Abstract:
A register of a processor is set to one value when a host is connected to the processor and to a second value when no host is connected. The processor then starts execution after reading the register contents, and if it finds that the second value is stored it writes a set value to a pointer storage location. When the one value is stored, it leaves the content of the pointer location unaffected.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method of operating an embedded system and to an embedded system, also to a method and apparatus for data communication between a host computer and a target computer system and more particularly but not exclusively between a host computer and an embedded target computer system.  
         BACKGROUND OF THE INVENTION  
         [0002]    During development and testing of an embedded computer system, especially an embedded microprocessor-based computer system, it is normally necessary to connect the system to a host computer so that an engineer can debug the embedded system. Debugging software on the host computer provides access by the engineer into the embedded system. Typically, the engineer uses the host computer and debugging software to set break points which stop threads of execution on the target embedded system, monitor registers in the target, monitor memory states and monitor input and output data. The engineer may also restart threads that have been previously stopped and perform a variety of other tasks including rebooting the target computer using the host.  
           [0003]    Although in normal operation the embedded system may be virtually self-sufficient in that it does not in normal use need to perform input/output, it may be necessary during debugging for such input/output to be performed. Examples of this are displaying messages at the host, reading characters from the host keyboard or reading data from a file stored on the host. Similar situations requiring input/output may occur where an embedded system is sold to an end user and the end user needs to customize the embedded system, for example writing information into programmable read only memory in the embedded system.  
           [0004]    Embedded systems may contain more than one application, for instance stored in a read only memory or downloaded over a link, at run-time. An example of the latter is a satellite transmission link. Each such application may need to perform input output of data to the host computer during debugging. Furthermore, applications may be interconnected so that one application that is executing may, at a virtually arbitrary point in time, cause another application to start executing. Although it is possible to provide input/output libraries during the construction of each application, such libraries, and as a result the protocol implemented by the application for input/output, may differ between applications.  
           [0005]    One problem when debugging a system is that of conveying to an application the fact that the host system is connected, and is in control. Absent this information, the application may be able to act in a way that results in it taking over control of the system, or which results in overwriting of data needed by the host.  
           [0006]    As used herein, the term ‘digital signal processor’ includes microprocessors in general and other like digitally operating processing devices.  
           [0007]    It is accordingly a primary object of the present invention to at least partially overcome the difficulties of the prior art.  
         SUMMARY OF THE INVENTION  
         [0008]    According to a first aspect of the present invention there is provided a method of operating a target system having a digital signal processor said digital signal processor having plural registers, the target system further having a link connectable to a host computer for debugging said target system, wherein said target system further has a memory having plural addressable locations, one of said locations being designated for storage therein of pointer information indicative of the entry point of a subroutine useable in debugging by said host computer, and other locations storing an application program, the method comprising:  
           [0009]    setting the contents of a predetermined one of said registers of said digital signal processor to a first value when said host is connected to said link, and to a second value when said host is not connected to said link;  
           [0010]    starting execution by said digital signal processor of said application program on said target system, wherein said step of starting execution comprises:  
           [0011]    reading the content of said predetermined one of said registers;  
           [0012]    when said one register stores said second value, writing a predetermined value to said one of said locations;  
           [0013]    when said one register stores said first value, leaving the contents of said one of said locations unchanged.  
           [0014]    According to a second aspect of the present invention there is provided a computer system having a digital signal processor, said digital signal processor having plural registers, the system further having a link connectable to a host computer for debugging said system, wherein said system further has a memory having plural addressable locations, one of said locations being designated for storage therein of pointer information indicative of the entry point of a subroutine useable in debugging by said host computer, and other locations storing an application program, the system comprising:  
           [0015]    reset circuitry for setting the contents of a predetermined one of said registers of said digital signal processor to a second value;  
           [0016]    circuitry responsive to a host connected to said link for setting the contents of said register to a first value;  
           [0017]    said digital signal processor comprising circuitry starting execution of said application, and further comprising determining circuitry for determining the content of said predetermined one of said registers; and writing circuitry for writing a predetermined value to said one of said locations when said one register stores said second value, whereby when said one register stores the first value, the contents of said one of said locations remains unchanged. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    A preferred but non-limiting embodiment of the invention will now be described with reference to the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 shows a block diagram of a host computer connected to a target computer for downloading an input/output library;  
         [0020]    [0020]FIG. 2 shows a schematic partial view of the target system showing the use of a pointer to indirectly access an entry point of a dynamically loaded library;  
         [0021]    [0021]FIG. 3 shows a block diagram of a target computer showing how data communication is effected using the invention;  
         [0022]    [0022]FIG. 4 shows a schematic partial view of the target system showing an alternative way of accessing an entry point of a dynamically loaded library.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    In the various figures like reference numerals refer to like parts.  
         [0024]    Referring first to FIG. 1, a host computer  1  includes an input device such as a keyboard  2  and runs debugging software  4  for a target embedded system  100 , the host and target being connected via a communications link  101 ,  102 . This link is shown figuratively in FIG. 1 as two separate paths with a first path  101  conveying data from the host machine  1  to the target machine  100  and a second path  102  connecting to the target computer  100  to the host computer  1 . It will be understood that the link may be of any conventional type that allows two-way communication. Such communication need not be simultaneous. The target system typically includes a microprocessor or other processing circuitry, hereinafter referred to as a digital signal processor.  
         [0025]    The host computer further includes a file  3 , hereinafter referred to as a library, containing a collection of subroutines and information that will be used by the target computer to implement a multi-layered communication protocol over the physical link  101 ,  102 .  
         [0026]    The link  101  outgoing from the host computer  1  to the target computer  100  is shown figuratively as connected to four portions of the target system, namely a first application  121 , a second application  122 , a block  123  whereby the embedded digital signal processor of the target system  100  may be reset, and to the volatile memory  200  of the target system.  
         [0027]    It will be understood by those skilled in the art that the debugging software  4  may communicate over the link  101  to interrogate or monitor the registers in the target, the memory in the target, to stop and restart threads of execution.  
         [0028]    The applications  121 ,  122  may be stored in read only memory or may be downloaded via a separate link (not shown) at run-time, eg into RAM  200 .  
         [0029]    Referring to FIG. 2, the target system  100  includes an exception handler  20  loaded into a memory area  201 , the exception handler  20  having a stack  21  in a reserved area of memory  202  the stack including a reserved location  205 . In the embodiment described, which has a negative-growing stack, the reserved location  205  is selected as the bottom of the stack. The host system  1  may dynamically load the exception handler stack into the target memory. The target includes a hard-wired location  190  accessible to both the target and the host, hereinafter referred to as a vector.  
         [0030]    As used in this document, a vector is a reserved area, typically at a hard-wired location, used to contain address data of a key system entity. A pointer is a general holder of the address of a selected (arbitrary) entity.  
         [0031]    At the start of debugging, the library  3  is accessed by the linker-loader  120  of the host which dynamically loads the library to a suitable vacant area of memory  203  of the target according to the usage of memory by applications, data and functions, to provide a loaded library  30  having an entry point  204  within the region of memory  203 . At the time of loading the library  30  into memory, the linker-loader  120  determines where in the memory of the target to load the stack. The linker-loader stores at the bottom stack location  205 , hereinafter referred to as the entry point pointer, information representing the entry point  204  and loads the location of the stack into the vector  190 . It will be understood by those skilled in the art that the information stored by the vector  190  may be of various forms, including the absolute address of the exception handler stack  202  or other information such as an offset.  
         [0032]    If the embedded system  100  is not connected to a host, no library will have been loaded and entry point pointer  205  is set to contain a predetermined value indicative of the fact that no library is available, as will be described below. This predetermined value may be a value that is not a valid address.  
         [0033]    When an application performs input/output, the following events take place:  
         [0034]    1. The application identifies the need to perform input/output, and calls a linked-in function setting up input/output data structures at the upper levels of the protocol.  
         [0035]    2. The linked-in function looks for a valid address at location  205  by reading the contents of vector  190  to find the exception handler stack and offsetting to the entry point pointer  205 . If the entry point vector  205  does not store a valid address but instead the “no library available” value, then the host is assumed to be disconnected. An appropriate error code is then produced.  
         [0036]    3. If the address is valid, it is assumed to be that of the library entry point. A call is made to that entry point and the code at this address will determine whether the host is connected and whether input/output is enabled. The method of checking the library code is described later herein. If the code checks indicate either that no host is connected, or that input/output is not enabled, an appropriate error code will be determined stating the reason for input/output failure.  
         [0037]    4. If the host is connected and input/output is enabled, the application will communicate with the debugger to carry out the input output operation and return an appropriate status code.  
         [0038]    Referring now to FIG. 3, the first application  121  may be initiated by the host computer  1  at an input  131 . If the first application  121  needs to input data from the host computer  1  then it must make use of the dynamic library  30 . To do this, application  121  reads the vector  190  and from its contents accesses the address of the exception handler stack  202 , offsets to the entry point pointer  205  which results in the entry point into the dynamic link library  30  being supplied to the application at input  141 . The application fetches information from the dynamic link library  30  at terminal  151 . The application  121  is thus able, by use of the required subroutines in the library  30  to “translate” information coming in over link  101  from the host computer.  
         [0039]    If the first application needs to output information to the host computer, then a similar process applies.  
         [0040]    Further inspection of FIG. 3 shows that the first application  121  is capable of starting the second application  122  over a path  132 . If during execution of the second application  122  there is a requirement for the second application to input or output data from or to the host computer  1 , the second application uses the vector  190  to load the entry point data from memory location  205  over the path  142  and accesses the library  30  over the path  152 .  
         [0041]    This in turn enables the second application  122  to input or output data as required.  
         [0042]    Each of the applications typically comprises a start up code. The system described uses a single region of memory  202  for the exception handler stack  21 . However, where more than one application exists in the embedded system, each may use a different region of memory for the stack of its exception handler. If this occurs, then when a host is connected, a region of memory known to the host must be reserved for this stack and the vector of the target updated to point to this region. The consequence is that all applications use a single area of stack for the particular exception. This ensures that the host has a single stack area, which it will use for the linker-loader  120  to store the address information of the library  30 .  
         [0043]    Without these measures being taken, each of the stack regions will be valid for a particular time period and the host will be unable to determine which of these stack regions to write the relevant data to at any one time.  
         [0044]    There are three ways in which an application may start. In the first, the host starts the application, in the second the application is started by the target following reset, eg power-up, and in the third, the application is started by another application, referred to here as the initiating application.  
         [0045]    For the first, use of the dynamic link library is necessary.  
         [0046]    For the second, the application should run correctly without recourse to the library, and if the application tries to call the library, an error should be indicated.  
         [0047]    However, for the third, the provision of the dynamic link library depends on how the initiating application was started. If the initiating application was started by the host, then the dynamic link library should be made available: if the initiating application started after target reset, then no library should be needed. (It will of course be understood by those skilled in the art that a whole chain of applications may exist, in which case the decision depends on how the first in the chain started up)  
         [0048]    To cope with the above situation, the embodiment initialises the contents of the entry point pointer  205  according to the following rules:  
         [0049]    If the host has started the application, the application start up code assumes that the entry point pointer value is valid—i.e. the host has set the value stored by the entry point pointer  205  to point to a dynamic library, which the host has downloaded to the target.  
         [0050]    If the application has been started following reset of the target, the application start up code will place the above-discussed “no library available” value in entry point pointer  205  indicating that there is no available dynamic library.  
         [0051]    If the application has been started by an initialising application then:  
         [0052]    a) If the host started the initialising application, then the value stored by entry point pointer  205  is considered valid and it is considered that there is a valid library in the system.  
         [0053]    b) If however there is no host connected, or if the host did not start the initialising application, then it is necessary to reset the contents of  205  to the “no library available” special value.  
         [0054]    To implement these rules, the target digital signal processor supports a mechanism whereby the host can set a single bit of state in a register of the target, which is initialised upon an input to reset logic  123 , i.e., a processor reset. This bit is hereinafter referred to as the “host-connected bit” and typically resides in a processor register or a memory-mapped register.  
         [0055]    As noted above, two system start-up situations exist:  
         [0056]    1. Ordinary, where the target executes a reset sequence, such as power up, and starts executing code at an address fixed, for example by contents of ROM within the embedded digital signal processor.  
         [0057]    2. Via the host, in which case the host controls the reset sequence and may also control the address of the first code to be executed.  
         [0058]    If the digital signal processor is reset using the ordinary sequence, the host-connected bit is initialised by the digital signal processor reset-logic. The start up code of the first application to be executed reads the host-connected bit, and because it is initialised determines that the host is regarded as not connected. The code then sets the contents of memory location  205  to the no-library value.  
         [0059]    If however the host starts the target, then:  
         [0060]    1. The host resets the target and holds it in a suspended state.  
         [0061]    2. The host downloads the exception handler stack and adjusts the vector to point to this stack.  
         [0062]    3. The host downloads the input/output library to the target and sets the entry point pointer  205  in the stack to contain the entry-point to this library.  
         [0063]    4. The host inverts the host-connected bit.  
         [0064]    5. The host releases the target from its suspended state.  
         [0065]    Note that the order may differ from the above. For example, the library may be dynamically loaded, whereby the host determines its entry point, and then the stack—containing the entry point pointer set to the entry point—may be dynamically loaded.  
         [0066]    In any event, the start up code of the first application then executes, reads the host-connected bit (which has been inverted) and decides that a host is connected. As a result, it is assumed that the host has loaded a library, and correctly set the entry point pointer. Thus the entry point pointer  205  is left unchanged.  
         [0067]    The dynamically loaded subroutine is called with two machine-word sized parameters. It returns a machine-word sized value. The first parameter is a value that represents an operation to perform, the second a parameter for that operation. Certain operations do not use the second parameter. Similarly, it is not necessary for an operation to utilise the return value. The application will have been provided at the time of writing, with the values representing the operations anticipated as necessary.  
         [0068]    These values are then used in calling the relevant subroutine from the library.  
         [0069]    Alternatively, the library may include a routine translating the values from the application into different values corresponding to those needed by the library.  
         [0070]    One such operation will determine whether the host is connected. This operation will be used by the application by calling the input/output library subroutine with the first parameter containing a reserved value, which represents ‘host connected operation’. The second parameter is not used by this operation so an arbitrary value may be passed. The subroutine will determine whether the host is connected with an internal action and return one of two possible values, which are ‘host-connected’ and ‘host not connected’.  
         [0071]    Similar operations are used to send and receive data from the host, and to specify callback functions in the application to be used by the input/output subroutine in special circumstances.  
         [0072]    The two-parameter model provides very good backward compatibility to legacy applications, which cannot be changed. For instance, a later dynamic library may provide additional features. Each additional feature may be accessed by using a value of the first parameter that was not available in earlier libraries. Features that were present in older libraries are provided in the newer libraries and each of these will be accessed with the same value for the first parameter in both old and new libraries. The second parameter remains a ‘feature-specific’ parameter.  
         [0073]    Having provided a mechanism for loading a dynamic library onto a remote system, the same approach is useable to satisfy a wide range of run-time requirements, including but not limited to input/output. An example of such a requirement is the need to provide a work round to a hardware bug that becomes manifest during a debug session. Such bugs may only be present on a particular version or revision of the silicon. Once the problem is discovered, a routine may be written that contains a feature that may be called by the application when appropriate to prevent the bug from occurring, or to mask the bug. This routine may be incorporated within a library on the host, and loaded dynamically into the target at the start of a debug session. Components that do not possess the bug will not require the routine and may be provided with a version of the library that does not have the bug work-round feature. Such a version provides a subset of library functionality such as input/output capability.  
         [0074]    In an alternative embodiment shown in FIG. 4, a processor register  1190 , which is large enough to contain a pointer, is reserved for use by the input/output system. The digital signal processor vector to an exception handler stack and the pointer at a reserved position of the exception handler stack are not necessary. The register is initialised by the application start-up code to contain the “no library available” value if the host is not connected. If the host is connected, the host places a value in this register pointing to the entry point  1204  of IO dynamic library  1300 , which the host has loaded onto the target.  
         [0075]    When the application wishes to perform input/output, it looks at the value in the reserved register  1190 . If this value is not the special ‘no library available’ value, it is assumed to be the address of the entry point  1204  to the input/output subroutine. The application calls the subroutine at this address to carry out input/output.