EMULATION MEANS IN A DATA PROCESSING SYSTEM

Abstract: Emulation apparatus allowing a first system, which is not designed as a functional duplicate of a second system, to emulate the second system. The input/output structure of the second system is emulated in the first system by means of routines stored therein and which direct the already existing input/output structure (12) to operate in the same manner as the input/output structure of the second system. The emula­ tion routines are in turn invoked through the non-maskable interrupt mechanism of the first system by a modification thereto which detects the occurrence of "foreign" input/­ output requests; that is, input/output requests occurring in programs originally written for the second system and which are not normally recognized by the first system. The input/output structure (12) of the first system includes input/output devices having ports (70, 72, 74, 76, 78, 48) occupying a first range of the system address space and the input/output devices of the second system have ports located within a second, differing range of address space. The means for detecting foreign input/output requests include means (29) for comparing input/output request port addresses to the range of port addresses occupied by the system input/output devices and indicating when a port address is within the range of address space occupied by the system input/output devices. The foreign request detection means generates a non-maskable interrupt to invoke an input/output emulation routine upon the occurrence of input/output requests which do not normally fall within the range of input/output ports supported by the first system.

Description: The present invention relates to the emulation of a first computer system by a second, substantially different such system and, in particular, in such a manner as to enable the second system to run, without modification, software programs developed for the first system. A recurring problem in computer systems is that of software availability and, in particular, the availability of applications software, that is, programs which are run on or executed by systems in order to perform particular tasks for users. Examples of such applications programs include word and data processing, data bases, graphics and spread sheets and encompass the full range of tasks and functions which a user would wish to accomplish with such a system. The potential range of applications for a particular system are such that it is extremely difficult for a single system manufacturer to develop more than a basic range of applications programs for its systems. The remainder, and often the majority of applications programs, are commonly developed and provided by companies specializing in the development of applications programs. For economic reasons, such applications program developers usually concentrate their available resources upon the more common computer systems. While applications program developers may wish to provide programs for the less common but still economically significant systems, their available resources may not allow the development of programs for a full range of different systems. As such, the range of applications programs for other than the few most common systems may be limited. This problem has frequently been described as one of program "transportability", that is, the ability to "transport" a program from one system to a second, differing system without the need to rewrite or extensively modify the program. A partial solution to this problem which has been adopted by the computer industry, and in particular for the smaller "personal" or "professional" computers, is the development of standardized operating systems which are used on a wide range of internally different computers. In this regard, it should be noted that operating systems essentially supervise, direct and control the overall operation of a computer system upon which it is "run". In addition, an operating system provides an interface between an applications program being run upon a system and the actual internal structure of the system itself. Essentially, the applications program "runs on" and "sees" the operating system and not the actual underlying computer system structure. The adoption of such common, standard operating systems, such as CP/M and MS-DOS, which are run upon a number of internally differing systems thereby theoretically allowing the development of programs which will also run upon those systems without modification. By way of example, both the Wang Laboratories Professional Computer and the IBM PC use essentially the same operating system, referred to as MS-DOS. An applications program developed for the IBM PC should therefore be theoretically able to run upon the Wang Professional Computer without modification, and the reverse. In practice, such operating systems frequently do not provide the full range of features and functions desired by applications program developers, or do not provide adequate performance in programs which interface with a system only through the operating system. Because of this, many if not most applications programs are in fact written so as to interface, at least in part, directly with the internal structures of the systems on which they are to be run. Because the internal structures of computer systems provided by different manufacturers differ, often widely, even applications programs written primarily to run on one of the standard operating systems are often not in fact "transportable" from one system to another. In addition, many manufacturers have modified the "standard" operating systems for enhanced performance upon their particular systems. For example, as described above, both Wang Laboratories and IBM use the same basic standard MS-DOS operating system in their personal and professional computers. In each case, however, MS-DOS has been modified to enhance performance on the particular systems designed by Wang and IBM and are no longer completely "standard". Such modifications are usually dependent upon the particular internal structural features of the particular systems and are subsequently reflected in the applications programs written for the particular systems. Consequently, even applications programs which interface only with a "standard" operating system running on a particular system are often not transportable to another system which uses yet another modified version of that "standard" operating system. One possible solution to these problems is for a manufacturer to essentially duplicate, without necessarily copying, the internal structures of a system to be emulated. This requires the development of both the hardware structure of the emulating system and the development of an operating system for the emulating system which essentially duplicates the functionality of the operating system of the system to be emulated. This approach may be undesirable, however, in that the system emulated may be obsolete or otherwise lacking or inferior in performance. Also, the emulating system and its operating system are essentially dedicated to a single purpose, that of emulating another system, rather than having broader utility. Yet another possible solution to the above problems is the emulation of a particular system by means of software, that is, emulation programs, running upon a system which is to emulate that system. This approach may, however, prove undesirable because of the possibly extensive "overhead", that is, memory space and processor operation time, required for the emulation function. From an article in AFIPS Conference Proceedings, 1978 National Computer Conference, Anaheim, California, 5.-8. June 1978, pages 379-384; D.A.Deel et al. "A Microprogrammed AN/UYK-20(V) Emulation", the emulation of a AN/UYK-20(V) computer on a Nanodata QM-1 computer, which has two levels of microprogram control, referred to as the micro level and the nano level is known. The emulation of the AN/UYK-20(V) by the QM-1 is performed entirely through software running on the QM-1, the software being comprised of QM-1 micro level programs which in turn invoke a number of nano level emulation programs. An emulation routine is invoked for each and every input/output operation and for each and every interrupt operation. Therefore, the software must be loaded or "unloaded" depending on whether the QM-1 system is to emulate the AN/UYK-20(V) or is to operate as a QM-1. The performance handicap and disadvantages of pure software emulation are discussed. US-A-3 955 180 by A.C. Hirtle for a Table Driven Emulation System, describes a system which emulates the input/output functions of another system by detecting all input/output requests occurring in the emulating system and executing corresponding emulation routines in a dedicated input/output control subsystem. This requires a complete, separate subsystem, including a processor unit and emulation routines residing in the subsystem, to perform the emulation. A single system CPU cannot be used. Such system detects and emulates each and every input/output operation not only those input/output requests which are "foreign" to the emulating system. US-A-3 938 101 by Lewis et al. for a Computer System With Post Execution I/O Simulation describes a system which includes a means for emulating different input/output devices which may be connected from the system. The emulation means is comprised of a "quasi I/O device" which is connected from the system to provide response signals which appear to come from the emulated I/O device while a set of emulation routines actually performs the input/output operation through another I/O device. This type of emulation means requires the use of a complete, separate "quasi I/O device" to aid in the emulation. The invention's object is to allow a first system, which is not designed as a functional dupulicate of a second ystem, to emulate the second system without incurring the hardware, software and operating penalities described above. In the invention as defined in the appended claims, the input/output structure of the second system is emulated in the first system by means of routines stored therein and which direct the already existing input/output structure to operate in the same manner as the input/output structure of the second system. The emulation routines are in turn invoked through the non-maskable interrupt mechanism of the first system by a modification thereto which detects the occurrence of "foreign" input/output requests; that is, input/output requests occurring in programs originally written for the second system and which are not normally recognized by the first system. In a presently preferred embodiment of the present invention, the input/output structure of the first system includes input/output devices having ports occupying a first range of the system address space and the input/output devices of the second system have ports located within a second, differing range of address space. The means for detecting foreign input/output requests includes means for comparing input/output request port addresses to the range of port addresses occupied by the system input/output devices and indicating when a port address is within the range of address space occupied by the system input/output devices. The foreign request detection means then generates a non-maskable interrupt to invoke an input/output emulation routine upon the occurrence of input/output requests which do not normally fall within the range of input/output ports supported by the first system. The emulation means of the present invention further includes means for "trapping", that is, storing, information pertaining to such foreign input/output requests at the time such requests are initiated. The trapped information is subsequently used by the emulation means in determining the particular input/output operation to be emulated. For example, if the request is for a write operation, the information to be written, which appears in association with the request, is similarly trapped to be subsequently written to the first system input/output port corresponding to the second system's port which was indicated by the request. In a yet further embodiment of the present invention, the first system further includes means for emulating the hardware interrupt mechanism of the second system. In the hardware interrupt emulation mechanism of the present system, the first system's software interrupt service routines are provided with an additional group of hardware interrupt emulation routines which emulate the operation of the hardware interrupt mechanism of the second system. The first system's interrupt vector table correspondingly contains additional vectors to the emulation routines and the first system's hardware interrupt service routines are altered to include references to these vectors. The emulation routines, vectors and modifications to the existing first system's hardware interrupt routines are enabled when the system is emulating the second system. The first system thereby emulates the hardware interrupt mechanism of the second system by performing hardware interrupt emulation operations in the same manner as software Interrupt operations. It should be noted that the added interrupt mechansim emulation routines are not limited to purely emulation routines, but may further include or be replaced by other routines providing additional functionality or enhanced performance to the system. Other advantages and features of the present invention will be understood by those of ordinary skill in the art after referring to the following detailed description of a preferred embodiment and drawings, wherein: Fig. 1 is a block diagram of a computer system of the "personal" or "professional" class; Fig. 2 is a block diagram of a video controller used in the system of Fig. 1; Fig. 3 is a diagrammic representation of the software and control structure of a system as illustrated in Figs. 1 and 2; Fig. 4 is a diagrammic representation of the address spaces of the system of Figs. 1 to 3; Figs. 5A and 5B are diagrammic representations of an I/O structure emulation means; and, Fig. 6 is a diagrammic representation of a hardware interrupt emulation mechanism. The following will first describe the general structure and operation of the hardware and software of a typical computer system of the "personal" or "professional" computer class, for example, a Wang Laboratories, Inc. Professional Computer, as will be well understood by one of ordinary skill in the art. The structural and operational features by which a first such system differs from a second system; for example, an IBM PC, and the effects of such differences with regard to the previously described "standard" operating systems and applications programs will then be described. Finally, the method and apparatus of the present invention by which a first such system is enabled to "run", without modification, programs originally written for a second such system, will be described. 1. Structure and Operation Of A "Personal" or "Professional" Class Computer A. Basic Hardware Structure and Operation (Figs. 1 and 2) Referring to Fig. 1, a block diagram of a "personal" or "professional" class computer system is shown, the example herein being a Wang Laboratories "Professional Computer". This system is typical and representative of the class of such systems and is described in, for example, "The Wang Professional Computer Technical Reference Manual" and other related publications available from Wang Laboratories, Inc. of Lowell, MA, which are incorporated herein by reference. The following description of the system of Fig. 1 will be brief in that the structure and operation of such systems is well understood by those of ordinary skill in the art, and that of the exemplary system presented herein is thoroughly described in the above referenced and incorporated publications. As indicated in Fig. 1, the major elements of the system include Central Processing Unit (CPU) 10 and Control-Input/Output Logic (CIO) 12, which are interconnected through the bus structure comprising System Address (SA) bus 14, System Data (SD) bus 16 and System Control (SC) 18. As indicated by their designations, SA bus 14 and SD bus 16 respectively communicate addresses and data between the elements of the system while SC bus 18 is essentially used to communicate system control information. 1 . Central Processing Unit (Fig. 1 ) Referring to CPU 10, arithmetic and logic operations, including major system control functions, are performed by Main Processor (MP) 20, assisted in arithmetic operations by Co-processor (COP) 22. MP 20 and COP 22 may respectively be, for example, an Intel 8086 microprocessor and an Intel 8087. MP 20 and COP 22 communicate addresses and data to, respectively, SA bus 14 and SD bus 16 through, respectively, Address Latch (AL) 24 and Data Transceivers (DT) 26 and memory and bus control information to SC bus 18 through Bus Controller (BC) 28. Associated with BC 28 is Input/Output Decode Logic (IODL) 29, which monitors addresses appearing on SA bus 14 and input/output control information appearing on SC bus 18. IODL 29 detects the occurrence of system input/output requests and provides corresponding control signal outputs to SC bus 18. The control outputs of IODL 29 are in turn used by the system I/O devices, as described further below, in performing system I/O operations. Associated with MP 20 and COP 22 are a Clock Generator (CG) 30, which is the source of clock signals for the system, and Bus Acquisition Logic (BAL) 32, which controls Direct Memory Access (DMA) operations, that is, the direct transfer of data to and from the system main memory, described below. Also associated with MP 20 and COP 22 is Wait State Logic (WSL) 34, which essentially monitors system operation and adjusts the time allowed to each so as to prevent conflicts in CPU 10 operations. Nonmaskable Interrupt Logic (NMIL) 36 controls a portion of the operation of the system's interrupt mechanism, which is described in further detail below. Finally, the various elements of CPU 10 described above are interconnected and communicate through CPU Local (CPUL) bus 38. 2. Control and Input/Output Unit (Fig. 1) Referring to CIO 12, the elements of CIO 12 are, as shown in Fig. 1, interconnected primarily through Buffered Address (BA) bus 40 and Buffered Data (BD) bus 42 and through SC bus 18. BA bus 40 is, as shown, connected from SA bus 14 through Address Buffer (AB) 44 while BD bus 42 is interconnected with SD bus 16 through Data Transceivers (DT) 46. CIO 12 includes Main Memory (MM) 48, which stores and provides data to be operated upon by the system and the programs which control the operation of the system, such as the operating system and applications programs. Associated with MM 48 are Main Memory Control Logic (MMCL) 50 and Main Memory Address Multiplexer (MMAM) 52, which are respectively connected from SC bus 18 and SA bus 14 and provide, respectively, control and address inputs to MM 48. The data input/output of MM 48 is interconnected with BD bus 42. Connected respectively from Buffered Address (BA) bus 40 and Buffered Data (BD) bus 42 are DMA Low Address Latch (DLAL) 54 and DMA High Address Latch (DHAL) 56. DLAL 54 and DHAL 56 are a part of the system's DMA mechanism and provide respectively the low and high order portions of DMA addresses to MM 48 through SA bus 14 and MMAM 52 during DMA operations. Associated with DLAL 54 and DHAL 56 is DMA Page Register (DPR) 58, which is connected from BA bus 40 and similarly provides a portion of DMA addresses to MM 48 through SA bus 14 and MMAM 52 during DMA operations, in this case the portion of the address identifying the memory page involved in the DMA operation. The primary control of DMA operations is provided by DMA Control Logic (DCL) 60 and DMA Bus Control Logic (DBCL) 62, which are connected from BA bus 40 and provide data outputs to BD bus 42 and control outputs to other portions of the system. The system control logic included in CIO 12 includes Counter and Timer Logic (CTL) 64 and Programmable Interrupt Controller (PIC) 66, which may comprise. respectively, an Intel 8253 and an Intel 8259A. CTL 64 essentially controls the timing of all major system operations while PIC 66, in conjunction with NMIL 36, comprises the system interrupt control logic, which will be described further below. Associated with MM 48 and the system interrupt control logic is Parity Error Logic (PEL) 67, which monitors data written into and read from MM 48 and provides a parity, or data, error signal to the interrupt control logic when such an error occurs. The system program and control logic further includes Erasable-Progammable Read-Only-Memory (EPROM) 68, which stores and provides programs used to load the system operating system and application programs from, for example, discs mounted in external disc drives into MM 48. The programs stored in EPROM 68 essentially comprise permanently resident programs which are used to initialize the operation of the system. The system program and control logic further includes Memory Control Store Decode (MCSD) 69, which operates in conjunction with EPROM 68 to select whether current system control is to be provided from EPROM 68, at system initialization, or from programs stored in MM 48, during normal operation. CIO 12 further includes a number of elements for communicating between the system and external devices, such as a keyboard, disc drives and printers. Included in these elements is Universal Asynchronous Receiver/Transmitter (UART) 70, which provides a serial interface with a keyboard and Floppy Disc Controller (FDC) 72, which provides an interface to a disc drive. Further included are an Extended Programmable Communications Interface (EPCI) 74; for example, an Intel 2661, which provides a programmable RS-232 I/O port, and Parallel Printer Interface (PPI) 76, for example, an Intel 8255A, which provides a parallel interface to various printers. Other input/output elements may include interface elements to, for example, Winchester hard disc drives and various remote telecommunications interfaces. Finally, CIO 12 includes I/O Control Store Decode (IOCSD) 75, which essentially stores and provides sequences of instructions; that is, routines, for controlling the input/operations of the system and, in particular, the operations of the I/O devices described above. 3. Video Controller Unit (Fig. 2) The system includes two primary user interfaces: the first, described above, is through a keyboard connected from UART 70 and the second is a CRT display. Referring to Fig. 2, a Video Controller (VC) 78 providing the interface between the system and a CRT is shown. While the system described herein has available a plurality of such video controllers for various displays, VC 78 is typical of this class of CRT interface elements and provides both a character set graphics display and a bit mapped display. Referring first to the character set portion of VC 78, VC 78 includes a Character Memory (CM) 80 for storing and providing ASCII type codes identifying alphanumeric characters, graphic symbols and associated textual symbols to be displayed on the CRT screen. These codes are stored in CM 80 in locations corresponding to the locations in which they are to appear on the display. Associated with CM 80 is Attribute Memory (AM) 82, which stores and provides, for each character code stored in CM 80, corresponding attribute codes identifying whether the corresponding character or symbol is to be, for example, highlighted, underlined, and so on. Information is written into CM 80 and AM 82 from SD bus 16 at addresses provided from SA bus 14 and under control of control inputs provided from SC bus 18. For these purposes, VC 78 includes a VC Address Buffer (VCAB) 84 connected from SA bus 14 and which provides corresponding address inputs to C/A Memory Input Address Multiplexer (CMIAM) 86. CMIAM 86 in turn provides address inputs to CM 80 and AM 82. As indicated in Fig. 2, VCAB 84 also provides address inputs to C/A Memory Output Multiplexer (CMOM) 88 which, as described further below, controls the reading of character and attribute codes from CM 80 and AM 82 to the logic driving the CRT display. VC 78 further includes a VC Data Transceiver (VCDT) 90 interconnected with SD bus 16 and which is used in part to write character and attribute codes into CM 80 and AM 82. As shown in Fig. 2, VCDT 90 is also provided with an output to CRT Controller (CRTC) 92, which in turn provides control signals to the CRT drive circuits, and an input from Font Memory (FM) 94, described further below, which allows the contents of FM 94 to be read onto SD bus 16. FM 94 is used to store bit patterns corresponding to the various character and symbol codes which may be represented on the CRT display. The codes stored in CM 80 and representing characters and symbols to be displayed on the CRT are read from CM 80 under control of addresses provided through CMIAM 86 from CRTC 92 and are provided as address inputs to FM 94. FM 94 in turn responds to these inputs by providing the corresponding bit patterns as an output to Bit Pattern Shift Register (BPSR) 96, which in turn performs a parallel to serial conversion and provides the resulting serial bit patterns to Attribute Logic (AL) 98. AL 98, as shown in Fig. 2, includes a further input from the output of AM 82. The attribute codes corresponding to the character/symbols codes stored in CM 80 are read from AM 82 in parallel with the corresponding codes read from CM 80 and are used by AL 98 to appropriately modify the bit patterns provided from BPSR 96 in such a manner as to provide the corresponding characters/symbols with their corresponding attributes. The output of AL 98 is provided as a text video output to the CRT driver circuits to result in a corresponding display on the CRT screen. Finally, VC 78 includes Timing Generator Logic (TGL) 100, which is connected from SC bus 18 and provides timing and control signals to the logic and circuitry of VC 78. Referring now to the bit mapped portion of VC 78, Bitmap Memory (BM) 102 stores and provides imagery to be displayed on the CRT screen in bit mapped form. The information to be displayed is written into BM 102, in bit mapped form, through the data bus connected from the output of VCDT 90 and under the control of addresses provided through an address bus connected from an output of Bitmap Memory Address Multiplexer (BMAM) 104. BMAM 104 in turn receives write address inputs from the address bus connected from the output of VCAB 84. BMAM 104 also receives read address inputs from the outputs of X Axis Address Generator (XAAG) 106 and Y Address Address Generator (YAAG) 108. The bit mapped imagery information stored in BM 102 is read from BM 102 under control of the read addresses provided from XAAG 106 and YAAG 108 and through Bit Mapped Shift Register (BMSR) 110, which performs a parallel to serial conversion, to be combined in Video OR Gate (VORG) 112 with possible image information output from AL 98. The output of VORG 112 is then provided as a video output to the CRT driver circuits to result in a corresponding display on the CRT screen. Detailed control of BM 102 is provided by BM Control Logic (BMCL) 114, which in turn is provided with control signals from TGL 100. B. Basic Software and Control Structure and Operation (Fig. 3) Referring to Fig. 3, a diagrammic representation of an exemplary software and control structure of a computer system of the type described above is shown. 1. Applications Programs, Operating System and Basic Input/Output System (Fig. 3) As indicated in Fig. 3, the software/control structure of a typical system is multi-leveled. The current applications program(s) (APs) 116 are the highest, most visible level. The system Hardware Structure (HS) 118, including CPU 10 and CIO 12 and associated External Devices (ED) 120, for example, CRTs, keyboards, disc drives, printers and telecommunications devices, is the lowest level. Interposed between APs 116 and HS 118 is the Operating System (OS) 120 and the Basic Input/Output System (BIOS) 122. As previously described, an OS 120 essentially supervises, directs and controls the overall operation of the computer system. In addition, OS 120 provides an interface between the APs 116 being run upon the system and the actual internal structure of the system itself, including HS 118. Essentially, APs 116 "run on" and "see" OS 120 rather than the actual underlying system structure. In many systems, OS 120, often referred to as the Disc Operating System (DOS), resides in MM 48 and is loaded therein from, for example, a disc. As also previously described, the underlying structures of computer systems differ, often widely, as a result of their designer's design choices, intended features and functions, and available implementation technology. To be standardized, an operating system cannot communicate directly to the system hardware but must specify an interface to hardware controlling routines provided by each hardware manufacturer. As a result, a "standard" operating system such as OS 120 generally cannot be run directly upon the system structure. For this reason, the system control structure further includes, as part of the internal structure of the system, the further level referred to as the Basic Input/Output System, that is, (BIOS) 122. BIOS 122 is essentially comprised of sequences of routines, that is, programs, which direct and control the functions and operations of the system structure at the lowest and most detailed level and forms an interface between the OS 120 and the system's internal structure. BIOS 122 also provides a primary means, described further below, by which the system elements, such as those comprising HS 118, communicate among themselves and with the higher levels of control. In addition, many BIOS's provide additional service routines or features which are not part of the standard operating system and which provide additional functionality to the system or enhance the performance of the system. BIOS 122 may reside in MM 48, being loaded therein from, for example, a disc, or may reside in a Read Only Memory (ROM) in the same manner as the system initialization routines residing in EPROM 68. In many systems, BIOS 122 resides partly in ROM and partly as loadable routines in MM 48. 2. Interrupt Mechanism (Fig. 3) The system structure includes a further control and communications element referred to as the Interrupt Mechanism (IM) 124. IM 124 is essentially a mechanism for handling events which occur during execution of a program but which are not usually performed as a direct step in the execution of the program. In the occurrence of such an event, execution of the program is "interrupted", the state of program execution saved, the event handled, and program execution resumed from the point at which it was interrupted. The two major classes of interrupt are software interrupts, initiated by AP 116 or OS 120 routines, and hardware interrupts, initiated by the operation of the elements comprising HS 118. That is, software interrupts are called by a program when the program wishes to have an operation performed by the operating system or by the system hardware while hardware interrupts are generated when an element of the system hardware requires servicing. The interrupt routines function upon essentially the same level of detail as the BIOS 122 routines and are usually, as in the exemplary system of Figs. 1 and 2, regarded as part of the routines residing in the BIOS 122. Depending upon the specific allotment of functions within BIOS 122 and IM 124, however, the interrupt routines may be regarded as comprising a separate set of routines. Again, the interrupt routines may reside in MM 48 as loadable routines, or in ROM, or a combination thereof. The handling of software interrupts, that is, the calling or initiating of the appropriate software interrupt handling routines, is performed in the same manner as the calling or initiating of BIOS 122 routines. That is, an AP 116 or OS 120 routine issues a request or instruction which addresses the appropriate interrupt handling routine stored in MM 48 or in a ROM. Hardware interrupts, which are generated by HS 118, are initiated through specific logic provided for this purpose and result from the operation of HS 118 elements. In the exemplary system of Figs. 1 and 2, this mechanism is provided by PIC 66 and NMIL 36. Essentially, PIC 66 and NMIL 36 are provided with specific control signal inputs for this purpose from the various elements of HS 118. When the occurrence of an interrupt condition in an HS 118 element is indicated to PIC 66 or NMIL 36 through such an interrupt input, PIC 66 or NMIL 36 provides a corresponding request or instruction which addresses a corresponding location in an interrupt vector table residing in memory. The interrupt vector table in turn contains vectors, or addresses, identifying the location of the corresponding interrupt handling routine in memory. A vector read from the table is then used to select and initiate the corresponding interrupt handling routine. PIC 66 and NMIL 36 initiate interrupt operations for two types of interrupt conditions. These are, respectively, maskable and non-maskable interrupts wherein non-maskable interrupts are essentially conditions which must be handled immediately while maskable interrupts are interrupt conditions whose handling may, if necessary, be delayed. For example, non-maskable interrupts include parity or data errors occurring in MM 48 reads and, as described below, the handling of I/O operations when the system is operating as an emulator of another system. Maskable interrupts, which are the larger group of interrupt conditions, are handled secondarily to non-maskable interrupts and, within the class, are prioritized, the relative priority of maskable interrupts being determined by the programming of PIC 66 as described further below. Maskable interrupts include, for partial example, keystroke inputs, disc I/O completion, printer I/O, and serial I/O operations. C. System Address Structure (Fig. 4) The address space of the exemplary system shown in Figs. 1 to 3 is essentially organized into two address spaces, the first being the general program, routine and data space (Program/Data) and the second being the input/output and system control (I/O - Control) space. Both address spaces are accessed through BA bus 40 and BD bus 42 and the space addressed is determined by the operation being performed. The Program/Data space is comprised, in general, of MM 48, and program ROMs, such as EPROM 68. This address space is generally used to store programs and routines, such as APs 116, OS 120, and BIOS 122, including the interrupt routines, and data to be operated upon or resulting from the operation of such programs and routines. Included in this space, as described below, is the memory space residing in VC 78 for video display I/O functions. The I/O-Control address space is comprised of registers and memories in the system control and input/output elements. This address space is used, for example, to store and provide information used in system control, for programming certain system control functions, and to transfer information between the system and external elements, for example, a disc drive or the keyboard. As described just above, however, certain I/O functions may reside in the Program/Data space. As regards system control, it should be noted that many system elements, such as PIC 66 or EPCI 74, are individually programmable to perform system operation support functions. The operation of such elements is programmed by addressing and writing appropriate commands and/or data into their internal memory spaces. These elements may in turn store and provide information used in the control of system operation when the memory spaces therein are appropriately accessed. For example, PIC 66 may be programmed to determine the relative priorities of system interrupt conditions and to indicate the appropriate interrupt routines to call for each such condition. In turn, information regarding interrupt conditions which have occurred is stored in registers therein and may be accessed by the interrupt handling routines to determine the appropriate action to be taken. Other system control elements, while not programmable, may also store and provide information regarding system operation which may be accessed and used for these purposes. In the system input/output functions, many elements such as FDC 72, EPCI 74 and PPI 76, may be similarly programmable and, in addition, include registers and memory spaces which comprise the paths by which information is transferred between the system and external elements. For example, FDC 72 contains registers which are used to store and buffer information being transferred between the system and a disc. These registers are addressed through BA bus 40 and information to be transferred to or from a disc written into or read from the registers through BD bus 42. In further example, UART 70 contains similar registers which are used to buffer and store keystroke information until read by the system. Referring to Fig. 4, a diagrammic representation of the system address space is shown. The various areas within the address space illustrated therein are delineated by the addresses, in hexidecimal notation, indicated on the left of the diagram. The function of each segment of address space is indicated on the right of the diagram. In the first, Program/Data Space, for example, address space 00000H through 9FFFFH (H = hexidecimal) is general address space accessible to CPU 10 and generally residing in MM 48. Within that space. addresses 0000H to 003FF are reserved and dedicated for, for example, the previously described tables used by IM 124. Address spaces E0000H to F3FFFH are assigned to video display memory, for example, the display memories residing in VC 78. In this regard, address space F0000H to F0FFFH corresponds to CM 80 and AM 82, F2000H to F3FFFH to FM 94, and E0000H to E7FFFH to BM 102. Finally, address space FC000H to FFFFFH is reserved and dedicated to, for example, EPROM 68 and the routines stored therein. In the second, Input/Output-Control Space, addresses 1000H to 1FFFH are used for input/output and control functions. Of this address range, the elements shown in Fig. 1 occupy addresses 1000H to 10FFH while addresses 1x00H to 1XFFH, x being equal to or greater than 1, are reserved for additional and optional elements, such as Winchester disc drives. Having described the structure and operation of a typical system of the "personal" or "professional" computer class, those factors effecting the emulation of one such system by another system will be described next below. 2. System Differences Effecting System Emulation As previously described, a primary problem in "software portability", that is, the ability to run a program written for a first system upon a second, different system without modification, arises from the practice among applications program developers of "bypassing" the operating system - that is, of including instructions and commands in the applications program which go directly to a system's BIOS, interrupt mechanism or hardware structure, rather than only to and through the operating system. As a result, and because of the possible differences in internal structure between two such systems, an applications program written for the first system and which provides instructions and commands directly to the first system's internal structure may, when run on the second system, fail to provide the expected result. The second system may differ, for example, in the structure and operation of its BIOS, in the mapping or location of programs and routines in its Program/Data address space, in its interrupt mechanism, in its I/O-Control Address space, or in its hardware devices. The two systems may use different devices for the same purpose, or may use the same device for different purposes. For example, PPI 76 is used to drive a parallel printer in the Wang PC, but the same device is used as a keyboard interface device in the IBM PC. In further example, one system may use a Signetics 2661 UART for serial I/O operations while another may use a National Semiconductor 8250 UART, each of which responds to different commands and uses different I/O addresses. In yet further example, two systems may have keyboards which differ in the keys provided and in the hardware code corresponding to the various keys. As previously described, a system's BIOS is essentially the bridge between the system's operating system and the system's hardware structure. As such, the BIOS's of two systems may differ accordingly and an instruction or command which is valid with respect to the first system's BIOS may be invalid with respect to the second system's BIOS. In further example, the organization of the Progam/Data address spaces of two systems may differ such that an applications program written for the first system may attempt to write data or routines into or modify a portion of the second system's Program/Data space which is reserved or dedicated to other functions. By way of example, the Wang PC Program/Data space assigned to storing video display information, that is, the memory spaces of VC 78, differ from the memory space assigned to this function in the IBM PC. As such, any program written for the IBM PC and providing video display outputs which write directly to VC 78 will fail to run properly on the Wang system, and the reverse. In yet further example, the interrupt mechanisms of the Wang and IBM PCs differ in two respects. First, and as implied above, the interrupt handling routines and tables of the two systems reside in different portions of the two systems address spaces, so that an IBM PC application program which makes a direct call to an interrupt routine will, again, fail to execute properly. In further difference, in the Wang PC a particular hardware interrupt routine may service interrupts from multiple devices, with the interrupt mechanism identifying which particular device requires service. In the IBM PC, each device is provided with an individual interrupt vector, that is, each device is effectively provided with a separate interrupt service routine. In addition, the software interrupt service routines of the two systems may have completely different specifications, that is, may be required to respond to completely different conditions. In a final example, the addresses or "ports" of the input/output devices of two systems may be mapped into different areas of their I/O-Control address spaces, may be differently organized within their respective spaces, or may service different I/O devices. 3. Emulation Of A First System By A Second System (Figs. 5A, 5B and 6) The emulation of a first system by a second system in such a manner as to allow programs written for the first system to run on the second system without modification thereby may require emulation of the first system in four aspects. These are the BIOS, the mapping of Program/Data address space, the interrupt mechanism, and the mapping, structure and functionality of I/O-Control address space. The function, operation and design of a BIOS coupling a particular operating system to the hardware and internal structure of a given system are well known to those of ordinary skill in the art and will not be described in further detail herein. It should be noted that the requirement that a particular BIOS emulate another BIOS operates as a design constraint on the emulating BIOS, rather than as a fundamental change in the nature of the emulating BIOS. Similarly, the remapping of an system's Program/Data address space to emulate that of another system is well known in the art and can be readily accomplished in many systems, usually under control of programs written for that purpose. For example, in emulating an IBM PC on a Wang Professional Computer, the primary remappings of the Wang system Program/Data space involve the allocation of address spaces for the IBM video display memory and certain reserved address spaces and the relocation of the Wang interrupt tables and routines. By way of example, Wang system address spaces B0000H to B0FFFH and B1000H to B1FFFH, which are otherwise free, are allocated respectively to contain a Frame Buffer, similar to CM 80 and AM 82 of VC 78, and a reserved address space corresponding to such a space in the IBM PC system. VC 78 may thereby receive and provide display data at the addresses expected by programs written for the IBM PC system. As previously described, the Wang system interrupt tables and routines reside in locations 00000H to 003FFH of Wang Program/Data space. Certain of these areas are not, however, protected or reserved in the IBM PC system and programs written for the IBM PC may write into or otherwise modify this area of the Wang system address space. Accordingly, certain Wang interrupt vectors, that is, addresses which point to the locations of interrupt handling routines for Wang PC hardware functions, are relocated from vector numbers 00080H to 00087H (addresses 200H to 21FH) to vector numbers 00050H to 00057H (addresses 140H to 15FH), which are reserved in the IBM PC system. The relocation of these interrupt vectors requires the reprogramming of PIC 66 so that PIC 66 provides the appropriate new addresses of the relocated vectors in response to the corresponding interrupt conditions. As previously described, PIC 66 is a programmable device and may be appropriately reprogrammed for this purpose by the emulation BIOS or other software provided for this purpose. The remaining two areas of system emulation, the mapping, structure and functionality of I/O-Control address space and the interrupt mechanism, are presented in greater detail next below. 1. I/O Structure Emulation (Figs. 5A and 5B) As previously described, the system I/O structure is comprised of the hardware elements and the controlling routines by which information is communicated between the system and external devices, such as disc drives, the video display, the keyboard, and serial and parallel input and output devices. As also previously described, an emulating system and the emulated system may often use different hardware devices to perform similar I/O functions and, even where the devices are similar, those I/O devices which are programmable may be programmed differently. In addition, the I/O device ports may, in the two systems, occupy different locations in their respective I/O-Control address spaces. For example, the I/O ports in the IBM PC are located in the address space from 000H to 3FFH while the I/O ports in the Wang Professional Computer are located, as described above, at addresses 1000H to 1FFFH. As a result, I/O requests generated by programs written for a first system will most probably be invalid when those programs are run on a second system. The second system must thereby emulate the I/O structure of the first system with regard to program generated I/O requests, and will preferable do so without requiring extensive or fundamental modification to its I/O structure. Referring to Figs. 5A and 5B, the mechanism by which the present system emulates the I/O structure and functionality of another system is represented. As indicated therein, the mechanism includes I/O Nonmaskable Interrupt Mechanism (IONMIM) 128, which in turn includes the system's previously existing IM 124, and I/O Emulation Mechanism (IOEM) 130. As described below, IONMIM 128 detects the occurrence of "foreign" I/O requests, that is, I/O requests generated by programs originally written for another system. As described further below, I/O requests outside the address range of 1000H to 1FFFH are considered to be foreign I/O requests. IOEM 130 provides control routines which direct the present system's I/O structure so as to emulate the I/O structure of the other system. Considering first the detection of foreign I/O requests, as previously described CPU 10 includes Input/Output Decode Logic (IODL) 29. IODL 29 essentially monitors system input/output requests and provides corresponding control signal outputs to SC bus 18 indicating, first, whether an I/O request has occurred, and, second, whether the I/O request is to a valid I/O port address location. Referring now to IONMIM 128, IONMIM 128 includes I/O Detect Logic (IODET) 132, which has inputs connected from SC bus 18 and, in particular, from the IODL 29 control outputs described above. IODET 132 is responsive to these control signals to provide a non-maskable interrupt (NMI) to the system's IM 124 upon the occurrence of an I/O request which is not to a valid I/O port address in the present system, that is, upon the occurrence of a foreign I/O request. IM 124, as previously described, provides an interrupt vector address (IVA) output which addresses a corresponding location in an Interrupt Vector Table 134 containing vectors, or addresses, identifying the locations of corresponding Interrupt Handling Routines (IHR) 136. When the present system is emulating another system, IM 124's IVT 134 and IHR 136 are additionally provided with the appropriate vectors and routines for controlling the operation of IOEM 130, described below. The programming of IM 124 and the creation of the appropriate routines for controlling the subsequent operation of IOEM 130 will be apparent to one of ordinary skill in the art after the following description of operation of IOEM 130. The I/O emulation routines, as described above, reside in and are provided to the system by IOEM 130 and are structured into two groups according to the type of operation to be performed. The first group is comprised of read operation emulation routines and resides in Read I/O Emulation Routines (RIOER) 138 while the second group is comprised of write operation emulation routines and resides in Write I/O Emulation Routines (WIOER) 140. As previously described with reference to IHR 136, RIOER 138 and WIOER 140 will generally reside in a portion of the system Program/Data space, either in memory as loaded code or in ROM. It should be noted that, as indicated in Fig. 5A, the I/O emulation routines residing in RIOER 138 and WIOER 140 include respectively a Read Default routine (RDEF) 142 and a Write Default Routine (WDEF) 144. These routines are provided for the occurrence of certain foreign I/O operations which may not be supported by the emulation routines provided in RIOER 138 and WIOER 140. Such requests are usually rarely seen, but do occur in certain applications programs. In such cases, RDEF 142 and WDEF 144 essentially terminate the unsupported requests without further action and in such a manner that system operation is not undesirably disrupted. Within each type of operation to be emulated, that is, read or write, the operations to be performed are organized into "classes" of operations, for example, having similar characteristics, and the routines residing in RIOER 144 and WIOER 146 are generally organized in this manner. The selection of a particular RIOER 144 or WIOER 146 routine for emulating a particular foreign I/O operation thereby requires the identification of the type of operation, the class of operation within that type, and the identification of the particular corresponding routine within that class. The selection of I/O emulation routines from RIOER 138 and WIOER 140 in response to a foreign I/O request is, as described above, controlled by routines residing in IHR 136. The actual selection of routines is performed through the mechanism comprised of Read and Write Dispatch Tables (RDT) 146 and (WDT) 148 and corresponding Read and Write Class Tables (RCT) 150 and (WCT) 152 under control of the routines provided by IHR 136. RDT 146 and RCT 150 are used to select read operation routines from those residing in RIOER 138 while WDT 148 and WCT 152 are used to select write operations from those residing in WIOER 140. As described below, RCT 150 and WCT 152 contain, respectively, Read and Write I/O Emulation Routine Pointers (RIOERPs and WIOERPs), or addresses, identifying the locations of and selecting the individual emulation routines residing in RIOER 138 and WIOER 140. As indicated in Fig. 5B, which is a diagrammic representation of the internal structures of RCT 150 and WCT 152, the pointers residing in RCT 150 and WCT 152 are structured into Classes 154 corresponding to the class groupings of emulation routines in RIOER 138 and WIOER 140. Each Class 154 contains the pointers, IOERs 156, identifying the locations of the individual corresponding emulation routines in RIOER 138 and WIOER 140. The pointers residing in RCT 150 and WCT 152 are, in turn, located and selected by address inputs provided, in part, from RDT 146 and WDT 148 and, in part, from certain IOEM 130 registers latching and storing information pertaining to foreign I/O operation requests. As will be described below, the addresses stored in and provided by RDT 146 and WDT 148 to, respectively, RCT 150 and WCT 152, are used as index addresses to select, within RCT 150 or WCT 152, the particular Classes 154 containing the IOERs pointing to the appropriate emulation routines in RIOER 138 and WIOER 140. The latched information pertaining to the foreign I/O request is used, in part, to address RDT 146 and WDT 148 to select the appropriate indexes stored therein to be provided to RCT 150 or WCT 152 and, in part, as an address offset input to RCT 150 and WCT 152 to select the individual IOERs within the corresponding Classes 154 selected by the index addresses provided by RDT 146 or WDT 148. Referring to the IOEM 130 registers for latching and storing information pertaining to foreign I/O requests, IOEM 130 includes I/O Address Register (IOA) 158 interconnected from SA bus 14, I/O Data Register (IOD) 160 interconnected from SD bus 16, and I/O Status Register (IOS) 162 interconnected from SC bus 18. Upon the initiation of a foreign I/O request, the NMI output of IODET 132 selects and initiates, as described above, the IHR 136 I/O emulation routine for directing the operation of IOEM 130 and concurrently latches certain information pertaining to the request into IOA 158, IOD 160 and IOS 162. In particular, the requested foreign I/O address is latched into IOA 158 and, if the request is for an I/O write operation, the data to be written is latched into IOD 160. The status information latched into IOS 162 includes the fact of occurrence of the request, whether the request was for a read or write, whether it was for a byte or word I/O access, and, if for a word, whether the address was at an odd address. In response to a foreign I/O operation interrupt, the system performs an interrupt state save operation as previously described with reference to the system IM 124. The resulting I/O emulation control routine provided from IHR 136 then reads the I/O request address information from IOA 158 and status information from IOS 162. The status information identifying whether the requested operation is a read or write and a first part of the address information read from IOA 158 are used as an address input to RDT 146 and WDT 148 to obtain the index identifying, in RCT 150 or WCT 152, the Class 154 containing the IOERP to the appropriate corresponding emulation routine residing in RIOER 138 or WIOER 140. In this respect, the read/write status information portion of the address into RDT 146 and WDT 148 selects whether the corresponding index lies in the RDT 146 or WDT 148 range of addresses. The index read from RDT 146 or WDT 148 is then used as a first part of an address into, respectively, RCT 150 or WCT 152 to select the Class 54 containing the IOERP to the emulation routine to be executed. The second part of the address into RCT 150 or WCT 152 is, as described above, comprised of a second part of the address information read from IOA 158. This portion of the RCT 150/WCT 152 address input is effectively used as an offset (Offset), relative to the index input, to select within the Class 154 selected by the index portion of the address the location of the IOERP to the emulation routine. The selected IOERP is then used by the I/O emulation control routine provided from IHR 136 to select and initiate the appropriate emulation routine from those residing in RIOER 138 or WIOER 140. The emulation routine may then, in turn, access the status information residing in IOS 162 as required to appropriately emulate the requested foreign I/O operation. If the requested operation was for a data write, the selected emulation routine will read and transfer the data latched into IOD 160 appropriately. Having described the means by which the present system emulates the I/O structure of another system, the means by which the present system emulates the interrupt structure of that other system will be described next below. 2. Interrupt Mechanism Emulation (Fig. 6) As previously described, there are two classes of interrupt operations in a system of the type described herein, software interrupts and hardware interrupts. Software interrupts are initiated by AP 116 or OS 120 routines, and hardware interrupts are initiated by the operation of the hardware elements comprising HS 118. Examples of hardware interrupts include: keyboard inputs initiated from the keyboard, serial and parallel port I/O operations, disc operations and timed sequences. The routines IHR 136 for handling interrupt operations function upon essentially the same level of detail as the BIOS 122 routines and are usually, as in the exemplary system of Figs. 1 and 2 represented in Fig. 5A, regarded as part of the routines residing in the BIOS 122. The handling of software interrupts, that is, the calling or initiating of the appropriate software interrupt handling routines, is performed in the same manner as the calling or initiating of BIOS 122 routines. That is, an AP 116 or OS 120 routine issues a request or instruction which addresses the appropriate interrupt handling routine stored in MM 48 or in a ROM. The emulation of the software interrupt operation of another system is thereby a matter of adding the appropriate interrupt handing routines to the BIOS emulating the other system, as described previously. Hardware interrupts, which are called by HS 118, are initiated through specific logic provided for this purpose and result from the operation of HS 118 elements. In the exemplary system of Figs. 1 and 2 and Fig. 5A, this mechanism is provided by PIC 66 and NMIL 36. Essentially, PIC 66 and NMIL 36 are provided with specific control signal inputs for this purpose from the various elements of HS 118. When the occurrence of an interrupt condition in an HS 118 element is indicated to PIC 66 or NMIL 36 through such an interrupt input, PIC 66 or NMIL 36 provides a corresponding address (IVA) which addresses a corresponding location in an interrupt vector table (IVT 134). IVT 134 in turn contains and provides vectors, or addresses, identifying the location of the corresponding interrupt handling routines in IHR 136. As previously described, the internal hardware structure and operation of two system may differ widely and, as a result, so may the operations of the systems hardware interrupt mechanisms. Accordingly, an applications program written for one system and which directly communicates with that systems interrupt mechanism will most probably fail to run correctly on another system. By way of example, a particular applications program may require the user to enter certain information by means of the keyboard and may expect the hardware interrupt routines handling keyboard input interrupts to present the information in a particular manner. As described above, the emulation of a second system's software interrupt mechanism by a first system can be performed by the first system's BIOS which emulates the second system and is a process well understood by those of ordinary skill in the art. The emulation of a second system's hardware interrupt mechanism by a first system, however, is similar in problems to that of emulating the second system's I/O structure. Referring to Fig. 6, a diagrammic representation of the means by which the present system emulates the hardware interrupt mechanism of a second system is shown. Indicated therein is the hardware interrupt mechanism of the present system, including PIC 66 and NMIL 36, IVT 134 and IHR 136, the operation of which has been previously described. In the hardware interrupt emulation mechanism of the present system, IHR 136 contains, in its software interrupt service routines (SISR's 164), an additional group of Hardware Interrupt Emulation Routines (HIER's) 166 which emulate the operation of the hardware interrupt mechanism of the second system. IVT 134 correspondingly contains additional HIER Vectors (HIERV's) 168 to HIER's 166 and the hardware interrupt service routines (HISR's 170) of IHR 136 are altered to include references to HIERV's 166. HIER's 166, HIERV's 168 and the altered HISR's 170 are enabled when the system is emulating the second system. The generation of a hardware interrupt in the system thereby results in the corresponding HISR 170 calling, through the corresponding HIERV 168, the corresponding HIER 166 residing in SISR 164. HIER's 166, HIERV's 168 and the altered HISR's 170 thereby effectively perform hardware interrupt operations in the same manner as software interrupt operations, described above. It should be noted that HIER 166 is not limited to purely emulation routines, but may further include or be replaced by other routines providing additional functionality or enhanced performance. Again, the design and operation of hardware interrupt emulation routines is similar to that of software interrupt routines and, being well understood by those of ordinary skill in the art, will not be discussed in further detail herein. It will be apparent to those of ordinary skill in the art that the present invention may be embodied in yet other specific forms without departing from the spirit or essential characteristics thereof. Thus, the present embodiments are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than be the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims: In a first data processing system including a means for storing programs, a CPU means responsive to the programs or processing data, and an input/output structure (12) responsive to the programs for communicating information between the system and external devices, a means for emulating the input/output structure of a second system, characterized by the input/output structure (130, 138, 140) of the first system having ports (48, 70, 72, 74, 76, 78) occupying a first range of the system address space, the input/output structure (12) of the second system having ports located in a second range of system address space that is different from the first range of system address space, an input/output request detection means (132) for comparing input/output port address to the first range of port addresses and providing a range output signal having a first state when an input/output request port address is within the first range of system address space and a second state when an input/output request port address is not within the first range of system address space, interrupt means (128) responsive to the occurrence of an input/output request and to the second state of the range output signal for generating thereon a non-maskable interrupt signal, and emulation routine means (158, 160, 162, 146, 148, 150, 152) for storing emulation routines and responsive to said non-maskable interrupt signal generated on occurence of an input/output request having a port address in the second range of system address space, said emulation routine means for selecting and initiating in response to said non-maskable interrupt signal a corresponding emulation routine for performing the input/output request of the second system. The emulation means of claim 1, wherein the interrupt means (128) includes means (134, 136) responsive to operation of the request detection means (132) detect the occurrence of input/output requests not accompanied by corresponding indications of port addresses within the range of address space occupied by the system input/output devices and provide the corresponding non-maskable interrupt outputs upon occurrence of each such foreign input/output request. The emulation means of claim 1, wherein the emulation routine means comprise: means (158, 160, 162) responsive to the non-maskable interrupts for storing information pertaining to the foreign input/output requests, the means (146, 148, 150, 152) for selecting and initiating emulation routines being further responsive to the stored input/output request information in selecting the corresponding input/output emulation routines. The emulation means of claim 3, wherein the means for storing request information includes first register means (158) for storing port addresses of foreign input/output requests, and second register means (162) for storing status information pertaining to foreign input/output requests, including an indication whether a foreign input/output requests is for a read or a write operation. The emulation means of claim 4, wherein the means for storing request information further includes: third register means (160) for storing the data to be written when an input/output request is for a write operation. The emulation means of claim 1, wherein the selecting and initiating means comprises: class table means (150, 152) for storing addresses of emulation routines stored in the routine storing means, the emulation routine addresses stored in the class table means (150, 152) being structured into classes, each class containing the addresses of a corresponding group of foreign input/output operations, and dispatch table means (146, 148) for storing index addresses, each index address corresponding to a group of emulation routines and identifying the location in the class table means (150, 152) of the corresponding class of emulation routine addresses, the dispatch table (146, 148) means being responsive to a first portion of each foreign input/output request for providing a corresponding index address of the corresponding class of addresses stored in the class table means (150, 152), and the class table means (150, 152) responsive to a second portion of each foreign input/output request and to the corresponding index address for providing a corresponding emulation routine address to select and initiate execution of the corresponding emulation routine. The emulation means of claim 6, wherein each foreign input/output request including a port address identifying the location of a corresponding input/output device within the address space of the other system and corresponding to the type of input/output operation requested, the means for storing request information includes first register means (158) for storing port addresses for foreign input/output requests, and the first portion of a foreign input/output request is a first portion of the request port address, and the second portion of a foreign input/output request is a second portion of the request port address. The emulation means of claim 7, wherein the means for storing request information further includes second register means (162) for storing status information pertaining to foreign input/output requests, including an indication whether a foreign input/output request is for a read or a write operation, and the index means (146, 148) is further responsive to the status information identifying whether a request is for a read or write operation in providing the corresponding index address. The emulation means of claim 1, wherein the emulation routine means further includes default routines corresponding to foreign input/output requests not having corresponding emulation routines, each default routine terminating the corresponding input/output request operation, the selecting and initiating means being responsive to requests not having corresponding emulation routines for selecting and executing a corresponding default routine. The means for emulating the input/output structure of a second system of claim 1, wherein the interrupt means (128) includes a first interrupt means (164) responsive to first interrupt requests provided by the programs directing operation of the system for providing corresponding first interrupt handling routines for directing corresponding first interrupt handling operations of the system, and a second interrupt handling means (170) responsive to second interrupt requests provided by the hardware elements of the system for providing corresponding second interrupt handling routines for directing corresponding second interrupt handling operations of the system, further characterized by the interrupt means (128) including a means for emulating the second interrupt handling means of the second system, including in the first interrupt means (164), a means for storing interrupt emulation routines for directing operation of the system to emulate the second interrupt handling operations of the second system, in the second interrupt means (170), a means for detecting second interrupt request of both the first and second systems, and a means (146, 148) for providing a mode signal indicating when the first system is to emulate the second system, the second interrupt means (170) being responsive to the mode signal and to second interrupt requests of the first system for providing corresponding second interrupt handling routines of the first system for directing corresponding second interrupt handling operations of the system when the first system is not emulating the second system, and responsive to the mode signal and to second interrupt requests of the first system for providing corresponding second interrupt handling routines of the second system for directing corresponding second interrupt handling operations of the system when the first system is emulating the second system.

Inventors: ALBRIGHT, LOREN O., ANGEL, DAVID J., KLOS, PATRICK, MOSKUN, JAMES P., TYLER, CAROL W.
Applicants: WANG LABORATORIES INC.
Application year: 1985
Main classification: G06F9
IPC codes: G06F9, G06F11, G06F13