Abstract:
An apparatus for counting microcode instruction execution in a microprocessor includes a first register, a second register, a comparator, and a counter. The first register stores an address of a microcode instruction. The microcode instruction is stored in a microcode memory of the microprocessor. The second register stores an address of the next microcode instruction to be retired by a retire unit of the microprocessor. The comparator compares the addresses stored in the first and second registers to indicate a match between them. The counter counts the number of times the comparator indicates a match between the addresses stored in the first register and the second register. The first register is user-programmable and the counter is user-readable. A mask register may be included to create a range of microcode memory addresses so that executions of microcode instructions within the range are counted.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to microprocessors, and more particularly to counting microcode instruction executions within a microprocessor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many modern microprocessors include microcode instruction sequences, or microcode, that implements complex and/or infrequently executed instructions of the microprocessor instruction set. A microcode memory within the microprocessor includes multiple microcode instruction sequences. When the microprocessor decodes one of the microcode-implemented instructions of the instruction set, rather than sending the instruction directly to the execution units of the microprocessor to be executed, the microprocessor transfers control to the appropriate microcode routine in the microcode ROM. The microprocessor then sends the microcode instructions to the execution units that execute the instructions to implement the complex and/or infrequently executed instruction. This allows the execution units (and other units of the microprocessor, such as a dependency checking unit or retire unit) to be less complex than they would be if they had to be capable of executing all the instructions of the microprocessor instruction set, including even the complex and/or infrequently executed instructions. 
         [0003]    Like other programs, microcode must be debugged. Furthermore, like other programs, it is desirable to optimize the performance of microcode, particularly since good performing microcode will likely improve the overall performance of programs that include microcode-implemented instructions of the microprocessor instruction set. However, because the microcode is within the microprocessor itself, unlike the fetching of user program instructions, typically the fetching of microcode instructions is not directly visible on the external pins of the microprocessor. This makes debugging and performance measurement of microcode more difficult than user programs. Furthermore, although microprocessors commonly provide debugging and performance measurement facilities for user programs (see, for example, Chapter 18 of the IA-32 Intel Architecture Software Developer&#39;s Manual, Volume 3B: System Programming Guide, Part 2, June 2006), they do not provide these facilities for microcode. 
         [0004]    Therefore, what is needed is an aid in debugging and measuring performance of microcode. 
       BRIEF SUMMARY OF INVENTION 
       [0005]    The present invention provides an apparatus for counting microcode instruction execution in a microprocessor. The apparatus includes a first register, configured to store an address of a microcode instruction. The microcode instruction is stored in a microcode memory of the microprocessor. The apparatus includes a second register, configured to store an address of the next microcode instruction to be retired by a retire unit of the microprocessor. The apparatus includes a comparator, coupled to the first and second registers, configured to indicate a match between the addresses stored in the first and second registers. The apparatus includes a counter, coupled to the comparator, configured to count the number of times the comparator indicates a match between the addresses stored in the first register and the second register. 
         [0006]    In one aspect, the present invention provides a method for counting microcode instruction execution in a microprocessor. The method includes storing to a first register an address of a microcode instruction stored in a microcode memory of the microprocessor. The method also includes storing to a second register an address of the next microcode instruction to be retired by a retire unit of the microprocessor. The method also includes comparing the addresses stored in the first register and the second register to determine whether a match occurs between the addresses stored in the first and second registers. The method also includes counting the number of times a match occurs between the addresses stored in the first register and the second register. 
         [0007]    In another aspect, the present invention provides a computer program product for use with a computing device. The computer program product includes a computer usable storage medium, having computer readable program code embodied in said medium, for specifying an apparatus for counting microcode instruction execution in a microprocessor. The computer readable program code includes first program code for specifying a first register, configured to store an address of a microcode instruction, wherein the microcode instruction is stored in microcode memory of the microprocessor. The computer readable program code includes second program code for specifying a second register, configured to store an address of the next microcode instruction to be retired by a retire unit of the microprocessor. The computer readable program code includes third program code for specifying a comparator, coupled to the first and second registers, configured to indicate a match between the addresses stored in the first and second registers. The computer readable program code includes fourth program code for specifying a counter, coupled to the comparator, configured to count the number of times the comparator indicates a match between the addresses stored in the first register and the second register. 
         [0008]    An advantage of the present invention is that it provides instrumentation for counting microcode execution in real time, without specialized external tools or probes into internal functions of a microprocessor. Therefore, microcode execution measurements can be made outside of a lab environment, such as in an end user installation for remote debug or performance measurement. 
         [0009]    Another advantage of the present invention is that it provides a way to measure microcode execution without impacting the actual execution of user programs executing on the microprocessor that include microcode-implemented instructions. The overhead required to commence measuring microcode execution and to subsequently obtain the measurements are a small number of writes/reads to/from control registers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram illustrating a microprocessor according to the present invention. 
           [0011]      FIG. 2  is a flowchart illustrating operation of the microprocessor  100  of  FIG. 1  according to the present invention. 
           [0012]      FIG. 3  is a block diagram illustrating a microprocessor according to an alternate embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring to  FIG. 1 , a block diagram illustrating a microprocessor  100  according to the present invention is shown. Microcode memory  104  stores microcode instructions  108  that are provided by the microcode memory  104  to execution units  112  in response to microprocessor  100  receiving user program instructions. Although not shown, microinstructions from the other sources are also provided to the execution units  112  for execution, such as from an instruction translator or instruction cache (not shown) of the microprocessor  100 . In one embodiment, the execution units  112  execute microinstructions in an out of order fashion. 
         [0014]    The microprocessor  100  also includes a reorder buffer  122  coupled to the execution units  112 . The microprocessor  100  allocates an entry  124 / 126  in the reorder buffer  122  for each microinstruction issued to the execution units  112 , such as microcode instructions  108 . Along with each microcode instruction  108 , the microprocessor  100  provides to the reorder buffer  122  the address of the microcode instruction  108  in the microcode memory  104  and an indication that the microcode instruction  108  was supplied by the microcode memory  104  rather than from another instruction source. After the execution units  112  execute microinstructions, they update the status  114  of the executed microinstructions within the reorder buffer  122 . This enables the reorder buffer  122  to insure that microinstructions are retired in program order. Specifically, each clock cycle, the reorder buffer  122  checks the status  114  of the oldest microinstruction therein to see whether it has completed execution and is therefore ready to be retired, shown in  FIG. 1  as the microinstruction in entry  126 . 
         [0015]    The reorder buffer  122  also contains a microcode instruction address register  128 . The microcode instruction address register  128  stores the address of a microcode instruction  108  in microcode memory  104  for which it is desired to measure the number of times the microcode instruction  108  is executed. The microcode instruction address register  128  is writeable by a user program. In one embodiment, when a program executes a write MSR (WRMSR) instruction, the execution units  112  write a microcode instruction address  118  specified by the WRMSR instruction to the microcode instruction address register  128 . 
         [0016]    A comparator  138  compares a compare address  136  provided from the microcode instruction address register  128  with a retire address  134  provided from the retired instruction entry  126  of the reorder buffer  122  to determine if the address of the microinstruction being retired matches the microcode memory address  136  programmed into the microcode instruction address register  128 . The comparator  138  produces a positive match  142  if the compare address  136  is the same as the retire address  134 , and produces a negative match  142  if the compare address  136  is not the same as the retire address  134 . An address match counter  144  increments its current count every time it receives a positive match  142 . In this way, the address match counter  144  stores a count equal to the number of times a microcode instruction  108  at a location in microcode memory  104  specified by the compare address  136  is retired. In one embodiment, the address match counter  144  is incremented if it receives a positive match  142  only if the above-mentioned indication indicates that the retired microinstruction  126  was sourced by the microcode memory  104 . In one embodiment, the reorder buffer  122  capable of retiring the oldest N microinstructions  126  in the reorder buffer  122 , where N is design dependent. In one embodiment, up to three microinstructions  126  are retired at the same time, thus generating N retire addresses  134 . In such an embodiment, the reorder buffer  122  includes N comparators  138 , each configured to compare a respective retire address  134  with the compare address  136 . If any of the comparators  138  generates a positive value, the counter  144  increments its count. 
         [0017]    The address match counter  144  provides its count  146  to the execution units  112 . In one embodiment, a user program executes a read MSR (RDMSR) instruction to read the matched addresses count  146  from the counter  144 . In one embodiment, the address match counter  144  is initialized to a count value of zero when the microcode instruction address  118  is programmed into the microcode instruction address register  128 . 
         [0018]    Referring now to  FIG. 2 , a flowchart illustrating operation of the microprocessor  100  of  FIG. 1  according to the present invention is shown. Flow begins at block  204 . 
         [0019]    At block  204 , a write MSR (WRMSR) instruction writes a microcode instruction address  118  to the microcode instruction address register  128 . The microcode instruction address  118  is the address of an instruction in microcode memory  104 . It is desired to count how many times the instruction at the microcode instruction address  118  is executed by the microprocessor  100 . The WRMSR instruction may be part of a user program. Flow proceeds to block  208 . 
         [0020]    At block  208 , in response to the write MSR (WRMSR) instruction writing a microcode instruction address  118  to the microcode instruction address register  128  in block  204 , the microprocessor  100  clears the address match counter  144 . Clearing the address match counter  144  initializes the count to a zero value. Flow proceeds to block  212 . 
         [0021]    At block  212 , a microsequencer of a microcode unit (not shown) of microprocessor  100  fetches microcode instructions  108  from the microcode memory  104  and sends the microcode instructions  108  to the execution units  112 . Flow proceeds to block  216 . 
         [0022]    At block  216 , the execution units  112  execute the microcode instructions  108  and subsequently update the status  114  of the executed microinstructions in their associated entries  124 / 126  of the reorder buffer  122 . Flow proceeds to block  218 . 
         [0023]    At block  218 , the reorder buffer  122  retires the oldest microinstruction  126  in reorder buffer  122 . In one embodiment, the reorder buffer  122  can simultaneously retire a plurality of microinstructions  126 , as discussed above. Flow proceeds to block  224 . 
         [0024]    At block  224 , the comparator  138  compares the retire address  134  of the retired microinstruction  126  with the compare address  136  in the microcode instruction address register  128  to generate the match signal  142  to indicate whether the address  134  of the retiring microinstruction  106  is the same as the compare address  136  in instruction address register  128 . Flow proceeds to decision block  228 . 
         [0025]    At decision block  228 , if the addresses compared at block  224  match, flow proceeds to block  232 ; otherwise, flow proceeds to block  212  where the process is repeated. 
         [0026]    At block  232 , the microprocessor  100  increments the address match counter  144 , in response to receiving a positive match  142  from the comparator  138 . Flow proceeds to block  212 , where the process is repeated. 
         [0027]    Referring now to  FIG. 3 , a block diagram illustrating a microprocessor  300  according to an alternate embodiment of the present invention is shown. The embodiment shown in  FIG. 3  is similar to the embodiment shown in  FIG. 1  and like-numbered elements are similar. Differences between the embodiment of  FIG. 3  and the embodiment of  FIG. 1  will now be described. 
         [0028]    In the embodiment of  FIG. 3 , the reorder buffer  122  contains an instruction mask register  308 . The instruction mask register  308  stores an address mask  312  that is used to mask off bits of the compare address  136  and the retire address  134  before being compared by the comparator  138 . The consequence is that a positive match  142  indicates that a microcode instruction  108  was retired whose microcode memory  104  address is within a range of addresses specified by the combination of the compare address  136  and the address mask  312 , rather than indicating that a microcode instruction  108  was retired whose microcode memory  104  address matches a particular address of the microcode memory  104  as with the embodiment of  FIG. 1 . 
         [0029]    The instruction mask register  308  is writeable by a user program. In one embodiment, when a program executes a WRMSR instruction, the execution units  112  write an instruction mask address  304  specified by the WRMSR instruction to the instruction mask register  308 . 
         [0030]    Although embodiments have been described in which the counter measures the actual execution of microcode instructions, other embodiments are contemplated in which the counter  144  measures the fetching of microcode instruction from the microcode memory  104 , which may be different from the actual execution thereof, such as due to speculative execution by the microprocessor  100 . Additionally, although embodiments are described that include a single microcode instruction address register  128 , comparator  138 , and address match counter  144 , other embodiments are contemplated in which the microprocessor  100  includes multiple of these elements to enable counting executions of more than one microcode instruction within the microcode memory  104 . 
         [0031]    While various embodiments of the present invention have been described herein, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant computer arts that various changes in form and detail can be made therein without departing from the scope of the invention. For example, software can enable, for example, the function, fabrication, modeling, simulation, description and/or testing of the apparatus and methods described herein. This can be accomplished through the use of general programming languages (e.g., C, C++), hardware description languages (HDL) including Verilog HDL, VHDL, and so on, or other available programs. Such software can be disposed in any known computer usable medium such as semiconductor, magnetic disk, or optical disc (e.g., CD-ROM, DVD-ROM, etc.). Embodiments of the apparatus and method described herein may be included in a semiconductor intellectual property core, such as a microprocessor core (e.g., embodied in HDL) and transformed to hardware in the production of integrated circuits. Additionally, the apparatus and methods described herein may be embodied as a combination of hardware and software. Thus, the present invention should not be limited by any of the herein-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Specifically, the present invention may be implemented within a microprocessor device which may be used in a general purpose computer. Finally, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the scope of the invention as defined by the appended claims.