Data processing system emulation in a window with a coprocessor and I/O emulation

• US 5,088,033 A
• Filed: 03/23/1990
• Issued: 02/11/1992
• Est. Priority Date: 04/28/1986
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Caro, Perry A., Vukkadala, Gaya, Fay, Charles R., Woods, Donald R., Kadifa, Abdo G., Wick, John D., Hooks, Hilary N., Neely, Everett T., Binkley, Joseph H., Lee, Jeffery W., Mock, Clayton W., Lynch, William C., Thompson, Geoffrey O., Dillon, John B., Gibbons, Jonathan, Tallan, Michael L.
• Primary Examiner(s): Lee, Thomas C.
• Assistant Examiner(s): Donaghue, Larry
• Application Number:
  US07/499,196
• Time in Patent Office: 690 Days
• Field of Search: 364/200 MS File, 364/900 MS File
• US Class Current: 703/24
• CPC Class Codes: G06F 13/105 where the programme performs an input/output emulation function

Abstract

An emulating data processor includes a host system and an emulating processor with outputs to and inputs from the host system. The emulating processor executes sequences of instructions executable by a PC being emulated, but a host processor independently executes sequences of its instructions which are different from PC instructions. Circuitry monitors the emulating processor outputs and provides information to the host system so that it can emulate the environment of the PC CPU, emulating both memory and I/O devices. The memory accesses of the emulating processor are mapped into the host system memory, so that the host processor is protected from defective PC software on the emulating processor. The display updates of the emulating processor are detected and provide information for the host processor in updating a part of its display which provides the information a PC display would provide simultaneously with the display characteristic of the host system. An input/output processor handles I/O operation requests of the emulating processor, using the host system I/O devices to emulate some of the PC I/O devices. Output operations to the printer may go either to a local printer or to a file for subsequent printing, so a buffer which can be unloaded to either destination emulates the PC printer. Floppy operations may be handled either by a floppy disk controller like that of the PC or by a software controller of a file in host rigid disk memory which may be accessed as a PC floppy disk, so that a data structure containing parameters of the operation is loaded and provided to the appropriate controller. Rigid disk operations are handled by another file in host rigid disk memory which may be accessed as a PC rigid disk, and an appropriate I/O operating system routine is provided so that the emulating processor can pass the operation parameters through to the host rigid disk controller in a group of registers. Keyboard input operations may come either from the host keyboard or directly from a data structure managed by the host processor, in each case converted to PC codes, and another buffer which can be loaded from either source emulates the PC keyboard. The host system emulates the environment of the emulating processor while emulating the user interface of the PC.

Claims

1. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation and for receiving output signals from the central processor while performing the target system I/O operation, the target system further including communication means for communicating the output signals from the central processor to the target system I/O device;

the emulating system comprising;

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, one of the signals being a target I/O output signal that is one of the output signals received by the target system I/O device while performing the target system I/O operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising I/O monitoring circuitry for detecting the target I/O output signal and for obtaining monitoring data relating to the target system I/O operation;

the environment means further comprising I/O device data from which the environment means can determine a current emulated state of the communication means;

the environment means further comprising device emulating means for emulating the target system I/O operation by using the I/O device data to determine the current emulated state of the communication means;

the device emulating means accessing the I/O device data based on the monitoring data in response to the target I/O output signal;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

2. The system of claim 1 in which the monitoring data include control data indicating an input operation, the device emulating means retrieving the control data and providing input signals to the emulating processor based on the I/O device data if the control data indicate an input operation.

3. The system of claim 1 in which the monitoring data include address data indicating a type of I/O operation, the I/O device data including respective data for the indicated type of I/O operation, the device emulating means retrieving the address data and emulating the type of I/O operation based on the respective data for the indicated type of I/O operation.

4. The system of claim 1 in which the target I/O output signal includes an address indicating a suboperation performed by the target system I/O device while performing the target system I/O operation;

the I/O device data including respective data for the indicated suboperation;

the monitoring data obtained by the I/O monitoring circuitry including the address;

the device emulating means emulating the target system I/O operation by accessing the respective data for the indicated suboperation and emulating the target system I/O device in response to the monitoring data that includes the address.

5. The system of claim 4 in which the device emulating means further identifies the suboperation performed by the target system I/O device from the address included in the monitoring data.

6. The system of claim 4 in which the target I/O output signal includes status data indicating whether the suboperation performed by the target system I/O device is an input I/O operation, the monitoring data obtained by the I/O monitoring circuitry including the status data;

the environment means further being for determining whether the status data included in the monitoring data indicates that the suboperation performed by the target system I/O device is an input I/O operation and, if so, for providing input data to the emulating processor.

7. The system of claim 6 in which the environment means further stops operation of the emulating processor whenever the status data included in the monitoring data indicates that an input I/O operation is requested and permits the emulating processor to resume operation after the requested I/O operation has been emulated, the emulating processor executing a routine for retrieving the input data when it resumes operation.

8. The system of claim 1 which the target I/O output signal includes an indication of whether the target system I/O device performs an input operation while performing the target system I/O operation, the I/O monitoring circuitry further obtaining form the target I/O output signal a read signal indicating whether the target system I/O device performs the input operation.

9. The system of claim 8 in which the I/O monitoring circuitry is further connected for providing an interrupt to the emulating processor, the I/O monitoring circuitry providing the interrupt if the read signal indicates the target system I/O device performs the input operation.

10. The system of claim 9 in which the emulating processor has a non-maskable interrupt input line, the I/O monitoring circuitry being connected to provide the interrupt on the non-maskable interrupt input line.

11. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation and for receiving output signals from the central processor while performing the target system I/O operation, the target system further including communication means for communicating the output signals from the central processor to the target system I/O device;

the emulating system comprising;

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals provided by the emulating processor including a target I/O output signal that is one of the output signals received by the target system I/O device while performing the target system I/O operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising a corresponding I/O device corresponding in function to the target system I/O device;

the environment means further comprising I/O device data from which the environment means can determine a current emulated state of the communication means;

the environment means further comprising device emulating means for emulating the target system I/O operation in response to the target I/O output signal by using the host processor and the corresponding I/O device and by using the I/O device data to determine the current emulated state of the communication means, the device emulating means accessing the I/O device data in response to the target I/O output signal;

the I/O device data comprising a data structure for transferring data between the emulating processor and the host processor during emulation of the target system I/O operation;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions;

the host processor being connected for accessing the data structure;

the sequence of the host system instructions including an operation accessing the data structure.

12. The system of claim 11 in which the corresponding I/O device is a printer controller, the data structure being a multi-character printer buffer for transferring characters to the printer controller, the device emulating means further managing the printer buffer to generate input signals for the emulating processor reflecting the status of the buffer.

13. The system of claim 11 in which the corresponding I/O device is a keyboard controller, the data structure being a multi-character keyboard buffer for transferring codes from the keyboard controller, the device emulating means further interrupting the emulating processor when the buffer contains at least one code.

14. The system of claim 11 which the corresponding I/O device is a display controller, the host system comprising a bitmap memory loaded by the host processor and read by the display controller, the data structure being for transferring display parameters to the display controller, the device emulating means further requesting the host processor to load display update information into the bitmap memory.

15. The system of claim 11 in which the device emulating means loads the data structure based on data received from the emulating processor in order to transfer the loaded data to the host processor and unloads from the data structure data received from the host processor in order to transfer the unloaded data to the emulating processor.

16. The system of claim 11 in which the target I/O operation signal includes status data indicating whether the suboperation performed by the target system I/O device is an input operation or an output operation, the monitoring data obtained by the I/O monitoring circuitry including the status data;

the corresponding I/O device being equivalent to the target system I/O device, the device emulating means further providing output data from the data structure to the corresponding I/O device when the status data indicates the requested operation is an output operation and retrieving input data from the corresponding I/O device when the status data indicates the requested operation is an input operation.

17. The system of claim 16 in which the target system I/O device is a floppy disk controller, the data structure including a data block for holding the input data to transfer floppy operation parameters to the floppy disk controller and for holding the output data to transfer floppy operation results to the emulating processor.

18. The system of claim 11 in which the environment means further comprises memory for storing the data structure, the environment means further comprising an input/output processor connected for accessing the data structure in the memory to transfer data between the emulating processor and the host processor;

the device emulating means operating the input/output processor to transfer data between the emulating processor and the host processor during emulation of the target system I/O operation.

19. The system of claim 18 in which the device emulating means further operates the input/output processor to load data from the emulating processor into the data structure and retrieve the loaded data from the data structure for transfer to the corresponding I/O device.

20. The system of claim 18 in which device emulating means operates the host processor to perform the host operation during emulation of the target system I/O operation;

the host processor operation providing a host I/O operation signal requesting an I/O operation and providing data from the data structure with the host I/O operation signal, the input/output processor further transferring the data provided with the host I/O operation signal to the corresponding I/O device.

21. The system of claim 20 in which the target I/O output signal includes output data, the device emulating means further operating the input/output processor to receive the output data, load the output data into the data structure, and receive the host I/O operation signal from the host processor.

22. A system for emulating a target system having a central processor for executing a set of target system instructions, the emulating system comprising:

a host system having a host processor for executing a set of host system instructions different from the target system instructions, the host processor providing a host I/O operation signal requesting an I/O operation during execution of the host system instructions;

the host system further comprising a set of I/O devices; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing an emulator I/O operation signal requesting an I/O operation during execution of the sequence of target system instructions;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising an input/output processor for responding to the host I/O operation signal and the emulator I/O operation signal by controlling the set of I/O devices to perform requested I/O operations for the emulating processor and the host processor;

the input/output processor further being for stopping the emulating processor while the I/O devices perform the requested I/O operation in response to the emulator I/O operation signal;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

23. The system of claim 22 in which the target system includes a set of I/O devices and the environment means comprises a memory accessible to the input/output processor and to the emulating processor, the emulator I/O operation signal being a request for one of the target system I/O devices;

the input/output processor responding to the emulator I/O operation signal by emulating the one target system I/O device to obtain a result and by loading the result into the memory so that the emulating processor can retrieve the result and continue to execute the sequence of target system instructions.

24. The system of claim 22 in which the environment means further comprises a bus shared by the input/output processor and the emulating processor, the environment means further comprising means for controlling which of the input/output processor and the emulating processor has control of the bus, the controlling means giving control of the bus to the emulating processor only after the input/output processor signals the controlling means to allow emulating processor control.

25. The system of claim 24 in which the host system further comprises memory having a part accessible to the input/output processor through the bus and a part accessible to the emulating processor through the bus, the input/output processor and the emulating processor each providing an address on the bus while accessing the respective part of the memory, the environment means further comprising mapping means for determining which of the input/output processor and the emulating processor is accessing memory and for mapping the address on the shared bus into the part of memory accessible to that processor.

26. The system of claim 25 in which the mapping means comprises mapping registers for providing information for mapping the address into the respective part of memory accessible to the processor accessing memory, the mapping register being accessible only to the input/out processor so that the emulating processor cannot change the part of memory which is accessible to it.

27. The system of claim 25 in which the part of memory accessible to the emulating processor is also accessible to the host processor, the mapping means further being for ensuring that the operations of the host processor in executing a sequence of host system instructions are not altered by the operations of the emulating processor in accessing memory.

28. The system of claim 25 in which the part of memory accessible to the input/output processor and the part of memory accessible to the emulating processor share an overlapping area of memory, the input/output processor and the emulating processor using the overlapping area of memory to transfer information to each other.

29. The system of claim 28 in which the set of I/O devices includes a rigid disk controller, the overlapping area of memory including a register block;

the input/output processor using the register block to transfer rigid disk operation results to the emulating processor, the emulating processor executing a loading routine for loading the register block and an unloading routine for unloading the register block.

30. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation, the emulating system comprising:

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals provided by the emulating processor including a target I/O operation signal requesting the target system I/O operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising a corresponding I/O device corresponding in function to the target system I/O device;

the environment means emulating the target system I/O operation in response to the target I/O operation signal by using the corresponding I/O device, the environment means further comprising a data structure for transferring data between the emulating processor and the corresponding I/O device during emulation of the target system I/O operation;

the environment means further comprising memory for storing the data structure, the environment means further comprising an input/output processor connected for accessing the data structure in the memory to transfer data between the emulating processor and the corresponding I/O device during emulation of the target system I/O operation;

the corresponding I/O device providing data for transfer to the emulating processor during emulation of the target system I/O operation, the host processor being connected for accessing the data structure in the memory, the host processor further loading the data provided by the corresponding I/O device into the data structure for transfer to the emulating processor;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

31. The system of claim 30 in which the input/output processor further provides the data from the corresponding I/O device to the host processor, retrieves the data loaded by the host processor from the data structure and provides the retrieved data to the emulating processor.

32. A data processing system for emulating a target data processing system having a central processor for executing a set of target system instructions, the central processor providing I/O operation requests to a set of target system I/O devices while executing a sequence of the target system instructions, the target system further including communication means for communicating I/O operation requests from the central processor to a first one of the target system I/O devices;

the emulating data processing system comprising;

a host system having a host processor for executing a set of host system instructions and a set of host system I/O devices, the host processor providing I/O operation requests requesting operations of the host system I/O devices while executing a sequence of the host system instructions; and

an emulating processor for executing a sequence of the target system instructions, the emulating processor providing I/O operation requests to the host system during execution of the target system instructions, a first one of the I/O operation requests being one of the I/O operation requests provided by the target system central processor to the set of target system I/O devices;

the host system further comprising input/output means for receiving the I/O operation requests from the host processor and from the emulating processor and for handling the I/O operation requests using the host system I/O devices so that the host processor continues to execute the sequence of host system instructions while the emulating processor executes the sequence of target system instructions;

the input/output means comprising I/O monitoring circuitry for detecting the first I/O operation request from the emulating processor and for obtaining monitoring data relating to the requested I/O operation;

the input/output means further comprising I/O device data from which the input/output means can determine a current emulated state of the communication means;

the input/output means further comprising device emulating means for emulating the first target system I/O device by using the I/O device data to determine the current emulated state of the communication means;

the device emulating means accessing the I/O device data based on the monitoring data.

33. The system of claim 32 in which the input/output means further comprises an input/output processor, the device emulating means operating the input/output processor to emulate the first target system I/O device so that the emulating processor continues to execute the sequence of target system instructions.

34. A data processing system for emulating a target data processing system having a central processor for executing a set of target system instructions and target interface means for providing external transfer of signals in a manner characteristic of the target system in response to the central processor, the emulating data processing system comprising:

a host system having host interface means for providing external transfer of signals and a host processor for controlling the host interface means to provide external transfer of signals in a manner characteristic of the host system and different from the manner characteristic of the target system;

the host interface means comprising a user interface for transferring signals between a user and the host system, the host processor controlling the user interface so that the signals are transferred in a manner characteristic of the host system user interface while simultaneously being transferred in a manner characteristic of a target system user interface;

the user interface comprising a display controlled by the host processor; and

an emulating processor for executing a sequence of the target system instructions, the emulating processor providing output signals and receiving input signals during execution of the target system instructions;

the host system receiving output signals from the emulating processor and providing input signals to that the emulating processor continues to execute the sequence target system instructions;

the host processor controlling the host interface means in response to the output signals to provide external transfer of signals in a manner characteristic of the host system while simultaneously providing external transfer of signals in a manner characteristic of the target system;

the host processor controlling the display so that the signals provided by the display to the user provide the appearance of a display characteristic of the host system while simultaneously providing within that display a display characteristic of the target system;

the display characteristic of the host system including a set of windows;

the host processor controlling the display to provide the display characteristic of the target system in one of the windows in the display characteristic of the host system.

35. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation, the emulating system comprising:

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, one of the signals being a target I/O operation signal requesting the target system I/O operation;

the target I/O operation signal including an indication of whether the requested I/O operation is an input operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising I/O monitoring circuitry for detecting the target I/O operation signal and for obtaining monitoring data relating to the target system I/O operation;

the I/O monitoring circuitry further obtaining from the target I/O operation signal a read signal indicating whether the requested I/O operation is an input operation;

the environment means emulating the target system I/O operation based on the monitoring data in response to the target I/O operation signal;

the I/O monitoring circuitry further being connected for providing an interrupt to the emulating processor if the read signal indicates the requested I/O operation is an input operation;

the environment means further being connected to hold and release the emulating processor, the environment means holding the emulating processor upon completion of execution of one of the sequence of target system instructions during which the target I/O operation signal was provided and then releasing the emulating processor after loading input data for the emulating processor, the emulating processor responding to the interrupt when the environment means releases it by retrieving the input data and using the retrieved input data to correct erroneous data resulting from the completion of execution of the target system instruction;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

36. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the set of target system devices including a target system memory and target system I/O devices;

the target system further including communication means for communicating signals from the central processor to a first one of the target system I/O devices;

the emulating system comprising;

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals including memory access signals and I/O operation signals;

each memory access signal requesting access to the target system memory;

each I/O operation signal requesting a respective I/O operation of the target system I/O devices;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising I/O device data from which the environment means can determine a current emulated state of the communication means;

the environment means further being for receiving a first one of the signals from the emulating processor;

the environment means further comprising device emulating means for emulating an I/O operation of the first target system device if the first signal is one of the I/O operation signals requesting an I/O operation of the first target system device;

the device emulating means accessing the I/O device data in response to the first signal and using the I/O device data to determine the current emulated state of the communication means;

the environment means further comprising memory;

the memory being accessed in response to the first signal if the first signal is one of the memory access signals;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions;

the host processor accessing the memory in executing the sequence of the host system instructions.

37. The system of claim 36 in which the memory includes a part that is accessible to the emulating processor, the environment means further being for mapping the first signal into the part of memory accessible to the emulating processor if the first signal is one of the memory access signals, the environment means further being for ensuring that the operations of the host processor in executing the sequence of the host system instructions are not altered by the operations of the emulating processor in accessing memory.

38. The system of claim 36 in which the host system further comprises interface means for providing external transfer of signals, the host processor accessing the memory to retrieve data stored by the emulating processor and controlling the interface means based on the retrieved data.

39. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation, the target system further including communication means for communicating signals requesting the target system I/O operation from the central processor to the target system I/O device;

the emulating system comprising;

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals provided by the emulating processor including a target I/O operation signal requesting the target system I/O operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising an equivalent I/O device to the target system I/O device;

the environment means further comprising means for simulating the equivalent I/O device;

the environment means further comprising I/O device data from which the environment means can determine a current emulated state of the communication means;

the environment means further comprising device emulating means for emulating the target system I/O operation in response to the target I/O operation signal by using the I/O device data to determine the current emulated state of the communication means, the device emulating means accessing the I/O device data in response to the target I/O operation signal;

the device emulating means determining in response to the target I/O operation signal whether to emulate the target system I/O operation by using the equivalent I/O device or by using the simulating means;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

40. The system of claim 39 in which the environment means comprises a memory and the target system I/O device is a controller for a memory medium, the environment means further managing a part of the memory to be accessible as if it were the memory medium, the simulating means simulating the equivalent I/O device by accessing the managed part of memory.

41. The system of claim 40 in which the target system I/O device is a floppy disk controller;

the environment means managing the part of the memory to be accessible as if it were a floppy disk.

42. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system I/O devices that provides an environment for the central processor;

the emulating system comprising;

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals including an I/O operation signal requesting an I/O operation of the target system I/O devices;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means further being for receiving the I/O operation signal from the emulating processor and for emulating the respective I/O operation of the target system devices in response to the I/O operation signal;

the environment means further stopping the emulating processor while emulating the respective I/O operation in response to the I/O operation signal;

the environment means further comprising a memory accessible to the emulating processor;

if the requested I/O operation is an I/O input operation, the environment means further being for loading input data into the memory while the emulating processor is stopped;

the environment means further being for permitting the emulating processor to resume operation after the requested I/O operation has been emulated;

the emulating processor further being for retrieving the input data from the memory when it resumes operation;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

43. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the set of target system devices including a target system memory and target system I/O devices;

the target system further including communication means for communicating signals from the central processor to a first one of the target system I/O devices;

the emulating system comprising;

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals including memory access signals and I/O operation signals;

each memory access signal requesting access to the target system memory;

each I/O operation signal requesting a respective I/O operation of the target system I/O devices;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising I/O device data from which the environment means can determine a current emulated state of the communication means;

the environment means further being for receiving a fist one of the signals from the emulating processor;

the environment means further comprising device emulating means for emulating an I/O operation of the first target system device if the first signal is one of the I/O operation signals requesting an I/O operation of the first target system device;

the device emulating means accessing the I/O device data in response to the first signal and using the I/O device data to determine the current emulated state of the communication means;

the environment means further comprising memory;

the memory being accessed in response to the first signal if the first signal is one of the memory access signals;

the memory being accessed in response to the first signal so that the emulating processor continues to execute the sequence of the target system instructions as if it were the central processor of the target system;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

44. The system of claim 43 in which the first signal includes a memory address if the first signal is one of the memory access signals;

the environment means further comprising mapping circuitry for receiving the first signal if the first signal is one of the memory access signals and for mapping the memory address in the first signal to a corresponding location in the memory so that the corresponding location is accessed in response to the first signal.

45. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation, the emulating system comprising:

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals provided by the emulating processor including a target I/O operation signal requesting the target system I/O operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising a printer controller corresponding in function to the target system I/O device;

the environment means emulating the target system I/O operation in response to the target I/O operation signal by using the printer controller, the environment means further comprising a multi-character printer buffer for transferring characters from the emulating processor to the printer controller during emulation of the target system I/O operation;

the environment means further managing the printer buffer to generate input signals for the emulating processor reflecting the status of the buffer;

the environment means further comprising a memory, the host processor being connected for storing a series of characters from the printer buffer in the memory for subsequent printing, the environment means determining whether to provide the characters from the buffer to the printer controller or to store the characters for subsequent printing;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

46. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation, the emulating system comprising:

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, the signals provided by the emulating processor including a target I/O operation signal requesting the target system I/O operation;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising a printer controller corresponding in function to the target system I/O device;

the environment means emulating the target system I/O operation in response to the target I/O operation signal by using the keyboard controller, the environment means further comprising a multi-character printer buffer for transferring characters from the keyboard controller to the emulating processor during emulation of the target system I/O operation;

the environment means further interrupting the emulating processor when the keyboard buffer contains at least one code;

the emulating processor executing an instruction causing it to provide an overflow signal if an input keyboard buffer under its control cannot accept a code from the multi-character keyboard buffer, the environment means further providing the code which could not be accepted to the emulating processor again in response to the overflow signal;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.

47. A system for emulating a target system having a central processor for executing a set of target system instructions and a set of target system devices that provides an environment for the central processor, the target system devices including a target system I/O device for performing a target system I/O operation the emulating system comprising:

a host system having a host processor for executing a set of host system instructions different from the target system instructions; and

an emulating processor for executing a sequence of the target system instructions;

the emulating processor providing signals during execution of the sequence of target system instructions, one of the signals being a target I/O operation signal requesting the target system I/O operation;

the target I/O operation signal includes an address corresponding to the target system I/O device;

the host system further comprising environment means connected for providing an environment for the emulating processor so that the emulating processor continues to execute the sequence of target system instructions and connected for providing an environment for the host processor so that the host processor executes the host system instructions;

the environment means comprising I/O monitoring circuitry for detecting the target I/O operation signal and for obtaining monitoring data relating to the target system I/O operation;

the monitoring data obtained by the I/O monitoring circuitry including the address;

the environment means emulating the target system I/O operation in response to the target I/O operation signal by emulating the target system I/O device based on the address in the monitoring data;

the environment means further comprising a memory storing an I/O operating system for enabling the emulating processor to execute the sequence of target system instructions and for causing the emulating processor to provide at least one non-target I/O operation signal not provided by the target system central processor, the non-target I/O operation signal requesting a non-target I/O operation that is not performed in the target system but that relates to emulation of the target system I/O operation;

the I/O monitoring circuitry further being for detecting the non-target I/O operation signal and for obtaining monitoring data relating to the non-target I/O operation;

the environment means further performing the non-target I/O operation in response to the non-target I/O operation signal based on the monitoring data;

the host processor independently executing a sequence of the host system instructions while the emulating processor executes the sequence of target system instructions.