for FIGS. 4A through 4L. That is, the MCP operating system is shown on the left-hand side of each sheet and the Windows NT operating system is shown on the right-hand of each sheet. With reference to FIG. 5C, there is shown on the left-hand side of the sheet an MCP Program Stack  239 . On the right-hand side of the sheet is shown an NT Procedure Call Stack  240 , a DLL Handle Stack  241 , a Server Proxy Table  242  and a Protocol Procedure Table  243 , all of which are stored within the NT section  244  of the memory  118 . The remaining drawings are divided up in a similar manner. 
     At this juncture of the description it is noted that the designations of Client Program in the O/S I, or Server Program in the O/S II is determined by which program makes a call to an initialization Generated Function. The program that makes such a call is designated the Client Program, and the other program is designated the Server Program. The designation is transient, and only applicable during the duration of each step of the above-described process. Either program may be both a Client Program and a Server Program at different times, as determined by the functions declared in the Interface Definition Language. In this way, it is possible for a program in the O/S I to call functions resident in a program in the O/S II, and for the same program in the O/S II to call functions resident in the program in the O/S I. That is, the programs may alternate the roles of Client Program and Server Program. 
     Further, these role reversals may be interleaved. That is, a program acting as a Server Program may begin acting as a Client Program, while still in the role of a Server Program. This process is known as callbacks in External Procedure Calls, and occurs when a called server function (while processing) calls a function resident in the original program that called the server function. Further, these call-back functions may be nested. That is, a program acting as a Client Program, and then acting as a Server Program during the scope of the Client Program call, may then act again as a Client Program during the scope of the server function execution. In this way, function calls may proceed back and forth between the programs, each building on the other, to an arbitrary depth. When calls are made in this fashion, most recently invoked functions are always completed and exited prior to any earlier invoked functions being completed. That is, functions are completed in a last-invoked first-completed fashion. 
     While there has been shown what is considered the preferred embodiment of the present invention, it will be manifest that many changes and modifications can be made therein without departing from the essential spirit and scope of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications which fall within the true scope of the invention.