Abstract:
The present invention provides memory structures for efficient tracking and recycling of physical register assignments. This approach reduces the size of the memory structures needed to track the usage of physical registers and the recycling of these registers.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to managing physical register usage and more particularly to a method and system for managing physical registers by reducing the sizes of the memory structures necessary for tracking and recycling the physical register assignments.  
         BACKGROUND OF THE INVENTION  
         [0002]    Most microprocessors contain registers that are used in executing instructions. When the microprocessors execute instructions, data values are read from the registers and results of operations are written into the registers. For example, the instruction “ADD R1, R2, and R3” adds the values held in resisters R1 and R2 and puts the resulting sum into register R3. R1, R2, and R3 are identifiers associated with respective registers.  
           [0003]    Traditional microprocessors sequentially execute instructions in a computer program (i.e., in the order that the instructions appear in the computer program). In the simplest case, an instruction is executed per a clock cycle. Many instructions, however, may take more than one clock cycle to execute.  
           [0004]    Superscalar processors, in contrast to traditional microprocessors, may issue and receive multiple instructions per clock cycle. With such processors, an instruction can be executed in a single clock cycle if there are no data dependencies, procedural dependencies, or resource conflicts that prevent execution of the instruction.  
           [0005]    High performance processors use out-of-order processing, that is the processors process the instructions non-sequentially. Such high performance processors typically do not have a fixed relationship between a register identified and the physical register referenced by the identifier. Thus, the identifier R1 may refer to a first physical register in one instruction and refer to a second physical register in another instruction. The identifiers reference logical resisters that are mapped to corresponding physical registers.  
           [0006]    One challenge with such processors is to manage and track the usage of register values. One conventional approach is to define an in-order state for each register. For example, if an instruction has been executed and all other previous instructions have also been executed, the values generated by the instructions are stored as the register&#39;s in-order state. The instruction is then considered “retired”.  
           [0007]    Another conventional approach to tracking and recycling physical register mappings is to create a single large structure to do all the work associated with tracking physical registers mappings. Such a structure would have to have one entry for each physical register. Each entry would have to contain information about that register&#39;s current status, i.e., whether it&#39;s free or in use. Maintaining the data in such a structure would be very complex because simultaneous reading and updating would have to be supported. Considering the size, it would be very prohibitive to make such a technique work effectively. Such a technique would require to much area, too much power, and would add too much complexity. This technique was conceivable in the past because the number of physical registers that were supported was much lower. With the advent of out of order processors, the number of supported physical registers has sky-rocketed.  
           [0008]    There are a variety of different solutions that utilize separate register files for holding register values. The transient values of the registers can be stored in the reader buffer or an instruction window, or a temporary register array. The major problem is that register values are transferred from transient register storage to a register array or a register file holding the in-order register values.  
           [0009]    The other aforementioned concepts are not efficient in handling the monitoring and tracking of physical registers. Therefore, there is a need to find a way to reduce the size of the memory structures needed for tracking and regulating the physical register assignments.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention addresses the above-described limitation by providing memory structures for efficient tracking and recycling of physical register assignments. This approach reduces the size of the memory structures needed to track the usage of physical registers and the recycling of these registers.  
           [0011]    In accordance with one aspect of the present invention, a method is provided in a microprocessor for managing a plurality of registers. In accordance with this method, a first structure is provided for holding information that identifies available physical registers that are free to be assigned to a plurality of destination operands for instructions executing in the microprocessor. The plurality of destination operands identifies where data resulting from an operation is to be stored. Also, the method comprises storing a physical register assignment in a second structure noting that a selected one of the physical registers is assigned to one of the plurality of destination operands for a selected instruction executing on the microprocessor. The method further comprises providing a third structure for holding information regarding available physical registers not utilized during execution of instructions.  
           [0012]    In accordance with another aspect of the present invention, a method is practiced in a microprocessor for managing a plurality of registers. In accordance with this method, a first structure is provided for holding information identifying available physical registers that are free to be assigned as a destination operand for instructions executing on the microprocessor. The destination operand identifies where data resulting from an operation is to be stored. A physical register assignment is stored in a second structure. The physical register assignment identifies that a selected one of the physical registers is assigned as a destination operand for a selected instruction executing on the microprocessor. A third structure is provided for holding information regarding available physical registers not utilized during execution of instructions. The physical register assignments of the selected physical registers are transferred from the second structure to the third structure after retirement of the selected instruction.  
           [0013]    In accordance with another aspect of the present invention, a microprocessor system having a plurality of physical registers for managing a plurality of physical register assignments is provided. The microprocessor includes a first module for providing a first structure for holding information identifying available physical registers that are free to be assigned as a destination operand for instructions executing on the microprocessor. The destination operand identifies where data resulting from an operation is to be stored. Also, the microprocessor includes a second module for storing a physical register assignment in a second structure noting that a selected one of the physical registers is assigned as a destination operand for a selected instruction executing on the microprocessor. The microprocessor includes a third module for providing a third structure for holding information regarding available physical registers not utilized during execution of instructions. The microprocessor also includes a first interface for transferring said physical register assignment of said selected physical register from said second structure to said third structure after retirement of said selected instruction, and a second interface for, when said selected physical register is assigned as a destination operand for a subsequent instruction, transferring information identifying said selected physical register as available to said first structure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    An illustrative embodiment of the present invention will be described below relative to the following drawings.  
         [0015]    [0015]FIG. 1 is a block diagram illustrating a microprocessor that is suitable for practicing the illustrative embodiment of the present invention.  
         [0016]    [0016]FIG. 2A illustrates a block diagram of the memory array elements used in the present invention.  
         [0017]    [0017]FIG. 2B illustrates another illustrative use of the APRM and RPRM structures in the present invention.  
         [0018]    [0018]FIG. 3 is a flowchart diagram illustrating the inventive memory structures tracking and recycling of physical registers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The illustrative embodiment of the present invention provides a microprocessor having a plurality of physical registers and data structures to track and recycle the physical registers. The data structures facilitate efficient management of the registers without hampering performance of the microprocessor. The data structures do not increase the memory requirements for tracking the various physical registers. As such, memory is conserved for other processing purposes.  
         [0020]    [0020]FIG. 1 illustrates a microprocessor suitable for practicing the illustrative embodiment. The microprocessor  2  includes register files  4 , execution units  6 , a memory array  8 , an instruction scheduler  10 , and retirement unit  12 . The microprocessor  2  is of the type designed to handle concurrent multiple instructions during a single processor execution cycle. The execution units  6  can execute instructions of different types. For example, the units  6  can access data from the memory unit  8 , e.g., load and store instructions of a program, perform arithmetic operations, and control execution flow.  
         [0021]    The register files  4  are used to store floating or fixed-point data. Also, the register files  4  contain information regarding the physical registers that are allocated for execution. Each physical register corresponds to an instance of a logical register. The retirement unit receives data from the execution units  6  regarding the execution of the instructions. The retirement unit  12  determines when it is safe to write results of each instruction into the register files  4 . The register files  4  store the final results of the instructions executed by the execution units  6 . However, the register files car directly receive or send from the execution units  6  executed data. The possibility of data being thrown out is high because exceptions or pipeline flushes may occur at any point in time. Thus, the retirement unit manages the exceptions and other possible computational processes for the register files.  
         [0022]    The memory array  8  is used to store signals representing instructions of programs and data which are processed by the microprocessor  2 . The programs may be types, including but not limited to, operating system programs and application programs.  
         [0023]    Also, the instruction scheduler  10  receives as input the instructions stored in the memory array  8 . The instruction scheduler schedules the received instructions for processing by the execution units  6 . Also, the instruction scheduler  10  receives retirement information from the retirement unit  12  for scheduling instructions based on when it is safe to write into the register files  4 .  
         [0024]    [0024]FIG. 2A illustrates a block diagram of the memory array elements used in the present invention. The microprocessor  2  has a number of A architectural registers, P number of supported physical registers, and W maximum number of instructions that may be active (i.e., scheduled to execute or currently executing) in the processor  2  at any given time. For illustrative purposes P is 180, A is 116, and W is 128.  
         [0025]    Furthermore, a typical microprocessor instruction may be of the following form: ADD R1, R2−&gt;R3  
         [0026]    In this case, the instruction adds two values together (R1 and R2) and stores the result as R3. The values being added together, R1 and R2, are referred to as source operands, while R3 is referred to as the destination register. R1, R2, and R3 are architectural register pointers that do not have a fixed correlation with a physical register in the processor  2 .  
         [0027]    For each incoming instruction into the instruction scheduler unit  10 , that requires a destination register, specific physical registers within the processor&#39;s register files  4  must be assigned to store the results of each instruction, i.e., acts as the temporary holding cell for each resultant value. The process of assigning physical registers to temporarily hold the values referenced by an architectural register is referred to as “register mapping”. Within the instruction scheduling unit, there are three structures used to perform this register mapping process, to track these mappings as instructions progress through the processor, and also to ensure that each physical register is reused when it is no longer needed.  
         [0028]    The first of these structures is the Free Physical Register Array  20  (FPRA)  20 . The FPRA  20  is a memory structure that keeps track of P-A registers that are not being used, and are thus “free” or “soon-to-be-free”., or available to be used as a destination register. The FPRA  20  therefore provides a physical register number for each instruction that requires a destination register. Once a physical register is assigned, or mapped, it is no longer considered “free” because it is now “in-use”.  
         [0029]    The assigned physical register number  21 A, along with its corresponding architectural register number  21 B, are sent to the Active Physical Register Mapper (APRM)  22 . The APRM  22  is a memory structure that stores the architectural to physical destination register mapping information for each of the W instructions that are currently active in the processor. The primary purpose of the APRM  22  is to act as a FIFO style buffer, (i.e., mapping information is read from the APRM  22  in the same order in which it was written).  
         [0030]    Once an instruction has retired, i.e., it has moved through the execution unit and its results have been written into the register file (thus updating the formal state of the processor), the mapping information (architectural register numbers  23 A and physical register numbers  23 B) of that instruction&#39;s destination register that resides in the APRM  22  is then moved to the Retired Physical Register Mapper (RPRM)  24 . The RPRM  24  is a memory structure that stores the architectural to physical destination mapping information for each of the A supported architectural registers. The RPRM  24  is organized as a RAM (random access memory) structure that stores physical register numbers  23 B, and is indexed by the architectural register number  23 A by way of a write decoder  25 A in the RAM. Therefore, by sending the RPRM  24  an architectural register as an address to a read address decoder  25 B, the RPRM  24  provides the corresponding physical register to which the architectural register is mapped.  
         [0031]    Each time an instruction that uses a destination register enters the instruction scheduling unit, the previous contents of that destination register will be overwritten when that instruction retires. Therefore, the contents of the physical register that is being used to store the value of that architectural register will soon be obsolete and can therefore be moved to the FPRA  20  and marked as “soon-to-be-free”. Once the instruction that overwrites this value retires, making the overwriting officially complete, the “soon-to-be-free” register in the FPRA  20  becomes a “free” register. Thus, the physical register can be re-used as a destination register. Therefore, the FPRA  20  essentially acts as a FIFO buffer with “soon-to-be-free” registers being rewritten into the FPRA  20 , and then being read out of the FPRA  20  as “free registers” in the same order.  
         [0032]    For a multi-threaded processor, the mapping information capacity of the FPRA and the RPRM must be increased to provide one full set of mappings for each supported thread. But the APRM need not be duplicated regardless of the number of supported threads.  
         [0033]    Furthermore, the contents of the FPRA  20  and the RPRM  24  must intialized at power-up such that one reference to each of the P supported physical registers is found in one or the other of these two structures. Likewise, the contents of the APRM  22  must be invalidated at power-up such that it contains no valid mapping information.  
         [0034]    [0034]FIG. 2B illustrates another use of the APRM  22  and RPRM  24  memory structures. These two memory structures can also be designed to support a secondary function to help increase the efficiency of the instruction scheduler unit  10 . The APRM  22  can also act as a CAM (content addressable memory) to provide register mapping information (physical register number  21  A and architectural register number  21 B) of instructions that are currently active in the pipeline. This function is useful for finding the physical register locations for an instruction&#39;s source operands. For example, if an incoming instruction uses an architectural register number  26 A such as the 5 th  architecture register (R5) as an operand, the value ‘5’ can be sent to the APRM  22  CAM, which will then provide the physical register source number  26 B mapped to the 5 th  architectural register if R5 is the destination register of an instruction that is currently active in the processor.  
         [0035]    At the same time, the architectural register number  26 A value can be sent to the RPRM  24 , which will return the physical register number  26 B for the architectural register. If no mapping information is found in the APRM  22 , then it can safely assume that the the instruction that provided the value stored has been retired, and the mapping information from the RPRM  24  should be used instead. In this way, the APRM  22  and the RPRM  24  can work together to provide the physical register pointers that are being used to stored the source operands for each new incoming instruction.  
         [0036]    [0036]FIG. 3 is a flowchart illustrating the inventive memory structures tracking and recycling of physical registers. As stated above, the FPRA  20  tracks  64  of the 180 physical registers. The FPRA  20  and RPRM  22  memory structures are initialized for processing as shown in step  32 . The FPRA  20  tags each of its physical register as “free”, meaning that they were currently available for use as a destination operand. The physical registers in FPRA  20  can also be tagged as “soon to be free”, meaning that they are no longer in use and will soon be free once the speculative conditions which initiated the change in status are confirmed to be correct as shown as step  34 . Once a physical register has been assigned as a destination operand by the FPRA  20 , that register assignment information is moved to the APRM  22  as shown in step  36 . Essentially, the APRM  22  tracks the physical register assignment information for instructions that are currently in the execution pipeline. The physical registers stored in the APRM  22  are marked as “in use, not yet retired” as shown in step  38 .  
         [0037]    The microprocessor  2  determines whether an instruction has retired from the execution pipeline as shown in step  40 . If the instruction retires from the execution pipeline, the physical register assignment information moves from the APRM  22  to the RPRM  24  as shown in step  42 . Otherwise, the APRM  22  maintains the physical assignment information. The RPRM  24  tracks physical register information for  116  registers. The physical registers in the RPRM  24  are essentially marked as “in-use, retired” as shown in step  44 . Once an architectural register in the RPRM  24  is reused as a destination register, the corresponding physical register is no longer “inuse”. Subsequently, the physical register is moved from the RPRM  24  to the FPRA  20 , and the FPRA  20  marks these physical registers as “soon to be free”. When the architectural register is officially overwritten, the contents of the “soon-to-be-free” register in the FPRA  20  become obsolete and the physical register is then marked as “free”. Subseqently, this makes the physical register available for use by another instruction as shown in step  46 .  
         [0038]    Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the invention. Details of the structure may vary substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved. It is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.