Abstract:
A method, apparatus, system, and signal-bearing medium that, in an embodiment, receive a request for an address watch breakpoint to a variable, determine that the variable is referenced via a basing pointer expression, and monitor for initialization of the basing pointer prior to monitor for a change to the contents of the variable that is pointed to by the basing pointer expression. In an embodiment, the basing pointer expression is implemented as an instruction that uses an indirect addressing mode. In an embodiment, the basing pointer is determined to be uninitialized, and thus needs monitoring for initialization, by detecting an error that results from adding an offset for the variable to the contents of the basing pointer, or by detecting an error when the result of the adding is used as a pointer. In an embodiment, the basing pointer expression comprises a plurality of basing pointers, each of which is monitored for initialization in turn. In this way, the user is freed from the difficulty involved in attempting to determine where and if the basing pointer is set prior to monitoring for a modification to a variable that is pointed to by the basing pointer.

Description:
FIELD 
     An embodiment of the invention generally relates to computers. In particular, an embodiment of the invention generally relates to address watch breakpoints that use basing pointers or pointer expressions. 
     BACKGROUND 
     The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely sophisticated devices, and computer systems may be found in many different settings. Computer systems typically include a combination of hardware, such as semiconductors and circuit boards, and software, also known as computer programs. As advances in semiconductor processing and computer architecture push the performance of the computer hardware higher, more sophisticated and complex computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago. 
     As the sophistication and complexity of computer software increase, the more difficult the software is to debug. Bugs are problems, faults, or errors in a computer program. Locating, analyzing, and correcting suspected faults in a computer program is a process known as “debugging.” Typically, a programmer uses another computer program commonly known as a “debugger” to debug the program under development. 
     Conventional debuggers typically support two primary operations to assist a computer programmer. A first operation supported by conventional debuggers is a “step” function, which permits a computer programmer to process instructions (also known as “statements”) in a computer program one-by-one and see the results upon completion of each instruction. While the step operation provides a programmer with a large amount of information about a program during its execution, stepping through hundreds or thousands of program instructions can be extremely tedious and time consuming and may require the programmer to step through many program instructions that are known to be error-free before a set of instructions to be analyzed is finally executed. 
     To address this difficulty, a second operation supported by conventional debuggers is a breakpoint operation, which permits a computer programmer to identify with a breakpoint a precise instruction for which it is desired to halt execution of a computer program during execution. As a result, when a computer program is executed by a debugger, the program executes in a normal fashion until the breakpoint is reached. The debugger then stops execution of the program and displays the results of the program and/or the state of the computer system to the programmer for analysis, typically via a debugger user interface. 
     Typically, step operations and breakpoints are used together to simplify the debugging process. Specifically, a common debugging operation is to set a breakpoint at the beginning of a desired set of instructions to be analyzed and then begin executing the program. Once the breakpoint is reached, the debugger halts the program, and the programmer then steps through the desired set of instructions line-by-line using the step operation. Consequently, a programmer is able to more quickly isolate and analyze a particular set of instructions without needing to step through irrelevant portions of a computer program. 
     But, sometimes the user does not know the address of the instruction to be analyzed. Instead, the user knows the name of a variable or an address of a storage location used by the program that the program may eventually modify, and the user would like the debugger to stop the program when the program modifies the contents at that storage location or in that variable. To address this need, some debuggers have a special breakpoint function, called an address watch breakpoint (AWB). When the program being debugged modifies the contents of the storage location specified by the address watch breakpoint, the debugger stops the program at the instruction that is modifying the storage. Thus, an address watch breakpoint instruction is a load or store instruction accessing a location within a memory page being monitored as an address watch breakpoint. 
     One of the problems with address watch breakpoints is when the user attempts to set an address watch breakpoint to a variable referenced by a basing pointer or pointer expression, which is not yet set (the basing pointer does not yet contain a valid address). Current debuggers typically use hardware watch capabilities to implement the address watch breakpoint, which require the debuggers to resolve the pointer expression to be watched to obtain a single address to watch. If the basing pointer is not yet set, the expression cannot be resolved, and the single address cannot be obtained. 
     In an attempt to work around this problem, users sometimes try to run the program being debugged until the basing pointer is set and then set the address watch breakpoint. Unfortunately, determining when a basing pointer has been set is no easy task. The user must make an initial guess where the basing pointer might be set, run the program for a time, perhaps stepping or setting breakpoints near where the pointer might be set, and then manually check to see if the pointer is set. If the pointer is not yet set, then the user must try again until the pointer is set. Further, if the user has allowed the program to run too far, the user must start the process over again with some different breakpoints and/or number of steps. Even worse, finally finding a location in the program where the basing pointer is set does not necessarily end the difficulty because timing issues can cause further complications. Timing problems caused by the delay in setup (hitting a breakpoint, then setting the watch) may cause the address watch breakpoint to not fire in the same way compared to the normal operating environment where the program runs without the breakpoint. Timing issues are particularly troublesome when the program has multiple threads or interacts with other programs, tasks, or jobs running concurrently. Thus, the timing issues may cause the error or bug under investigation to be difficult to reproduce when address watch breakpoints are used with basing pointers. 
     Hence, without a better way to handle address watch breakpoints directed to variables that are referenced via basing pointers, developers will continue to experience delay, frustration, and difficulty when debugging programs. 
     SUMMARY 
     A method, apparatus, system, and signal-bearing medium are provided that, in an embodiment, receive a request for an address watch breakpoint to a variable, determine that the variable is referenced via a basing pointer expression, and monitor for initialization of the basing pointer prior to monitor for a change to the contents of the variable that is pointed to by the basing pointer expression. In an embodiment, the basing pointer expression is implemented as an instruction that uses an indirect addressing mode. In an embodiment, the basing pointer is determined to be uninitialized, and thus needs monitoring for initialization, by detecting an error that results from adding an offset for the variable to the contents of the basing pointer, or by detecting an error when the result of the adding is used as a pointer. In an embodiment, the basing pointer expression comprises a plurality of basing pointers, each of which is monitored for initialization in turn. In this way, the user is freed from the difficulty involved in attempting to determine where and if the basing pointer is set prior to monitoring for a modification to a variable that is pointed to by the basing pointer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  depicts a block diagram of an example system for implementing an embodiment of the invention. 
         FIG. 2  depicts a block diagram of selected components of the example system, according to an embodiment of the invention. 
         FIG. 3A  depicts a block diagram of example debug commands, according to an embodiment of the invention. 
         FIG. 3B  depicts a block diagram of an example dcode program resulting from a debug command, according to an embodiment of the invention. 
         FIG. 4A  depicts a block diagram of an example page in memory with a basing pointer that is not yet set, according to an embodiment of the invention. 
         FIG. 4B  depicts a block diagram of an example page in memory with a basing pointer that has been set, according to an embodiment of the invention. 
         FIG. 5  depicts a block diagram of an example data structure for pending watch records, according to an embodiment of the invention. 
         FIG. 6  depicts a flowchart of example processing for handling events, according to an embodiment of the invention. 
         FIG. 7  depicts a flowchart of interpreting debug commands, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the Drawing, wherein like numbers denote like parts throughout the several views,  FIG. 1  depicts a high-level block diagram representation of a computer system  100  connected to a network  130 , according to an embodiment of the present invention. The major components of the computer system  100  include one or more processors  101 , a main memory  102 , a terminal interface  111 , a storage interface  112 , an I/O (Input/Output) device interface  113 , and communications/network interfaces  114 , all of which are coupled for inter-component communication via a memory bus  103 , an I/O bus  104 , and an I/O bus interface unit  105 . 
     The computer system  100  contains one or more general-purpose programmable central processing units (CPUs)  101 A,  101 B,  101 C, and  101 D, herein generically referred to as a processor  101 . In an embodiment, the computer system  100  contains multiple processors typical of a relatively large system; however, in another embodiment the computer system  100  may alternatively be a single CPU system. Each processor  101  executes instructions stored in the main memory  102  and may include one or more levels of on-board cache. 
     The main memory  102  is a random-access semiconductor memory for storing data and programs. The main memory  102  is conceptually a single monolithic entity, but in other embodiments the main memory  102  is a more complex arrangement, such as a hierarchy of caches and other memory devices. For example, memory may exist in multiple levels of caches, and these caches may be further divided by function, so that one cache holds instructions while another holds non-instruction data, which is used by the processor or processors. Memory may further be distributed and associated with different CPUs or sets of CPUs, as is known in any of various so-called non-uniform memory access (NUMA) computer architectures. 
     The memory  102  includes a debug user interface  154 , pending watch records  156 , a parser  158 , an interpreter  160 , a watch trap handler  162 , a breakpoint trap handler  164 , a debug hook  166 , a program  168 , and dcode  170 . Although the debug user interface  154 , the pending watch records  156 , the parser  158 , the interpreter  160 , the watch trap handler  162 , the breakpoint trap handler  164 , the debug hook  166 , the program  168 , and the dcode  170  are illustrated as being contained within the memory  102  in the computer system  100 , in other embodiments some or all of them may be on different computer systems and may be accessed remotely, e.g., via the network  130 . The computer system  100  may use virtual addressing mechanisms that allow the programs of the computer system  100  to behave as if they only have access to a large, single storage entity instead of access to multiple, smaller storage entities. Thus, while the debug user interface  154 , the pending watch records  156 , the parser  158 , the interpreter  160 , the watch trap handler  162 , the breakpoint trap handler  164 , the debug hook  166 , the program  168 , and the dcode  170  are illustrated as being contained within the main memory  102 , these elements are not necessarily all completely contained in the same storage device at the same time. 
     In an embodiment, the debug user interface  154  and the interpreter  160  include instructions capable of executing on the processor  101  or statements capable of being interpreted by instructions executing on the processor  101  to perform the functions as further described below with reference to  FIGS. 6 and 7 . In another embodiment, the debug user interface  154  and the interpreter  160  may be implemented in microcode. In another embodiment, the debug user interface  154  and the interpreter  160  may be implemented in hardware via logic gates and/or other appropriate hardware techniques. 
     The program  168  is a program being debugged. The parser  158  evaluates debug commands from the debug user interface  154  to create the dcode  170 , which the interpreter  160  interprets for execution on the processor  101 . The dcode  170 , when executed by the interpreter  160  on the processor  101 , modifies an address that is being watched by an address watch breakpoint, which the user requests via the debug user interface  154 . The interaction of the debug user interface  154 , the parser  158 , the interpreter  160 , the watch trap handler  162 , the breakpoint trap handler  164 , and the debug hook  166  are further described below with reference to  FIG. 2 . An example of the dcode  170  is further described below with reference to  FIG. 3B . The address watch breakpoint is further described below with reference to  FIGS. 4A and 4B . 
     The memory bus  103  provides a data communication path for transferring data among the processor  101 , the main memory  102 , and the I/O bus interface unit  105 . The I/O bus interface unit  105  is further coupled to the system I/O bus  104  for transferring data to and from the various I/O units. The I/O bus interface unit  105  communicates with multiple I/O interface units  111 ,  112 ,  113 , and  114 , which are also known as I/O processors (IOPs) or I/O adapters (IOAs), through the system I/O bus  104 . The system I/O bus  104  may be, e.g., an industry standard PCI bus, or any other appropriate bus technology. 
     The I/O interface units support communication with a variety of storage and I/O devices. For example, the terminal interface unit  111  supports the attachment of one or more user terminals  121 ,  122 ,  123 , and  124 . The storage interface unit  112  supports the attachment of one or more direct access storage devices (DASD)  125 ,  126 , and  127  (which are typically rotating magnetic disk drive storage devices, although they could alternatively be other devices, including arrays of disk drives configured to appear as a single large storage device to a host). The contents of the main memory  102  may be stored to and retrieved from the direct access storage devices  125 ,  126 , and  127 . 
     The I/O and other device interface  113  provides an interface to any of various other input/output devices or devices of other types. Two such devices, the printer  128  and the fax machine  129 , are shown in the exemplary embodiment of  FIG. 1 , but in other embodiment many other such devices may exist, which may be of differing types. The network interface  114  provides one or more communications paths from the computer system  100  to other digital devices and computer systems; such paths may include, e.g., one or more networks  130 . 
     Although the memory bus  103  is shown in  FIG. 1  as a relatively simple, single bus structure providing a direct communication path among the processors  101 , the main memory  102 , and the I/O bus interface  105 , in fact the memory bus  103  may comprise multiple different buses or communication paths, which may be arranged in any of various forms, such as point-to-point links in hierarchical, star or web configurations, multiple hierarchical buses, parallel and redundant paths, etc. Furthermore, while the I/O bus interface  105  and the I/O bus  104  are shown as single respective units, the computer system  100  may in fact contain multiple I/O bus interface units  105  and/or multiple I/O buses  104 . While multiple I/O interface units are shown, which separate the system I/O bus  104  from various communications paths running to the various I/O devices, in other embodiments some or all of the I/O devices are connected directly to one or more system I/O buses. 
     The computer system  100  depicted in  FIG. 1  has multiple attached terminals  121 ,  122 ,  123 , and  124 , such as might be typical of a multi-user “mainframe” computer system. Typically, in such a case the actual number of attached devices is greater than those shown in  FIG. 1 , although the present invention is not limited to systems of any particular size. The computer system  100  may alternatively be a single-user system, typically containing only a single user display and keyboard input, or might be a server or similar device which has little or no direct user interface, but receives requests from other computer systems (clients). In other embodiments, the computer system  100  may be implemented as a personal computer, portable computer, laptop or notebook computer, PDA (Personal Digital Assistant), tablet computer, pocket computer, telephone, pager, automobile, teleconferencing system, appliance, or any other appropriate type of electronic device. 
     The network  130  may be any suitable network or combination of networks and may support any appropriate protocol suitable for communication of data and/or code to/from the computer system  100 . In various embodiments, the network  130  may represent a storage device or a combination of storage devices, either connected directly or indirectly to the computer system  100 . In an embodiment, the network  130  may support Infiniband. In another embodiment, the network  130  may support wireless communications. In another embodiment, the network  130  may support hard-wired communications, such as a telephone line or cable. In another embodiment, the network  130  may support the Ethernet IEEE (Institute of Electrical and Electronics Engineers) 802.3x specification. In another embodiment, the network  130  may be the Internet and may support IP (Internet Protocol). In another embodiment, the network  130  may be a local area network (LAN) or a wide area network (WAN). In another embodiment, the network  130  may be a hotspot service provider network. In another embodiment, the network  130  may be an intranet. In another embodiment, the network  130  may be a GPRS (General Packet Radio Service) network. In another embodiment, the network  130  may be a FRS (Family Radio Service) network. In another embodiment, the network  130  may be any appropriate cellular data network or cell-based radio network technology. In another embodiment, the network  130  may be an IEEE 802.11B wireless network. In still another embodiment, the network  130  may be any suitable network or combination of networks. Although one network  130  is shown, in other embodiments any number (including zero) of networks (of the same or different types) may be present. 
     It should be understood that  FIG. 1  is intended to depict the representative major components of the computer system  100  at a high level, that individual components may have greater complexity that represented in  FIG. 1 , that components other than or in addition to those shown in  FIG. 1  may be present, and that the number, type, and configuration of such components may vary. Several particular examples of such additional complexity or additional variations are disclosed herein; it being understood that these are by way of example only and are not necessarily the only such variations. 
     The various software components illustrated in  FIG. 1  and implementing various embodiments of the invention may be implemented in a number of manners, including using various computer software applications, routines, components, programs, objects, modules, data structures, etc., referred to hereinafter as “computer programs,” or simply “programs.” The computer programs typically comprise one or more instructions that are resident at various times in various memory and storage devices in the computer system  100 , and that, when read and executed by one or more processors  101  in the computer system  100 , cause the computer system  100  to perform the steps necessary to execute steps or elements comprising the various aspects of an embodiment of the invention. 
     Moreover, while embodiments of the invention have and hereinafter will be described in the context of fully functioning computer systems, the various embodiments of the invention are capable of being distributed as a program product in a variety of forms, and the invention applies equally regardless of the particular type of signal-bearing medium used to actually carry out the distribution. The programs defining the functions of this embodiment may be delivered to the computer system  100  via a variety of signal-bearing media, which include, but are not limited to: 
     (1) information permanently stored on a non-rewriteable storage medium, e.g., a read-only memory device attached to or within a computer system, such as a CD-ROM readable by a CD-ROM drive; 
     (2) alterable information stored on a rewriteable storage medium, e.g., a hard disk drive (e.g., DASD  125 ,  126 , or  127 ) or diskette; or 
     (3) information conveyed to the computer system  100  by a communications medium, such as through a computer or a telephone network, e.g., the network  130 , including wireless communications. 
     Such signal-bearing media, when carrying machine-readable instructions that direct the functions of the present invention, represent embodiments of the present invention. 
     In addition, various programs described hereinafter may be identified based upon the application for which they are implemented in a specific embodiment of the invention. But, any particular program nomenclature that follows is used merely for convenience, and thus embodiments of the invention should not be limited to use solely in any specific application identified and/or implied by such nomenclature. 
     The exemplary environments illustrated in  FIG. 1  are not intended to limit the present invention. Indeed, other alternative hardware and/or software environments may be used without departing from the scope of the invention. 
       FIG. 2  depicts a block diagram of selected components of the example system, according to an embodiment of the invention. Illustrated are the debug user interface  154 , the parser  158 , the interpreter  160 , the watch trap handler  162 , the breakpoint trap handler  164 , and the debug hook  166 . 
     The debug user interface  154  provides address watch break-points to be established by interacting with a user. In some embodiments, the user may define these break-points by issuing a debugger command that refers to high-level language (HLL) references in the program  168  ( FIG. 1 ), such as line or statement numbers or software object references such as a program or module name, from which the physical storage address may be cross referenced. In various embodiments, the debugger command may be issued on a command line or through a graphical user interface. 
     The parser  158  parses this debugger command using a table that was produced by a compiler or interpreter stored with the computer program  170  to map the line number in the debugger command to the actual physical storage address in the memory  102 . The dcode interpreter  160  stores this information in an unillustrated breakpoint table. The dcode interpreter  160  further runs a dcode program to interpret instructions in the dcode  170  and to set the address watch break-points in the program  168 . 
     After the breakpoints are set, the user provides an input to the debug user interface  154  that resumes execution of the program  168 . Execution of the program  168  eventually results in an encounter of a break-point. In the illustrative embodiment, this is accomplished by an add instruction that fails, causing a system exception. The watch trap handler  162  or the breakpoint trap handler  164  passes information regarding the exception or interrupt to the debug hook  166 . 
     Then, the debug user interface  154  utilizes the debug hook  166  in order to obtain debugger commands, especially when a break-point has halted program execution. The additional step of the debug hook  166  is illustrated for instances where an interface is required between the debug user interface  154  and the watch trap handler  162  and/or the breakpoint trap handler  164 . In an embodiment, the debug hook  166  may utilize an unillustrated result buffer to cache data for the debug user interface  154 . 
       FIG. 3A  depicts a block diagram of example debug commands  200 , according to an embodiment of the invention. The debug commands  200  are entered via the debug user interface  154 . The example debug commands include commands  305  and  310 , which are both pointer expressions. The statement  305  is a single pointer reference and indicates that the variable “X” is to be found at an address contained in the basing pointer “PTR.” The statement  310  is a double pointer reference and indicates that the variable X is to be found at an address contained in the basing pointer “PTR 1 ,” and the basing pointer PTR 1  is to be found at an address contained in the basing pointer “PTR.” Pointer references may have any number of basing pointers. If the user desires to set an address watch breakpoint on the contents of the variable X, the pointer PTR must be initialized before the interpreter  160  can find the address of the variable X for statement  305 . Further, the pointers PTR and PTR 1  must be initialized before the interpreter  160  can find the address of the variable X for the statement  310 . Those skilled in the art will appreciate that in some languages, the basing pointer need not be specified to access the variable (Eval X). In such languages, locating the correct basing pointer can be a challenge. 
       FIG. 3B  depicts a block diagram of an example dcode program  170  resulting from a debug command, according to an embodiment of the invention. The example dcode program  170  includes instructions  315 ,  320 ,  325 , and  330 , which the parser  158  created from the expression  305  and which the interpreter  160  interprets. The instruction  315  is a load instruction using direct addressing that loads an address of the pointer PTR. The instruction  320  is a load instruction with a pointer type that uses indirect addressing to load the contents of the pointer PTR. The instruction  325  is a load constant instruction that loads the immediate value of POFFSET. The instruction  330  is an add instruction that adds the value of the pointer PTR and the constant POFFSET to obtain an address of the variable X. If the pointer PTR was not set to a valid memory address at the time the instruction  320  is executed, the add instruction  330  will fail. In another embodiment, instead of the add instruction  330  failing, a later instruction that attempts to dereference the pointer PTR fails since the pointer PTR does not contain a valid address. 
       FIG. 4A  depicts a block diagram of an example page in the memory  102 - 1  (an instance of the memory  102  from  FIG. 1 ) with a basing pointer PTR  405 - 1  that is not yet set, according to an embodiment of the invention. The basing pointer PTR  405 - 1  (corresponding to the statement  305  and the instruction  320 ) is illustrated at memory address “C022AF34,” and the contents of this memory address are “0,” which indicates that the pointer PTR  405 - 1  is not yet set or initialized. In another embodiment, any appropriate contents may be used to indicate that a pointer is not set or initialized. The variable X is not shown in the memory  102 - 1  in  FIG. 4A  because the interpreter  160  does not yet know the address of the variable X since the pointer PTR  405 - 1  is not yet set. The addresses and data illustrated in  FIG. 4A  are examples only, and in other embodiments any appropriate addresses and data may be used. 
       FIG. 4B  depicts a block diagram of an example page in the memory  102 - 2  (an instance of the memory  102  from  FIG. 1 ) with a basing pointer PTR  405 - 2  that has been set, according to an embodiment of the invention. In this example, the basing pointer PTR  405 - 2  has contents of “C053AD2A,” which is the address of storage location  410  in the memory  102 - 2 , which contains the variables X  415  and Y  420 . Using the example of  FIG. 3B , the POFFSET  325  for the variable X  415  is zero, and the POFFSET  325  for the variable Y  420  is one since those are the values that need to be added to the contents of the PTR  410  (by the instruction  330 ), in order to access X  415  and Y  420 , respectively. 
       FIG. 5  depicts a block diagram of an example data structure for the pending watch records  156 , according to an embodiment of the invention. The example pending watch records  156  include records  505  and  510 , but in other embodiments any number of records with any appropriate data may be present. Each of the records  505  and  510  include an address being watched field  515 , an expression field  520 , and a pending field  525 , but in other embodiments, more or fewer fields may be present. The address being watched field  515  includes the address of the memory location (a pointer) that is currently being watched for modification. The expression field  520  includes the pointer expression that, such that after the memory contents at the address being watched  515  is modified, its result is to be watched next. 
     The pending field  525  indicates (true/false or yes/no) whether a watch is pending. If the pending field  525  indicates true or yes, then the address being watched  515  includes an address that is currently being watched for modification, and the expression field  520  contains a pointer expression whose result is to be watched next. If the pending field is false, then the memory contents at the address being watched  515  has already been modified, or was established correctly the first time the watch was set, so its watch is no longer pending. 
     In the example shown in  FIG. 5 , the pending field  525  for the record  505  contains “true,” which indicates that an address watch is currently pending at “C022AF34,” which is the address of the pointer PTR  405 - 1  ( FIG. 4A ), and the pointer PTR  405 - 1  has not yet been set to a valid address. Once the pointer PTR  405 - 2  ( FIG. 4B ) is set to a valid address (“C053AD2A” in the example of  FIG. 4B ), the interpreter  160  retrieves the pointer expression  520 , evaluates it to yield the valid address “C053AD2A” and initiates an address watch breakpoint at the memory location  410  ( FIG. 4B ), as further described below with reference to  FIGS. 6 and 7 . 
       FIG. 6  depicts a flowchart of example processing for handling events by the debug user interface  154 , according to an embodiment of the invention. Control begins at block  600 . Control then continues to block  605  where the debug user interface  154  receives an event. Control then continues to block  610  where the debug user interface  154  determines whether the event previously received at block  605  is a watch fired event. 
     If the determination at block  610  is false, then the received event is not a watch fired event, so control continues to block  615  where the debug user interface  154  processes other events, such as an event requesting a breakpoint at an instruction in the program  168  or an address watch breakpoint for a variable. For an address watch breakpoint request, the parser  158  parses the watch expression in the request to create the dcode program  170 , which is then sent to the dcode interpreter  160 . The parser  158  saves an indication that the interpreter  160  can check later at block  710  ( FIG. 7 ). Control then returns to block  605 , as previously described above. 
     If the determination at block  610  is true, then the received event is a watch fired event (address watches are initiated at blocks  635  of  FIG. 6  and block  745  of  FIG. 7 , as further described below), so control continues to block  620  where the debug user interface  154  retrieves a watch record associated with the address that causes the watch breakpoint firing from the pending watch records  156  ( FIG. 5 ). Control then continues to block  625  where the debug user interface  154  determines whether a watch is pending for this address by checking the pending field  525  ( FIG. 5 ) in the retrieved record. 
     If the determination at block  625  is true, then a watch is pending for this address (the pending field  525  indicates true), so control continues to block  635  where the debug user interface  154  uses the expression  520  in the retrieved record from the pending watch records  156  to re-establish a watch. Thus, after the basing pointer has been initialized (described below with reference to  FIG. 7 ), then the debug user interface  154  initiates a monitor or watch for a change to a variable via the basing pointer expression that was previously saved (described below with reference to  FIG. 7 ). In another embodiment, the debug user interface  154  initiates a monitor or watch for a change to another basing pointer, which handles the scenario where a pointer expression contains multiple basing pointers (e.g., the pointer expression  310  of  FIG. 3A ), so each of the basing pointers is monitored or watched in turn. The debug user interface  154  further sets the pending field  525  to false. Control then returns to block  605 , as previously described above. 
     It the determination at block  625  is false, then a watch is not pending for this address (the pending field  525  indicates false), so control continues to block  630  where the debug user interface  154  signals that the watch has fired on the user interface. Control then returns to block  605 , as previously described above. 
       FIG. 7  depicts a flowchart of interpreting commands in the debug commands  200  by the interpreter  160 , according to an embodiment of the invention. The logic of  FIG. 7  is invoked once for each command (e.g., the command  305  or  310  illustrated in  FIG. 3A ) in the debug commands  200 . Control begins at block  700 . Control then continues to block  705  where the interpreter  160  enters a loop executed once for each instruction in the dcode  170  created for the command. For example, the loop is executed once each for the instructions  315 ,  320 ,  325 , and  330 , which were created from the command  305 , as previously described above with reference to  FIGS. 3A and 3B . 
     So long as an instruction unprocessed by the loop remains, control continues from block  705  to block  710  where the interpreter  160  determines whether an address watch breakpoint is being set, as previously described above with reference to block  615  in  FIG. 6 . If the determination at block  710  is true, then an address watch breakpoint has been set, so control continues to block  715  where the interpreter  160  determines whether the current instruction is an instruction that uses indirect addressing to fetch an address. An example of an indirect instruction is the LDI (load indirect) instruction  320  in  FIG. 3B , which in an embodiment comes from a variable being referenced via a basing pointer expression, but in other embodiments any appropriate instruction that uses indirect addressing may be used. 
     If the determination at block  715  is true, then the current instruction is an indirect instruction, so control continues to block  720  where the interpreter  160  saves the address to be dereferenced by the current instruction and also saves the basing pointer expression of the current instruction. For example, if the current instruction is the LDI (load indirect) PTR  320  ( FIG. 3B ), then the address to be dereferenced is the memory address of the pointer “PTR,” which is illustrated in  FIGS. 4A and 4B  as “C022AF34.” Thus, in this example at block  720 , the interpreter  160  saves the address “C022AF34” and also saves the pointer expression “PTR-&gt;A.” Control then continues to block  725  where the interpreter  160  processes the current instruction, including directing the processor  101  to execute the current instruction. 
     Control then continues to block  730  where the interpreter  160  determines whether execution of the current instruction resulted in an error. Thus, the interpreter  160  determines whether an error results from use of a basing pointer that has not been set. 
     If the determination at block  730  is true, then the current instruction when executed on the processor  101  resulted in an error, so control continues to block  735 . 
     The add instruction  330  ( FIG. 3B ) results in an error if the pointer PTR  405 - 1  ( FIG. 4A ) does not contain a valid memory address since the pointer PTR  405 - 1  has not yet been set. In response to the error, at block  735 , the interpreter  160  creates a record (e.g., the record  505  or  510  illustrated in  FIG. 5 ) in the pending watch records  156 , sets the address previously saved at block  720  into the address field  515 , and sets the pending field  525  in the record to true. Control then continues to block  740  where the interpreter  160  sets the expression field  520  in the record to be the pointer expression previously saved at block  720 . Control then continues to block  745  where the interpreter  160  sets a hardware watch on the saved address (the address of the pointer), which in this example is the address of PTR (C022AF34 in this example). Thus, the interpreter  160  monitors for an initialization of the basing pointer. By setting the hardware watch, the interpreter  160  directs the processor  101  to raise an interrupt when the contents at the address (C022AF34 in this example) are modified, which in this example is the PTR  405 - 2 . Control then continues to block  799  where the logic of  FIG. 7  returns. 
     If the determination at block  730  is false, then the current instruction executed successfully, so control returns to block  705  where the current instruction is set to be the next instruction in the dcode  170 , and execution of the loop continues, as previously described above. 
     If the determination at block  715  is false, then the current instruction is not an indirect instruction, so control continues to block  725 , as previously described above. 
     If the determination at block  710  is false, then an address watch breakpoint has not been set, so control continues to block  725 , as previously described above. 
     After every instruction has been processed by the loop that begins at block  705 , control continues from block  705  to block  799 , where the logic of  FIG. 7  returns. 
     In the previous detailed description of exemplary embodiments of the invention, reference was made to the accompanying drawings (where like numbers represent like elements), which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments were described in sufficient detail to enable those skilled in the art to practice the invention, but other embodiments may be utilized and logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention. Different instances of the word “embodiment” as used within this specification do not necessarily refer to the same embodiment, but they may. The previous detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     In the previous description, numerous specific details were set forth to provide a thorough understanding of the invention. But, the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown in detail in order not to obscure the invention.